Category Archives: College of the Redwoods

Earthquake Report: Greece

Well, I was about to head to town and noticed a magnitude M = 5.0 earthquake in Greece. I thought to myself, I wonder if that is a foreshock. It was.

Then, the M 6.8 mainshock hit while i was out and about, followed by a M = 5.2 aftershock.

Before I looked more closely, I thought this sequence might be related to the Kefallonia fault. I prepared some earthquake reports for earthquakes here in the past, in 2015 and in 2016.

Both of those earthquakes were right-lateral strike-slip earthquakes associated with the Kefallonia fault.

However, today’s earthquake sequence was further to the south and east of the strike-slip fault, in a region experiencing compression from the Ionian Trench subduction zone. But there is some overlap of these different plate boundaries, so the M 6.8 mainshock is an oblique earthquake (compressional and strike-slip). Based upon the sequence, I interpret this earthquake to be right-lateral oblique. I could be wrong.

There are records of tsunami observed on tide gage data.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

The poster below includes earthquakes that represent the different plate boundaries and tectonic regimes.

  • The 1999 M = 7.6 Izmit earthquake was quite damaging and deadly earthquake on the North Anatolian fault. To the east, the majority of this plate boundary has ruptured in the 20th century. The last portion of the fault to rupture is to the west of this M = 7.6 earthquake and those who live in Istanbul would do well to invest in earthquake resilient building design. The Iszmit earthquake generated a tsunami with run up elevations about 2 meters, though had localized larger run ups due to a submarine landslide.
  • The 1981 M = 7.2 earthquake shows that this dextral (right-lateral) strain extends through the region into eastern Greece.
  • The 2015 M = 6.5 earthquake I mention above is plotted, showing the right-lateral sense of motion associated with the Kefallonia fault. There was a tsunami observed following this earthquake, probably associated with a landslide also observed (dust was seen and photographed).
  • The 2008 M = 6.9 earthquake is a thrust earthquake and represents the convergence (compression) associated with the convergent plate boundary associated with the Ionian Trench.
  • The 2017 M = 6.6 earthquake is an interesting earthquake that shows the upper plate deformation in the Anatolia plate in western Turkey is extending. Geologic structural cross sections in this region shows that this extension has been ongoing for millions of years. Here is my earthquake report for this 2017 M 6.6 earthquake. There was a tsunami observed as a result of this earthquake, believe it or not.
  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>
  • I include the faults from the NOA Digital Database for Active faults in Greece (Ganas et a., 2013) as red lines.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a small scale map showing the major tectonic fault systems in the eastern Mediterranean (Taymaz et al., 2007). The large black arrows show relative plate motions. I place a blue star in the general location of today’s earthquake sequence.
  • In the lower left corner is a generalized view of the tectonic regimes as interpreted by Taymaz et al. (2007). Today’s earthquake is in the SW Aegena/Peloponnisos plate, a region of compression associated with the Ionian Trench subduction zone. Today’s earthquake was probably right-lateral oblique, given the spatial relations between the different earthquakes.
  • In the upper right corner is a figure that shows GPS plate motion vectors (Ganas and Parsons, 2009). NOt how the vectors in the northeast are parallel to the North Anatolian fault and, as one moves to the southwest, they become normal (perpendicular) to the Ionian trench.
  • In the lower right corner is a more detailed map showing an interpretation of the faulting in the region (Kokkalas et al., 2006).
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a century’s seismicity plotted.

  • Here is the tide gage data from Katakolo, which is ony 65 km from the M 6.8 epicenter.

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the large scale tectonic setting map (Taymaz et al., 2007) with their figure below.

  • Summary sketch map of the faulting and bathymetry in the Eastern Mediterranean region, compiled from our observations and those of Le Pichon & Angelier (1981), Taymaz (1990), Taymaz et al. (1990, 1991a, b); S¸arogˇlu et al. (1992), Papazachos et al. (1998), McClusky et al. (2000) and Tan & Taymaz (2006). Large black arrows show relative motions of plates with respect to Eurasia (McClusky et al. 2003). Bathymetry data are derived from GEBCO/97–BODC, provided by GEBCO (1997) and Smith & Sandwell (1997a, b). Shaded relief map derived from the GTOPO-30 Global Topography Data taken after USGS. NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; NEAF, North East Anatolian Fault; EPF, Ezinepazarı Fault; PTF, Paphos Transform Fault; CTF, Cephalonia Transform Fault; PSF, Pampak–Sevan Fault; AS, Apsheron Sill; GF, Garni Fault; OF, Ovacık Fault; MT, Mus¸ Thrust Zone; TuF, Tutak Fault; TF, Tebriz Fault; KBF, Kavakbas¸ı Fault; MRF, Main Recent Fault; KF, Kagˇızman Fault; IF, Igˇdır Fault; BF, Bozova Fault; EF, Elbistan Fault; SaF, Salmas Fault; SuF, Su¨rgu¨ Fault; G, Go¨kova; BMG, Bu¨yu¨k Menderes Graben; Ge, Gediz Graben; Si, Simav Graben; BuF, Burdur Fault; BGF, Beys¸ehir Go¨lu¨ Fault; TF, Tatarlı Fault; SuF, Sultandagˇ Fault; TGF, Tuz Go¨lu¨ Fault; EcF, Ecemis¸ Fau; ErF, Erciyes Fault; DF, Deliler Fault; MF, Malatya Fault; KFZ, Karatas¸–Osmaniye Fault Zone.

  • This figure shows GPS velocities in the region (Taymaz et al., 2007).

  • GPS horizontal velocities and their 95% confidence ellipses in a Eurasia-fixed reference frame for the period 1988–1997 superimposed on a shaded relief map derived from the GTOPO-30 Global Topography Data taken after USGS. Bathymetry data are derived from GEBCO/97–BODC, provided by GEBCO (1997) and Smith & Sandwell (1997a, b). Large arrows designate generalized relative motions of plates with respect to Eurasia (in mm a21) (recompiled after McClusky et al. 2000). NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; NEAF, North East Anatolian Fault; EPF, Ezinepazarı Fault; CTF, Cephalonia Transform Fault; PTF, Paphos Transform Fault; CMT, Caucasus Main Thrust; MRF, Main Recent Fault.

  • Finally their summary figure showing the tectonic regimes (Taymaz et al., 2007).

  • Schematic map of the principal tectonic settings in the Eastern Mediterranean. Hatching shows areas of coherent motion and zones of distributed deformation. Large arrows designate generalized regional motion (in mm a21) and errors (recompiled after McClusky et al. (2000, 2003). NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; NEAF, North East Anatolian Fault; EPF, Ezinepazarı Fault; CTF, Cephalonia Transform Fault; PTF, Paphos Transform Fault.

  • This is a tectonic summary figure from Kokkalas et al. (2006).

  • Simplified map showing the main structural features along the Hellenic arc and trench system, as well as the main active structures in the Aegean area. The mean GPS horizontal velocities in the Aegean plate, with respect to a Eurasia-fixed reference frame, are shown (after Kahle et al., 1998; McClusky et al., 2000). The lengths of vectors are
    proportional to the amount of movement. The thick black arrows indicate the mean motion vectors of the plates. The polygonal areas on the map (dashed lines) define the approximate borders of the five different structural regions discussed in the text. The borders between structural regions are not straightforward, and wide transitional zones probably exist between them. The inset shows a schematic map with the geodynamic framework in the eastern Mediterranean area (modified from McClusky et al., 2000). DSF—Dead Sea fault; EAF—East Anatolia fault; HT—Hellenic trench; KFZ— Kefallonia fault zone; MRAC—Mediterranean Ridge accretionary complex; NAF—North Anatolia fault; NAT—North Aegean trough.

  • Here is their detailed view of the faulting in the region (Kokkalas et al., 2006)

  • General simplified structural map of Greece showing the main currently active structures in the five structural regions along the Hellenic Arc, as well as some main pre-existing lineaments. Insets illustrate the main structural features of each region and the period of activity of these structures (for further details see discussion). KFZ—Kefallonia Fault zone; MCL—Mid-Cycladic lineament; NAFZ—North Anatolia fault zone; NAT—North Aegean trough; PF—Pelagonian fault.

  • Here is an even more detailed view of this region (Kokkalas et al., 2006). Note how the Convergent plate boundary “Ionian thrust” overlaps with the strike-slip faulting of the Kefallonia fault. Today’s M 6.8 happened south of where these authors map the Ionian thrust extending south from Zakynthos Island.

  • Schematic structural map of the central Hellenic Peninsula (Region II), with stress nets showing the orientation of principal stress axes. Stress net explanation as for Figure 3. Also included are cross-sections showing the geometry and kinematics of the External Hellenides in the area (A-A′) and the evolution of the synorogenic basin in the Paleros area (B-B′-B′′). AG—Abelon graben; ALG—Almyros graben; AMG—Amvrakikos graben; CG—Corinth graben; KB—Kymi basin; KF—Klenia fault zone; KFZ—Kefalonia fault zone; LF—Lapithas fault; MG—Megara graben; NG—Nedas graben; P—Parnitha area; PG—Pyrgos graben; PLB—Paleros basin; PTG—Patras graben; RG—Rio graben; S-A.G— Sperchios-Atalanti graben; SEG—South Evoikos graben; TB—Thiva basin; TG—Tithorea graben; TRG—Trihonis graben; VF—Vounargos fault.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    References:

  • Ganas, A., and T. Parsons (2009), Three-dimensional model of Hellenic Arc deformation and origin of the Cretan uplift, J. Geophys. Res., 114, B06404, doi:10.1029/2008JB005599
  • Ganas, A., Oikonomou, I.A., and Tsimi, C., 2013. NOAFAULTS: A Digital Database for Active Faults in Greece in Bulletin of the Geological Society of Greece, v. XLVII, Proceedings fo the 13th International Cogfress, Chania, Sept, 2013
  • Kokkalas, S., Xypolias, P., Koukouvelas, I., and Doutsos, T., 2006, Postcollisional contractional and extensional deformation in the Aegean region, in Dilek, Y., and Pavlides, S., eds., Postcollisional tectonics and magmatism in the Mediterranean region and Asia: Geological Society of America Special Paper 409, p. 97–123, doi: 10.1130/2006.2409(06)
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • Taymaz, T. , Yilmaz, Y., and Dilek, Y., 2007. The geodynamics of the Aegean and Anatolia: introduction in TAYMAZ, T., YILMAZ, Y. & DILEK, Y. (eds) The Geodynamics of the Aegean and Anatolia. Geological Society, London, Special Publications, 291, 1–16. DOI: 10.1144/SP291.1 0305-8719/07

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Earthquake Report: Explorer plate

Last night I had completed preparing for class the next day. I was about to head to bed. I got an email from the Pacific Tsunami Warning Center notifying me that there was no risk of a tsunami due to an earthquake with a magnitude M 6.6. I noticed it was along the Sovanco fault, a transform fault (right-lateral strike-slip). Strike slip faults can produce tsunami, but they are smaller than tsunami generated along subduction zones. The recent M = 7.5 Donggala Earthquake in Sulawesi, Indonesia is an example of a tsunami generated in response to a strike-slip earthquake (tho coseismic landslides may be part of the story there too).

I thought I could put together a map in short time as I already had a knowledge base for this area (e.g. earthquake reports from 2017.01.07 and 2016.03.18). However, as I was creating base maps in Google Earth, before I completed making a set (the posters below each take 4 different basemaps displayed at different transparencies), there was the M 6.8 earthquake. Then there was the M 6.6 earthquake. I had to start all over. Twice. Heheh.

This region of the Pacific-North America plate boundary is at the northern end of the Cascadia subduction zone (CSZ). To the east, the Explorer and Juan de Fuca plates subduct beneath the North America plate to form the megathrust subduction zone fault capable of producing earthquakes in the magnitude M = 9 range. The last CSZ earthquake was in January of 1700, just almost 319 years ago.

The Juan de Fuca plate is created at an oceanic spreading center called the Juan de Fuca Ridge. This spreading ridge is offset by several transform (strike-slip) faults. At the southern terminus of the JDF Ridge is the Blanco fault, a transtensional transform fault connecting the JDF and Gorda ridges.

At the northern terminus of the JDF Ridge is the Sovanco transform fault that strikes to the northwest of the JDF Ridge. There are additional fracture zones parallel and south of the Sovanco fault, called the Heck, Heckle, and Springfield fracture zones.

The first earthquake (M = 6.6) appears to have slipped along the Sovanco fault as a right-lateral strike-slip earthquake. Then the M 6.8 earthquake happened and, given the uncertainty of the location for this event, occurred on a fault sub-parallel to the Sovanco fault. Then the M 6.5 earthquake hit, back on the Sovanco fault.

So, I would consider the M 6.6 to be a mainshock that triggered the M 6.8. The M 6.5 is an aftershock of the M 6.6.

Based upon our knowledge of how individual earthquakes can change the stress (or strain) in the surrounding earth, it is unlikely that this earthquake sequence changed the stress on the megathrust. Over time, hundreds of these earthquakes do affect the potential for earthquakes on the CSZ megathrust. But, individual earthquakes (or even a combination of these 3 earthquakes) do not change the chance that there will be an earthquake on the CSZ megathrust. The chance of an earthquake tomorrow is about the same as the chance of an earthquake today. Day to day the chances don’t change much. However, year to year, the chances of an earthquake get higher and higher. But of course, we cannot predict when an earthquake will happen.

So, if we live, work, or play in earthquake country, it is best to always be prepared for an earthquake, for tsunami, and for landslides.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

I include the earthquake mechanisms for 2 special earthquakes that happened in the past two decades along this plate boundary system. In 2001 the M 6.8 Nisqually earthquake struck the Puget Sound region of Washington causing extensive damage. This earthquake was an extensional earthquake in the downgoing JDF plate. The damage was extensive because the earthquake was close to an urban center, where there was lots of infrastructure to be damaged (the closer to an earthquake, the higher the shaking intensity).

In 2012 was a M = 7.8 earthquake along the northern extension of the CSZ. The northern part of the CSZ is a very interesting region, often called the Queen Charlotte triple junction. There are some differences than the Mendocino triple junction to the south, in northern California. There continues to be some debate about how the plate boundary faults are configured here. The Queen Charlotte is a right lateral strike slip fault that extends from south of Haida Gwaii (the large island northwest of Vancouver Island) up northwards, where it is called the Fairweather fault. There are several large strike-slip earthquakes on the Queen Charlotte/Fairweather fault system in the 20th century. However, the 2012 earthquake was a subduction zone fault, evidence that the CSZ megathrust (or some semblance of this subduction zone) extends beneath Haida Gwaii (so the CSZ and QCF appear to over lap).

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly northeast-southwest trends of these red and blue stripes in the JDF and Pacific plates. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying North America plate (and accretionary prism) mask the evidence for the JDF plate.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004). I place a blue star in the general location of today’s seismicity.
  • In the upper left corner is a map showing the plate boundary faults associated with the northern CSZ and to the north (including the Queen Charlotte fault; Braunmiller and Nabalek, 2002). I place a red star in the general location of today’s seismicity. These earthquakes occurred in the region east of the Explorer rift. This region of the world still contains some major tectonic mysteries and this is quite exciting. This shows the Winona Block as a microplate between the Pacific and North America plates, north of the Explorer plate. The Winona Block is labeled “WIN BLOCK” on the map. Note that there are two spreading ridges on the western and central part of this block. It is possible that the Explorer ridge-rift system extends into the Winona Block to form a third spreading ridge in the Winona Block.
  • In the lower left corner is a map from Dziak (2006). Dziak (2006) used bathymetric and seismologic data to evaluate the faulting in the region and discussed how the Explorer plate is accommodating a reorganization of the plate boundary.
  • Here is the map with a month’s seismicity plotted.


  • Here is the map with a century’s seismicity plotted.


  • Here is a video showing the earthquake epicenters for the period of 1900-2017 for USGS earthquakes with magnitudes M ≥ 5.0. Here is a link to the embedded video below (2.5 MB mp4). Note how the earthquakes that happen between the northern terminus of the JDF Ridge and the southern terminus of the Queen Charlotte fault form a wide band (not a stepwise patter that might reflect steps in ridges and spreading centers). This pattern is key to unravelling the mysteries of the western Explorer plate.
    • Here is the map with the seismicity from 1900-2017 plotted. These are USGS earthquakes with magnitudes M ≥ 7.0 for this time period. I include the moment tensors from the 2012 and 2013 earthquakes (the only earthquakes for this time period that have USGS moment tensors). The 2012 earthquake generated a tsunami. I discuss the 2012 “Haida Gwaii” earthquake here.


Other Report Pages

Some Relevant Discussion and Figures

  • Here is the general tectonic map of the region (Braunmiller and Nabalek, 2002). Today’s earthquakes happened in a place that suggest the Explorer ridge extends further to the north into the Winona Block. Below I include the text from the original figure caption in blockquote.

  • Map of Explorer region and surroundings. Plate boundaries are based on Riddihough’s [1984] and Davis and Riddihough’s [1982] tectonic models. Solid lines are active plate boundaries (single lines are transform faults, double lines are spreading centers, barbed lines are subduction zones with barbs in downgoing plate direction). The wide double line outlines the width of the Sovanco fracture zone, and the dots sketch the Explorer-Winona boundary. Plate motion vectors (solid arrows) are from NUVEL-1A [DeMets et al., 1994] for Pacific-North America motion and from Wilson [1993] for Pacific-Juan de Fuca and Juan de Fuca-North America motion. Open arrows are Explorer relative plate motions averaged over last 1 Myr [Riddihough, 1984] (in text, we refer to these most recent magnetically determined plate motions as the ‘‘Riddihough model’’). Winona block motions (thin arrows), described only qualitatively by Davis and Riddihough [1982], are not to scale. Abbreviations are RDW for Revere-Dellwood- Wilson, Win for Winona, FZ for fault zone, I for island, S for seamount, Pen for peninsula.

  • Here is the larger scale figure that shows the details of the plate boundary in this region (Braunmiller and Nabalek, 2002). Below I include the text from the original figure caption in blockquote.

  • Close-up of the Pacific-Explorer boundary. Plotted are fault plane solutions (gray scheme as in Figure 3) and well-relocated earthquake epicenters. The SeaBeam data are from the RIDGE Multibeam Synthesis Project (http://imager.ldeo.columbia.edu) at the Lamont-Doherty Earth observatory. Epicenters labeled by solid triangles are pre-1964, historical earthquakes (see Appendix B). Solid lines mark plate boundaries inferred from bathymetry and side-scan data [Davis and Currie, 1993]; dashed were inactive. QCF is Queen Charlotte fault, TW are Tuzo Wilson seamounts, RDW is Revere-Dellwood-Wilson fault, DK are Dellwood Knolls, PRR is Paul Revere ridge, ER is Explorer Rift, ED is Explorer Deep, SERg is Southern Explorer ridge, ESM is Explorer seamount, SETB is Southwest Explorer Transform Boundary, SAT is Southwestern Assimilated Territory, ESDZ is Eastern Sovanco Deformation Zone, HSC is Heck seamount chain, WV is active west valley of Juan de Fuca ridge, MV is inactive middle valley.

  • This is the figure that shows an interpretation of how this plate boundary formed over the past 3 Ma (Braunmiller and Nabalek, 2002). Below I include the text from the original figure caption in blockquote.

  • Schematic plate tectonic reconstruction of Explorer region during the last 3 Myr. Note the transfer of crustal blocks (hatched) from the Explorer to the Pacific plate; horizontal hatch indicates transfer before 1.5 Ma and vertical hatch transfer since then. Active boundaries are shown in bold and inactive boundaries are thin dashes. Single lines are transform faults, double lines are spreading centers; barbed lines are subduction zones with barbs in downgoing plate direction. QCF is Queen Charlotte fault, TW are the Tuzo Wilson seamounts, RDW is Revere-Dellwood-Wilson fault, DK are the Dellwood Knolls, ED is Explorer Deep, ER is Explorer Rift, ERg is Explorer Ridge, ESM is Explorer Seamount, SOV is Sovanco fracture zone, ESDZ is Eastern Sovanco Deformation Zone, JRg is Juan de Fuca ridge, and NF is Nootka fault. The question mark indicates ambiguity whether spreading offshore Brooks peninsula ceased when the Dellwood Knolls became active (requiring only one independently moving plate) or if both spreading centers, for a short time span, where active simultaneously (requiring Winona block motion independent from Explorer plate during that time).

  • Below I include some inset maps from Audet et al. (2008 ) and Dziak (2006). Each of these authors have published papers about the Explorer plate. Dziak (2006) used bathymetric and seismologic data to evaluate the faulting in the region and discussed how the Explorer plate is accommodating a reorganization of the plate boundary. Audet et al. (2008 ) use terrestrial seismic data to evaluate the crust along northern Vancouver Island and present their tectonic map as part of this research (though they do not focus on the offshore part of the Explorer plate). I include these figures below along with their figure captions. Today’s earthquakes happened at the northwestern portion of these maps from Dziak (2006).
  • Dziak, 2006

  • This map shows the shape of the seafloor in this region and there is an inset map that shows the major fault systems here.

  • Bathymetric map of northern Juan de Fuca and Explorer Ridges. Map is composite of multibeam bathymetry and satellite altimetry (Sandwell and Smith, 1997). Principal structures are labeled: ERB—Explorer Ridge Basin, SSL—strike-slip lineation. Inset map shows conventional tectonic interpretation of region. Dashed box shows location of main figure. Solid lines are active plate boundaries, dashed line shows Winona-Explorer boundary, gray ovals represent seamount chains. Solid arrows show plate motion vectors from NUVEL-1A (DeMets et al., 1994) for Pacific–North America and from Wilson (1993) for Pacific–Juan de Fuca and Juan de Fuca–North America. Open arrows are Explorer relative motion averaged over past 1 m.y. (Riddihough, 1984). Abbreviations: RDW—Revere-Dellwood-Wilson,Win—Winona block, C.O.—Cobb offset, F.Z.—fracture zone. Endeavour segment is northernmost section of Juan de Fuca Ridge.

  • This map shows the line work Dziak (2006) used to delineate the structures shown in the bathymetric map.

  • Structural interpretation map of Explorer–Juan de Fuca plate region based on composite multibeam bathymetry and satellite altimetry data (Fig. 1). Heavy lines are structural (fault) lineations, gray circles and ovals indicate volcanic cones and seamounts, dashed lines are turbidite channels. Location of magnetic anomaly 2A is shown; boundaries are angled to show regional strike of anomaly pattern.

  • This map shows the seismicity patterns (this matches the patterns in the animation above).

  • Earthquake locations estimated using U.S. Navy hydrophone arrays that occurred between August 1991 and January 2002. Focal mechanisms are of large (Mw>4.5) earthquakes that occurred during same time period, taken from Pacific Geoscience Center, National Earthquake Information Center, and Harvard moment-tensor catalogs. Red mechanism shows location of 1992 Heck Seamount main shock.

  • Here Dziak (2006) shows how they interpret that this plate boundary is being reconfigured with time. Like the rest of the adjacent plate boundary (Queen Charlotte/Fairweather, Cascadia, San Andreas), there is an overall dextral (right-lateral) shear couple between the North America and Pacific plates. Some of the existing structures represent the orientation of faults from an earlier strain field. Eventually through going faults will align with the band of seismicity in the above map and above animation. At least, that is one hypothesis. Seems reasonable to me, given the very short record of earthquakes.

  • Tectonic model of Explorer plate boundaries. Evidence presented here is consistent with zone of shear extending through Explorer plate well south of Sovanco Fracture Zone (SFZ) to include Heck, Heckle, and Springfield seamounts, and possibly Cobb offset (gray polygon roughly outlines shear zone). Moreover, Pacific– Juan de Fuca–North American triple junction may be reorganizing southward to establish at Cobb offset. QCF—Queen Charlotte fault.

  • From Audet et al. (2008), here is another view of the fault system in this part of the plate boundary.

  • Identification of major tectonic features in western Canada. BP—Brooks Peninsula, BPfz—Brooks Peninsula fault zone, NI— Nootka Island, QCTJ—Queen Charlotte triple junction. Dotted lines delineate extinct boundaries or shear zones. Seismic stations are displayed as inverted black triangles. Station projections along line 1 and line 2 are plotted as thick white lines. White triangles represent Alert Bay volcanic field centers. Center of array locates town of Woss. Plates: N-A—North America; EXP—Explorer; JdF—Juan de Fuca; PAC—Pacific.

  • Speaking of the Queen Charlotte/Fairweather fault system, here is another map that shows the tectonics of this region. Hyndman (2015) shows the region where the 2012 Haida Gwaii earthquake ruptured. I include two more figures below. This figure Below I include the text from the original figure caption in blockquote.

  • The Queen Charlotte fault (QCF) zone, the islands of Haida Gwaii and adjacent area, and the locations of the 2012 Mw 7.8 (ellipse), 2013 Mw 7.5 (solid line), and 1949 Ms 8.1 (dashed) earthquakes. The along margin extent of the 1949 event is not well constrained.

  • This map shows the main and aftershocks from the 2012 Haida Gwaii earthquake sequence (Hyndman, 2015). This 2012 sequence is interesting because, prior to these earthquakes, it was unclear whether the fault along Haida Gwaii was a strike-slip or a thrust fault. For example, Riddihough (1984) suggests that there is no subduction going on along the Explorer plate at all. Turns out it is probably both. When this 2012 earthquake happened, I took a look at the bathymetry in Google Earth and noticed the Queen Charlotte Terrace, which looks suspiciously like an accretionary prism. This was convincing evidence for the thrust fault earthquakes. Below I include the text from the original figure caption in blockquote.

  • Aftershocks of the 2012 Mw 7.8 Haida Gwaii thrust 13 earthquake (after Cassidy et al., 2013). They approximately define the rupture area. The normal-faulting mechanisms for two of the larger aftershocks are also shown. Many of the aftershocks are within the incoming oceanic plate and within the overriding continental plate rather than on the thrust rupture plane.

  • This is a great version of this figure that shows how there are overlapping subduction (thrust) and transform (strike-slip) faults along the Haida Gwaii region (Hyndman, 2015). Below I include the text from the original figure caption in blockquote.

  • Model for the 2012 Mw 7.8 earthquake rupture and the partitioning of oblique convergence into margin parallel motion on the Queen Charlotte transcurrent fault and nearly orthogonal thrust convergence on the Haida Gwaii thrust fault.

  • Here is a figure that shows two ways of interpreting the Queen Charlotte triple junction region (Kreemer et al., 1998). Note the 1900-2017 seismicity map above, which supports the interpretation in the right panel (B). Something of trivial nature is that this article is from the pre-computer illustration era (see the squiggly hand drawn arrow in the right panel B). Below I include the text from the original figure caption in blockquote.

  • (A) Major tectonic features describing the micro-plate model for the Explorer region. The Explorer plate (EXP) is an independent plate and is in convergent motion towards the North American plate (NAM). V.I. D Vancouver Island; PAC D the Pacific plate; JdF D the Juan the Fuca plate. The accentuated zone between the Explorer and JdF ridges is the Sovanco transform zone and the two boundary lines do not indicate the presence of faults but define the boundaries of this zone of complex deformation. (B) The key features of the pseudo-plate model for the region are a major plate boundary transform fault zone between the North American and Pacific plates and the Nootka Transform, a left-lateral transform fault north of the Juan the Fuca plate.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.


    References:

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Braunmiller, J. and Nabelek, J., 2002. Seismotectonics of the Explorer region in JGR, v. 107, NO. B10, 2208, doi:10.1029/2001JB000220, 2002
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Audet, P., Bostock, M.G., Mercier, J.-P., and Cassidy, J.F., 2008., Morphology of the Explorer–Juan de Fuca slab edge in northern Cascadia: Imaging plate capture at a ridge-trench-transform triple junction in Geology, v. 36, p. 895-898.
  • Clarke, S. H., and Carver, G. C., 1992. Late Holocene Tectonics and Paleoseismicity, Southern Cascadia Subduction Zone, Science, vol. 255:188-192.
  • Dziak, R.P., 2006. Explorer deformation zone: Evidence of a large shear zone and reorganization of the Pacific–Juan de Fuca–North American triple junction in Geology, v. 34, p. 213-216.
  • Flück, P., Hyndman, R. D., Rogers, G. C., and Wang, K., 1997. Three-Dimensional Dislocation Model for Great Earthquakes of the Cascadia Subduction Zone, Journal of Geophysical Research, vol. 102: 20,539-20,550.
  • Heaton, f f., Kanamori, F. F., 1984. Seismic Potential Associated with Subduction in the Northwest United States, Bulletin of the Seismological Society of America, vol. 74: 933-941.
  • Hyndman, R. D., and Wang, K., 1995. The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime, Journal of Geophysical Research, vol. 100: 22,133-22,154.
  • Keemer, C., Govers, R., Furlong, K.P., and Holt, W.E., 1998. Plate boundary deformation between the Pacific and North America in the Explorer region in Tectonophysics, v. 293, p. 225-238.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • McPherson, R. M., 1989. Seismicity and Focal Mechanisms Near Cape Mendocino, Northern California: 1974-1984: M. S. thesis, Arcata, California, Humboldt State University, 75 p
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
  • Riddihough, R., 1984. Recent Movements of the Juan de Fuca Plate System in JGR, v. 89, no. B8, p. 6980-6994.
  • Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D., and Sagiya, T., 2003. A revised dislocation model of interseismic deformation of the Cascadia subduction zone Journal of Geophysical Research, B, Solid Earth and Planets v. 108, no. 1.

Return to the Earthquake Reports page.

Earthquake Report: New Britain!

Just a few hours there was a subduction zone megathrust earthquake along the New Britain Trench in the western equatorial Pacific Ocean.

In this region of the world, the Solomon Sea plate and the South Bismarck plate converge to form a subduction zone, where the Solomon Sea plate is the oceanic crust diving beneath the S.Bismarck plate.

The subduction zone forms the New Britain Trench with an axis that trends east-northeast. To the east of New Britain, the subduction zone bends to the southeast to form the San Cristobal and South Solomon trenches. Between these two subduction zones is a series of oceanic spreading ridges sequentially offset by transform (strike slip) faults.

Earthquakes along the megathrust at the New Britain trench are oriented with the maximum compressive stress oriented north-northwest (perpendicular to the trench). Likewise, the subduction zone megathrust earthquakes along the S. Solomon trench compress in a northeasterly direction (perpendicular to that trench).

There is also a great strike slip earthquake that shows that the transform faults are active.

This earthquake was too small and too deep to generate a tsunami.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 7.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed. These stripes make evident the spreading centers south of the Solomon Sea plate, forming the Woodlark Basin. Note how the color bands along the spreading centers (orange arrows pointing in direction of plate motion). What color are they? Why?

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a general overview of the plate boundaries and mapped faults in the region (Koulali et al., 2015). I place a blue star in the general location of the M 7.0 epicenter.
  • In the upper right corner is a more detailed tectonic map of the region, showing the ways that the S. Bismark plate is dissected by strike-slip faults. The active volcanoes are shown as red stars.
  • In the lower left corner are a couple figures Dr. Stephen Hicks prepared in response to an earthquake sequence earlier in 2018. On the left is a map showing recently observed seismicity. The seismicity that lies within the dashed box is used to plot the earthquakes with depth (the hypocenters). The March 2018 earthquakes are in yellow and orange. Today’s M 7.0 is shown as a blue star on both plots. The location of profile A-A’ is located in the general location on the earthquake interpretive poster.
  • Above these figures is a figure pair from Holm & Richards (2013). On the left is a map that shows land in green and the subducting Solomon Sea plate in black that turns orange with depth. The image on the right is a low angle oblique view of the slab, shoing the shape of the plate in 3-D. Note the tear in the slab. Read more about this below.
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a century’s seismicity plotted.

Other Report Pages

Some Relevant Discussion and Figures

  • Here are the figures presented by Dr. Hicks.
  • On March 26, 2018 there was an M 6.6 earthquake. Steve prepared these figures. 3 days later there was an M 6.9, which made the M 6.6 a foreshock.
  • Today’s earthquake sequence also included a foreshock-mainshock sequence. There was an M 6.1 and 3 minutes later there was the M 7.0, making the M 6.1 a foreshock. We do not know if an earthquake is a foreshock until there is a larger magnitude earthquake later.

Here is a visualization of the seismicity as presented by Dr. Steve Hicks.

  • Here is the generalized tectonic map of the region from Holm et al., 2015. I include the figure caption below as a blockquote.

  • Topography, bathymetry and regional tectonic setting of New Guinea and Solomon Islands. Arrows indicate rate and direction of plate motion of the Australian and Pacific plates (MORVEL, DeMets et al., 2010); Mamberamo thrust belt, Indonesia (MTB); North Fiji Basin (NFB).

  • Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • 3-D model of the Solomon slab comprising the subducted Solomon Sea plate, and associated crust of the Woodlark Basin and Australian plate subducted at the New Britain and San Cristobal trenches. Depth is in kilometres; the top surface of the slab is contoured at 20 km intervals from the Earth’s surface (black) to termination of slabrelated seismicity at approximately 550 km depth (light brown). Red line indicates the locations of the Ramu-Markham Fault (RMF)–New Britain trench (NBT)–San Cristobal trench (SCT); other major structures are removed for clarity; NB, New Britain; NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; TLTF, Tabar–Lihir–Tanga–Feni arc. See text for details.

  • Here are the forward models for the slab in the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • Forward tectonic reconstruction of progressive arc collision and accretion of New Britain to the Papua New Guinea margin. (a) Schematic forward reconstruction of New Britain relative to Papua New Guinea assuming continued northward motion of the Australian plate and clockwise rotation of the South Bismarck plate. (b) Cross-sections illustrate a conceptual interpretation of collision between New Britain and Papua New Guinea.

  • This map shows plate velocities and euler poles for different blocks. Note the counterclockwise motion of the plate that underlies the Solomon Sea (Baldwin et al., 2012). I include the figure caption below as a blockquote.

  • Tectonic maps of the New Guinea region. (a) Seismicity, volcanoes, and plate motion vectors. Plate motion vectors relative to the Australian plate are surface velocity models based on GPS data, fault slip rates, and earthquake focal mechanisms (UNAVCO, http://jules.unavco.org/Voyager/Earth). Earthquake data are sourced from the International Seismological Center EHB Bulletin (http://www.isc.ac.uk); data represent events from January 1994 through January 2009 with constrained focal depths. Background image is generated from http://www.geomapapp.org. Abbreviations: AB, Arafura Basin; AT, Aure Trough; AyT, Ayu Trough; BA, Banda arc; BSSL, Bismarck Sea seismic lineation; BH, Bird’s Head; BT, Banda Trench; BTFZ, Bewani-Torricelli fault zone; DD, Dayman Dome; DEI, D’Entrecasteaux Islands; FP, Fly Platform; GOP, Gulf of Papua; HP, Huon peninsula; LA, Louisiade Archipelago; LFZ, Lowlands fault zone; MaT, Manus Trench; ML, Mt. Lamington; MT, Mt. Trafalgar; MuT, Mussau Trough; MV, Mt. Victory; MTB, Mamberamo thrust belt; MVF, Managalase Plateau volcanic field; NBT, New Britain Trench; NBA, New Britain arc; NF, Nubara fault; NGT, New Guinea Trench; OJP, Ontong Java Plateau; OSF, Owen Stanley fault zone; PFTB, Papuan fold-and-thrust belt; PP, Papuan peninsula; PRi, Pocklington Rise; PT, Pocklington Trough; RMF, Ramu-Markham fault; SST, South Solomons Trench; SA, Solomon arc; SFZ, Sorong fault zone; ST, Seram Trench; TFZ, Tarera-Aiduna fault zone; TJ, AUS-WDKPAC triple junction; TL, Tasman line; TT, Trobriand Trough;WD, Weber Deep;WB, Woodlark Basin;WFTB, Western (Irian) fold-and-thrust belt; WR,Woodlark Rift; WRi, Woodlark Rise; WTB, Weyland thrust; YFZ, Yapen fault zone.White box indicates the location shown in Figure 3. (b) Map of plates, microplates, and tectonic blocks and elements of the New Guinea region. Tectonic elements modified after Hill & Hall (2003). Abbreviations: ADB, Adelbert block; AOB, April ultramafics; AUS, Australian plate; BHB, Bird’s Head block; CM, Cyclops Mountains; CWB, Cendrawasih block; CAR, Caroline microplate; EMD, Ertsberg Mining District; FA, Finisterre arc; IOB, Irian ophiolite belt; KBB, Kubor & Bena blocks (including Bena Bena terrane); LFTB, Lengguru fold-and-thrust belt; MA, Mapenduma anticline; MB, Mamberamo Basin block; MO, Marum ophiolite belt; MHS, Manus hotspot; NBS, North Bismarck plate; NGH, New Guinea highlands block; NNG, Northern New Guinea block; OKT, Ok Tedi mining district; PAC, Pacific plate; PIC, Porgera intrusive complex; PSP, Philippine Sea plate; PUB, Papuan Ultramafic Belt ophiolite; SB, Sepik Basin block; SDB, Sunda block; SBS, South Bismarck plate; SIB, Solomon Islands block; WP, Wandamen peninsula; WDK, Woodlark microplate; YQ, Yeleme quarries.

  • This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). The New Britain region is in the map near the A and B sections. I include the figure caption below as a blockquote.

  • Oblique block diagram of New Guinea from the northeast with schematic cross sections showing the present-day plate tectonic setting. Digital elevation model was generated from http://www.geomapapp.org. Oceanic crust in tectonic cross sections is shown by thick black-and-white hatched lines, with arrows indicating active subduction; thick gray-and-white hatched lines indicate uncertain former subduction. Continental crust, transitional continental crust, and arc-related crust are shown without pattern. Representative geologic cross sections across parts of slices C and D are marked with transparent red ovals and within slices B and E are shown by dotted lines. (i ) Cross section of the Papuan peninsula and D’Entrecasteaux Islands modified from Little et al. (2011), showing the obducted ophiolite belt due to collision of the Australian (AUS) plate with an arc in the Paleogene, with later Pliocene extension and exhumation to form the D’Entrecasteaux Islands. (ii ) Cross section of the Papuan peninsula after Davies & Jaques (1984) shows the Papuan ophiolite thrust over metamorphic rocks of AUS margin affinity. (iii ) Across the Papuan mainland, the cross section after Crowhurst et al. (1996) shows the obducted Marum ophiolite and complex folding and thrusting due to collision of the Melanesian arc (the Adelbert, Finisterre, and Huon blocks) in the Late Miocene to recent. (iv) Across the Bird’s Head, the cross section after Bailly et al. (2009) illustrates deformation in the Lengguru fold-and-thrust belt as a result of Late Miocene–Early Pliocene northeast-southwest shortening, followed by Late Pliocene–Quaternary extension. Abbreviations as in Figure 2, in addition to NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; (U)HP, (ultra)high-pressure.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

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Earthquake Report: Sulawesi (Celebes), Indonesia

Well, around 3 AM my time (northeastern Pacific, northern CA) there was a sequence of earthquakes including a mainshock with a magnitude M = 7.5. This earthquake happened in a highly populated region of Indonesia.

This area of Indonesia is dominated by a left-lateral (sinistral) strike-slip plate boundary fault system. Sulawesi is bisected by the Palu-Kola / Matano fault system. These faults appear to be an extension of the Sorong fault, the sinistral strike-slip fault that cuts across the northern part of New Guinea.

There have been a few earthquakes along the Palu-Kola fault system that help inform us about the sense of motion across this fault, but most have maximum magnitudes mid M 6.

GPS and block modeling data suggest that the fault in this area has a slip rate of about 40 mm/yr (Socquet et al., 2006). However, analysis of offset stream channels provides evidence of a lower slip rate for the Holocene (last 12,000 years), a rate of about 35 mm/yr (Bellier et al., 2001). Given the short time period for GPS observations, the GPS rate may include postseismic motion earlier earthquakes, though these numbers are very close.

Using empirical relations for historic earthquakes compiled by Wells and Coppersmith (1994), Socquet et al. (2016) suggest that the Palu-Koro fault system could produce a magnitude M 7 earthquake once per century. However, studies of prehistoric earthquakes along this fault system suggest that, over the past 2000 years, this fault produces a magnitude M 7-8 earthquake every 700 years (Bellier et al., 2006). So, it appears that this is the characteristic earthquake we might expect along this fault.

Based on what we know about strike-slip fault earthquakes, the portions of the fault to the north and south of today’s sequence may have an increased amount of stress due to this earthquake. Stay tuned for a Temblor.net report about this earthquake where I discuss this further.

There are reports of a local tsunami with a run-up about 2 meters. However, the UNESCO Sea Level Monitoring Facility (website) does not show any tsunami observations on tide gage data in the region.

Most commonly, we associate tsunamigenic earthquakes with subduction zones and thrust faults because these are the types of earthquakes most likely to deform the seafloor, causing the entire water column to be lifted up. Strike-slip earthquakes can generate tsunami if there is sufficient submarine topography that gets offset during the earthquake. Also, if a strike-slip earthquake triggers a landslide, this could cause a tsunami. We will need to wait until people take a deeper look into this before we can make any conclusions about the tsunami and what may have caused it.

Did you feel this earthquake? If so, fill out the USGS “Did You Feel It?” form here. If not, why not? Probably because you were too far away. The closer to an earthquake, the more strong the shaking intensity and the larger chance of infrastructure damage (roads, houses, etc.). The USGS PAGER alert for this earthquake shows that there are ~282,000 people living in Palu, a city near the epicenter. The estimate for shaking intensity is a MMI VI, which could result in light damage for resistant structures and moderate damage for vulnerable structures. More about USGS PAGER alerts here. There exists a possibility that there were more than 100 fatalities from this earthquake.

UPDATE 2018.09.28 23:00

  • There have been tsunami waves recorded on a tide gage over 300 km to the south of the epicenter, at a site called Mumuju. Below is a map and a plot of water surface elevations from this source.


UPDATE 2018.09.29 07:00


I awakened this morning (my time, obviously) to find that there are over 380 reported deaths from this earthquake and tsunami. More on this later in the day (clouds are preparing to our and i need to put some of my stuff under tarps).

I prepared a report for Temblor where we present results of static coulomb stress modeling. Here is that report.

UPDATE 2018.09.29 10:45

Here is a (200 MB) video that I edited slightly. Download here. This was originally posted here.

UPDATE 2018.09.30 17:00

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.0 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Australia plate.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a map from Bellier et al. (2016) that shows the plate boundary faults in the region. Relative senses of motion across these faults is shown as red arrows. The M 7.5 epicenter is shown as a blue star (as in other figures).
  • In the upper right corner is a larger scale map showing the strike-slip fautls that transect the island of Sulawesi, Indonesia (Bellier, et al., 2006)
  • In the lower right corner is a low angle oblique view of the subducting plates in this region (Hall, 2011). Note the orientation of the Sorong fault and the Sulawesi faults.
  • In the lower left corner is a large scale map showing detailed versions of these fault systems in Sulawesi. Earthquake fault mechanisms are plotted for historic earthquakes. Today’s M 7.5 occurred just to the north of the spatial extent of this map.
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a centuries worth of seismicity plotted.

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the low angle oblique view of the plate boundaries in this region (Hall., 2011).

  • 3D cartoon of plate boundaries in the Molucca Sea region modified from Hall et al. (1995). Although seismicity identifies a number of plates there are no continuous boundaries, and the Cotobato, North Sulawesi and Philippine Trenches are all intraplate features. The apparent distinction between different crust types, such as Australian continental crust and oceanic crust of the Philippine and Molucca Sea, is partly a boundary inactive since the Early Miocene (east Sulawesi) and partly a younger but now probably inactive boundary of the Sorong Fault. The upper crust of this entire region is deforming in a much more continuous way than suggested by this cartoon.

  • Here is the map from Bellier et al. (2006) that shows the plate boundary faults, along with some other tectonic information.

  • Regional geodynamic sketch that presents the present day deformation model of Sulawesi area (after Beaudouin et al., 2003) and four main deformation systems around the Central Sulawesi block, highlighting the tectonic complexity of Sulawesi. Approximate location of the Central Sulawesi block rotation pole (P) [compatible with both GPS measurements (Walpersdorf et al., 1998a) and earthquake moment tensor analyses (Beaudouin et al., 2003)], as well as the major active structures are reported. Central Sulawesi Fault System (CSFS) is formed by the Palu–Koro and Matano faults. Arrows correspond to the compression and/or extension directions deduced from both inversion and moment tensor analyses of the focal mechanisms; arrow size being proportional to the deformation rate (e.g., Beaudouin et al., 2003).We also represent the focal mechanism provided by the Harvard CMT database [CMT data base, 2005] for the recent large earthquake (Mw=6.2; 2005/1/23; lat.=0.92° S; long.=120.10° E). The box indicates the approximate location of the Fig. 6 that corresponds to the geological map of the Palu basin region. The bottom inset shows the SE Asia and Sulawesi geodynamic frame where arrows represent the approximate Indo-Australian and Philippines plate motions relative to Eurasia.

  • The is the larger scale map showing the general layout of the strike-slip faults in Sulawesi (Bellier et al., 2006).

  • Sketch map of the Cenozoic Central Sulawesi fault system. ML represents the Matano Lake, and Leboni RFZ, the Leboni releasing fault zone that connects the Palu–Koro and Matano Faults. Triangles indicate faults with reverse component (triangles on the upthrown block). On this map are reported the fault kinematic measurement sites.

  • Here is a spectacular photo/sketch pair that demonstrates the excellent geomorphic evidence for this strike-slip fault (Bellier et al., 2006).The stream channels that flow down the alluvial fan in this photo are typical of the features that were used to evaluate the Holocene slip rate. There is a modest amount of vertical motion across this fault in places, causing the formation of basins like the Palu Palu Basin (a graben). The city of Palu is in the center of the Palu Palu Basin.

  • West-looking view of the Palu–Koro fault escarpment SSW of the Palu basin showing faceted spurs and a left-lateral offset of an alluvial fan. At the bottom, sketch of the photograph where white arrows point to the fault trace and black arrows point to the cumulate fan offset along the fault traces.

  • This map shows how Palu is situated relative to this fault system (Bellier et al., 2006).

  • Simplified geological map of the Palu domain (modified after Sukamto, 1973) where are reported the locations of fission-track samples. 1 — Holocene alluvial deposits; 2 — Quaternary coral reef terraces; 3 — Mio-quaternary molasses, 4 — Mio-quaternary granitic rocks and granodiorites, 5 — Middle to Upper Eocene Tinombo Formation metamorphism, 6 — Tinombo Formation magmatism, 7 and 8 — metamorphic bedrock (7 — Cretaceous Latimonjong Formation; 8 — Triassic-Jurassic Gumbasa Formation).

  • Some early GPS analysis was conducted by Waldpersdorf et al. (1998). Below is a map showing the location of these GPS observations relative tot he Palu-Koro fault.

  • The area of convergence of the Eurasian, Philippine and Australian plate is characterized by the Sula block motion. Active block boundaries are the North Sulawesi trench *(1)., the Palu-Koro (2), and the Matano (3) faults. The Palu transect is indicated buy the box, with a zoom presented in the inset. Furthermore, the two largest earthquakes (CMT) occurring during the observation period are indicated.

  • Here is a map that shows the GPS velocities as vectors in the region of Palu, Indonesia (Waldpersdorf et al., 1998).

  • Velocities of the Palu transect stations, with respect to the PALU station. Error ellipses correspond to formal uncertainties of the global solution with a confidence level of 90%.

  • Here are the Waldpersdorf et al. (1998) velocities plotted on a chart.

  • Transect station velocity components parallel to the fault, with the co-seismic deformation due to the Jan. 1996 earthquake removed. They are indicated in function of their distance to the fault. The dark grey line shows best model values (5.5 cm/yr total velocity, 12 km locking depth). Lighter grey lines correspond to locking depths of 8 and 16 km, marking an uncertainty of +-4 km.

  • Below are updated results from GPS and block model analyses from Socquet et al. (2006). Arrows are vectors that represent plate motion velocity in mm/yr (scale in upper right corner). Note how the velocities are different on either side of the Palu-Koro fault.

  • GPS velocities of Sulawesi and surrounding sites with respect to the Sunda Plate. Grey arrows belong to the Makassar Block, black arrows belong to the northern half of Sulawesi, and white arrows belong to non-Sulawesi sites (99% confidence ellipses). Numbers near the tips of the vectors give the rates in mm/yr. The main tectonic structures of the area are shown as well.

  • This map shows models plate motion velocities as informed by their block model.

  • Rotational part of the inferred velocity field in the Sulawesi area (relative to the Sunda Plate) as predicted by the Euler vectors of the best fit model (model 2). Error ellipses of predicted vectors show the 99% level of confidence. Also shown are poles of rotation and error ellipses (with respect to the Sunda Plate) from the best fit model. Curved arrows indicate the sense of rotation, and numbers indicate the rotation rate. MAKA, Makassar Block; MANA, Manado Block; ESUL, East Sula Block; NSUL, North Sula Block.

  • Here is a map that shows the plate boundary slip velocities as color (Socquet et al., 2006).

  • Best fit block model derived from both GPS and earthquakes slip vector azimuth data. Center: Observed (red) and calculated (green) velocities with respect to the Sunda Block (shown are 20% confidence ellipses, after GPS reweighting; see text). The slip rate deficit (mm/yr) for the faults included in the model is represented by a color bar. The profile of Figure 7 is located by the dashed black line. The black rectangles around Palu and Gorontalo faults localize the insets. Top right and bottom left insets show details of the measured and modeled velocities across the Gorontalo and Palu faults. The bottom right inset shows residual GPS velocities with respect to the model. The value of the coupling ratio, j, for the faults included in the model is represented by the color bar. Light blue dots represent the locations of the fault nodes where the coupling ratio is estimated. Nodes along the block boundaries are at the surface of the Earth, and the others are at depth along the fault plane. In this model, j is considered uniform along strike and depth for all the faults, except for Palu Fault and Minahassa Trench, where it is allowed to vary along strike.

  • This plot is similar to the one above, which shows how different GPS observations have different plate motion velocities relative to the faults in the area (Socquet et al., 2006).

  • Velocity profile across Makassar Trench, Palu Fault, and Gorontalo Fault (profile location in Figure 6) in Sunda reference frame. Observed GPS velocities are depicted by dots with 1-sigma uncertainty bars, while the predicted velocities are shown as curves. The profile normal component (approximately NNW) (i.e., the strike-slip component across the NW trending faults) is shown with black dots and solid line, while the profile-parallel component (normal or thrust component across the fault) is shown with grey dots and a dashed line. Where the profile crosses the faults and blocks is labeled.

  • Here are their results plotted on a map (Socquet et al., 2006).

  • (top) GPS velocities in Palu area relative to station WATA. STRM topography is used as background. (bottom) Four parallel elastic dislocations that fit best the velocities in the Palu fault zone. The fault-parallel component of the GPS velocities (with 1-sigma error bars) is plotted with respect to their distance to the main fault scarp, in the North Sula Block reference frame. The black curve represents the fault-parallel modeled velocity of the four strand model. For comparison, the fault-parallel modeled velocity predicted by the single fault model is also plotted (grey dashed curve). The location of the modeled dislocation is represented as vertical bars for each model (black and dashed grey lines, respectively).

  • Here is the fault map from Watkinson and Hall (2017).

  • Central Sulawesi overview digital elevation model (SRTM), CMT catalogue earthquakes, 35 km depth and structures that show geomorphic evidence of Quaternary tectonic activity. Rivers marked in white. Illumination from NE.

  • Here is another fantastic view of the geomorphology associated with the Palu-Koro fault (Watkinson and Hall, 2017). The hanging valley is evidence for normal displacement (extension) along this fault. Wine glass canyons are evidence for differential uplift.

  • (a) The Palu and Sapu valleys showing structures that with geomorphic evidence of Quaternary tectonic activity, plus topography and drainage. Mountain front sinuosity values in bold italic text. For location, see Figure 4. Major drainage basins for Salo Sapu and Salo Wuno are marked, separated by uplift at the western end of the Sapu valley fault system. (b) View of the Palu–Koro Fault scarp from the Palu valley, showing geomorphic evidence of Quaternary tectonic activity.

  • In this case, we can see how the river meanders are controlled by the fault. Places where stream offsets were used to measure slip rate are also shown (Watkinson and Hall, 2017).

  • Evidence of a cross-basin fault system within the Palu valley Quaternary fill. (a) Overview ASTER digital elevation model draped with ESRI imagery layer. Illumination from NW. Palu River channels traced from six separate images from 2003 to 2015. Inset shows fault pattern developed in an analogue model of a releasing bend, modified after Wu et al. (2009), reflected and rotated to mimic the Palu valley. Sidewall faults and cross-basin fault system are highlighted in the model and on the satellite imagery. (b, c) Laterally confined meander belts, interpreted as representing minor subsidence within the cross-basin fault system. (d) Laterally confined river channels directly along-strike from a Palu–Koro Fault strand seen to offset alluvial fans in the south of the valley. (c, d, e) showESRI imagery.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    References:

  • Bellier, O., Sebrier, M., Beaudouin, T., Villenueve, M., Braucher, R., Bourles, D., Siame, L., Putranto, E., and Pratomo, I., 2001. High slip rate for a low seismicity along the Palu-Koro active fault in central Sulawesi (Indonesia) in Terra Nova, v. 13, No. 6, p. 463-470
  • Bellier, O., Sebrier, M., Seward, D., Beaudouin, T., Villenueve, M., and Putranto, E., 2006. Fission track and fault kinematics analyses for new insight into the Late Cenozoic tectonic regime changes in West-Central Sulawesi (Indonesia) uin Tectonophysics, v. 413, p. 201-220, doi:10.1016/j.tecto.2005.10.036
  • Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • Socquet, A., W. Simons, C. Vigny, R. McCaffrey, C. Subarya, D. Sarsito, B. Ambrosius, and W. Spakman (2006), Microblock rotations and fault coupling in SE Asia triple junction (Sulawesi, Indonesia) from GPS and earthquake slip vector data, J. Geophys. Res., 111, B08409, doi:10.1029/2005JB003963.
  • Watkinson, I.M. and Hall, R., 2017. Fault systems of the eastern Indonesian triple junction: evaluation of Quaternary activity and implications for seismic hazards in Cummins, P. R. &Meilano, I. (eds) Geohazards in Indonesia: Earth Science for Disaster Risk Reduction, Geological Society, London, Special Publications, v. 441, https://doi.org/10.1144/SP441.8,
  • Walpersdorf, A., Rangin, C., and Vigny, C., 1998. GPS compared to long-term geologic motion of the north arm of Sulawesi in EPSL, v. 159, p. 47-55
  • Zahirovic, S., Seton, M., and Müller, R.D., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014

Earthquake Report: Hokkaido, Japan

Following the largest typhoon to strike Japan in a very long time, there was an earthquake on the island of Hokkaido, Japan today. There is lots on social media, including some spectacular views of disastrous and deadly landslides triggered by this earthquake (earthquakes are the number 1 source for triggering of landslides). These landslides may have been precipitated (sorry for the pun) by the saturation of hillslopes from the typhoon. Based upon the USGS PAGER estimate, this earthquake has the potential to cause significant economic damages, but hopefully a small number of casualties. As far as I know, this does not incorporate potential losses from earthquake triggered landslides [yet].

This earthquake is in an interesting location. to the east of Hokkaido, there is a subduction zone trench formed by the subduction of the Pacific plate beneath the Okhotsk plate (on the north) and the Eurasia plate (to the south). This trench is called the Kuril Trench offshore and north of Hokkaido and the Japan Trench offshore of Honshu.

The okhotsk plate is considered part of the North America plate on some maps. The location of the plate boundary of the Okhotsk plate are not well understood (e.g. using GPS plate motion velocities, it is difficult to find the northern boundary with the North America plate).

Many of the earthquakes in this region are related to the subduction zone. Most notably is the 2011 Tohoku-oki M 9.1 tsunamigenic earthquake. More background information about the 2011 earthquake can be found here and information about the tsunami can be found here.

The 2011 earthquake had lots of aftershocks and was quite complicated. One interesting thing that happened is that there was an extensional earthquake in the Pacific plate to the west of the Japan Trench. This M 7.7 earthquake happened along faults formed as the Pacific plate bends near where it meets the trench. Similar subduction zone / outer rise earthquake pairs are known, including some along the New Hebrides Trench in the western equatorial Pacific ocean, as well as further north along the Kuril subduction zone. I spend time discussing the 2006/2007 Kuril earthquake pair in this report.

There was also a subduction zone earthquake in 2003, the Tokachi-oki earthquake, that triggered submarine landslides. These landslides transformed into turbidity currents and these were directly observed with offshore instrumentation.

One of the interesting things about this region is that there is a collision zone (a convergent plate boundary where two continental plates are colliding) that exists along the southern part of the island of Hokkaido. The Hidaka collision zone is oriented (strikes) in a northwest orientation as a result of northeast-southwest compression. Some suggest that this collision zone is no longer very active, however, there are an abundance of active crustal faults that are spatially coincident with the collision zone.

Today’s M 6.6 earthquake is a thrust or reverse earthquake that responded to northeast-southwest compression, just like the Hidaka collision zone. However, the hypocentral (3-D) depth was about 33 km. This would place this earthquake deeper than what most of the active crustal faults might reach. The depth is also much shallower than where we think that the subduction zone megathrust fault is located at this location (the fault formed between the Pacific and the Okhotsk or Eurasia plates). Based upon the USGS Slab 1.0 model (Hayes et al., 2012), the slab (roughly the top of the Pacific plate) is between 80 and 100 km. So, the depth is too shallow for this hypothesis (Kuril Trench earthquake) and the orientation seems incorrect. Subduction zone earthquakes along the trench are oriented from northwest-southweast compression, a different orientation than today’s M 6.6.

So today’s M 6.6 earthquake appears to have been on a fault deeper than the crustal faults, possibly along a deep fault associated with the collision zone. Though I am not really certain. This region is complicated (e.g. Kita et al., 2010), but there are some interpretations of the crust at this depth range (Iwasaki et al., 2004) shown in an interpreted cross section below.

I present more about the basics behind ground shaking, triggered landslides, and possible earthquake triggering on Temblor here:

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

I also include active crustal faults from the Coordinating Committee for Geoscience Programmes in East and Southeast Asia (CCOP). Note the abundance of north-northwest oriented yellow lines to the east of today’s earthquakes. While today’s earthquake was not on those crustal faults, the earthquakes and these faults are responding to similarly oriented tectonic stresses.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • Note the parallel magnetic anomalies to the east of Japan. These were formed about 150 million years ago at the spreading center where this portion of the Pacific plate was created. More can be found about the creation of the Pacific plate in Boschman and van Hinsbergen, 2(016).

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper right corner is a low angle oblique view of the tectonic configuration in this region. Note how many subduction zones are that are interacting in different ways. This is from the AGU blog, “Trembling Earth.” I place a blue star in the general location of today’s earthquakes (same for other figures in this poster).
  • In the lower right corner is a plate tectonic map of this part of the world (Liu et al., 2013). The major plate boundary faults are shown, along with the volcanoes in the magmatic arcs. Also, seismicity is shown (the 2011 earthquake as a small blue star) and the slab contours for the Pacific and Philippine Sea plates. Color shows the age of the oceanic crust. These authors place the southern boundary of the Okhotsk plate further to the south (dashed black line), where the Izu Collision Zone intersects Japan (near the intersection of the magmatic arc associated with the Izu-Bonin Trench, with Japan).
  • In the lower left corner is a geologic map of Japan (van Horne et al., 2016). Note the orientation of the rocks in Hokkaido as they are oriented in a northwest-southeast direction in the area labeled Hidaka Collision. These rocks are oriented this way due to the northeast-southwest convergence. This map places the southern boundary of the Okhotsk plate near where the Hidaka Collision is. Compare this with the Liu map to the right.
  • In the upper left corner is a large scale portion of a figure from NUMO (Kurikami et al., 2009), a publication put together by the N to evaluate the suitability of sites for high level radioactive waste. They considered various geologic hazards in this report. This map shows some key tectonic features and geologic data. I include the legend to the right of the map. The magmatic arc is shown as a red line. The Hidaka Collision Zone is shown as a dashed blue line with arrows showing the direction of collision. The blue arrows show the direction of maximum stress, the stress field. These arrows are pointed in the direction of compression. The convergence direction along the collision zone is oriented well with today’s earthquakes, but the stress field data are not perfectly oriented.
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a centuries seismicity plotted.

Some Relevant Discussion and Figures

  • This map shows the current tectonic configuration of this region, along with some inherited features from the tectonic past (e.g. green lines). This is from NUMO’s report: “Evaluating Site Suitability for a HLW Repository (Scientific Background and Practical Application of NUMO’s Siting Factors), NUMO-TR-04-04.”

  • Also from the NUMO report, this shows the Niigata-Kobe fold and thrust belt. In addition, this map shows a northwest striking convergent plate boundary along the southeastern boundary of Hokkaido. However, it cannot explain the interesting orientation of the M 6.2 deep (240 km) earthquake.

  • Here is a great figure from Itoh et al. (2005) that shows how they interpret the Hidaka Collision Zone.

  • Maps showing tectonic context around the Japanese Islands (a) and geologic belts in Hokkaido (b; after Kato et al., 1990).

  • This map (also from Itoh et al., 2005) shows the active faults and folds mapped in the region, along with the geology.

  • Geologic map around the Umaoi anticline redrawn from Geological Survey of Japan (2002). Location of active fault and/or fold scarps (after Ikeda et al., 2002) are also shown. buQ and bdQ attached on fault traces are upthrown and downthrown sides of faults, respectively. Sampling points of surface paleomagnetic data is after Kodama et al. (1993).

  • Here is more evidence for the thrust faults associated with the Hidaka Collision Zone (Iwasaki et al., 2004). These authors used seismic refraction and seismic reflection experiments to interpret the deep crustal structures associated with the collision here. The profile shown in the next figure is denoted by the east-west oriented black arrows in the lower part of this figure.

  • Geological map of Central Hokkaido with our seismic refraction/wide-angle reflection profiles and shot points (stars). Seismic reflection lines of the Hokkaido Transect were laid out from shot L-2 to M-5 on the wide-angle line. Reflection lines carried out from 1994 to 1997 in the southernmost part of the HCZ and refraction/wide-angle reflection lines in 1984 and 1992 are also shown. SYB: Sorachi-Yezo Belt; KMB: Kamuikotan Metamorphic Belt; IB: Idon’nappu Belt; HMB: Hidaka Metamorphic Belt; HB: Hidaka Belt; YB: Yubetsu Belt; TB: Tokoro Belt; HMT: Hidaka Main Trust.

  • Here is the interpreted cross section from Iwasaki et al. (2004). Note (1) the thrust faults and (2) the depths for these different structures. There are still regions that are poorly understood. Recall the depth of the M 6.6 earthquake is about 33 km.

  • Geological interpretation of the seismic model. KMB: Kamuikotan Metamorphic Belt; IB: Idon’nappu Belt; HMB: Hidaka Metamorphic Belt; Yz: Yezo Super Group; Sr: Sorachi Group; HMT: Hidaka Main Thrust.

  • Here is the cool tectonic map from Liu et al. (2013). We all like cool maps! (right?)

  • Tectonic settings of the study region (black box). The solid sawtooth lines and the black dashed line denote the plate boundaries (Bird 2003). The red triangles denote the active volcanoes. The blue dashed lines and the pink lines denote the depth contours to the upper boundary of the subducting Pacific slab and that of the subducting Philippine Sea slab, respectively (Hasegawa et al. 2009; Zhao et al. 2012). The topography data are derived from the GEBCO_08 Grid, version 20100927, http://www.gebco.net. The ages of oceanic plates are from M¨uller et al. (2008).

  • This is a very cool figure (also from Liu et al., 2013) that shows a plot of earthquakes from 3 different perspectives. First is the map view. To the right of the map is a plot of earthquakes shown as viewed from the east of the map and this shows the hypocenters. The profile below the map shows a cross section of seismicity as viewed from the south looking north. The original figure includes more maps (A and B).

  • (c) Distribution of the 4803 earthquakes used in
    this study. The black crosses denote 3818 events (Group-1) that occurred under the seismic network. The green dots show 228 events (Group-2) that occurred outside the seismic network, selected from the events relocated by Gamage et al. (2009) using sP depth phases. The red dots denote 757 suboceanic earthquakes (Group-3) that are newly relocated in this work using P-wave, S-wave and sP depth-phase data. (d) East–west and (e) north–south vertical cross-sections of the earthquakes shown in (c).

  • Here is a map from the recent update of the Japan National Seismic Hazard Maps, resulting from knowledge gained following the 2011 M 9.1 earthquake (Fujiwara et al., 2012). The color represents the chance that a region will experience ground shaking at or greater that Japan Meteorological Agency (JMA) seismic intensity 6 in the next 30 years. JMA intensity is a scale of shaking intensity similar to the Modified Mercalli Intensity (MMI) Scale. The numbers are different, so they are difficult to compare. The JMA intensity 6 is similar to MMI X. Today’s earthquakes are in a region of slightly elevated chance of ground shaking (between 6-26%). Today’s M 6.6 earthquake may have reached

  • This is a map from the National Research Institute for Earth Science and Disaster Resilience, where data from the Strong-motion Seismograph Networks in Japan are located. This shows measurements of JMA intensity. It appears that a site near the epicenter (red star) reached JMA intensity 7.

  • This is an animation from the same source showing observations of JMA intensity recorded at the surface throughout Japan. h/t to Jascha Polet for sharing this on twitter.
  • Here is the upper figure showing the tectonic setting (Kurikami et al., 2009). Note how the Okhotsk plate has a strike-slip fault that terminates near the Hidaka Collision Zone (called a forearc-sliver fault, formed because the plate convergence is oblique to the subduction zone fault). I include their figure caption as a blockquote.

  • Tectonic setting of Kyushu within the Japanese island arc. The locations of active faults and volcanoes that have been active in the last 10,000 years are also shown.

  • This is a fantastic educational video from IRIS that discusses the plate tectonics and mentions some earthquakes in the region of Japan.

  • Here is a USGS poster than summarizes the earthquake history and plate geometry for this region. This is the USGS Open File Report 2010-1083-D (Rhea et al., 2010).

Earthquake Triggered Landslides

  • Here is the aerial video from NHK that shows some of the landslides triggered by this sequence of earthquakes today. This comes from a tweet below.
  • Well, here is a great figure from Keefer (1984) that shows that the larger the magnitude of an earthquake, the larger an area can be subject to triggering of landslides from the ground shaking from that earthquake.

  • Area affected by landslides in earthquakes of different magnitudes. Numbers beside data points are earthquakes listed in Table 1. Dots = onshore earthquakes; x = offshore earthquakes. Horizontal bars indicate range in reported magnitudes. Solid line is approximate upper bound enclosing all data.

  • In 2008 there was an earthquake in China with a magnitude M 7.9. Unfortunately this earthquake caused many deaths. Using satellite imagery, geologists identified about 60,000 individual landslides (Gorum et al., 2011). Below is a map that shows the faults in the region, as well as epicenters from the earthquakes from this sequence.

  • Location and 12May 2008Wenchuan earthquake fault surface rupturemap, and focalmechanisms of the main earthquake (12May) and two of the major aftershocks (13 May and 25 May). Also the epicenters of historic earthquakes are indicated. The following faults are indicated: WMF: Wenchuan–Maowen fault; BF: Beichuan–Yingxiu fault; PF: Pengguan fault; JGF: Jiangyou–Guanxian fault; QCF: Qingchuan fault; HYF: Huya fault;MJF:Minjian fault based on the following sources: (Surface rupture: Xu et al., 2009a,b; Epicenter and aftershocks: USGS 2008; Historic earthquakes: Kirby et al., 2000; Li et al., 2008; Xu et al., 2009a,b).

  • This map shows the region where there was a high density of landslides (Fan et al., 2012). Note how the majority of landslides are located near the larger earthquakes (the larger circles in the above map).

  • Distribution of landslide dams triggered by the Wenchuan earthquake, China. The high landslide density zone is defined by a landslide area density >0.1 km−2; also shown are epicenters of historical earthquakes (USGS, 2008) and the historical Diexi landslide dams (Dahaizi, Xiaohaizi and Diexi). White polygons are unmapped due to the presence of clouds and shadows in post-earthquake imagery. WMF: Wenchuan–Maowen fault; YBF: Yingxiu–Beichuan fault; PF: Pengguan fault; JGF: Jiangyou–Guanxian fault; QCF: Qingchuan fault; HYF: Huya fault; MJF: Minjiang fault (after X. Xu et al., 2009). MJR: Minjiang River; MYR: Mianyuan River; JJR: Jianjiang River; QR: Qingjiang River.

  • Many of these landslides dammed rivers, causing an additional hazard. These earthen dams block rivers, leading to a large lake forming upstream of these dams. The dams can be overtopped when the lakes fill with water. once the water reaches the top of the dam, they can overflow and rapidly down cut back to the level of the river prior to the dam emplacement. If this happens too rapidly, a flood can occur, putting those downstream at risk of flooding.

  • Comparison of densities of blocking and non-blocking landslides. (a) Landslide density. (b) Landslide dam point density. White dashed lines are 240-km by 25-km swath profiles. (c). Mean normalized landslide and landslide dam densities along the SW–NE profile. Red lines are Yingxiu-Beichuan fault (YBF) and Pengguan fault (PF). Yellow dash lines are the boundary of the P1–P7 watersheds in the Pengguan Massif. YX, WC, HW, BC, and QC are the cities of Yingxiu, Wenchuan, Hanwang, Beichuan and Qingchuan, respectively. MJR, JJR, FJR, and QR represent Minjiang, Jianjiang, Fujiang and Qingjiang rivers, respectively.

  • In 1959, there was an earthquake in southwestern Montana, the M 7.2 Hebgen Lake Earthquake. This earthquake triggered a landslide that dammed the Madison River. This dam created a lake now called “Earthquake Lake.” I was actually driving on a road trip following my graduation from Oregon State University in 2014. I drove to this area and arrived the day that the Earthquake Lake Visitor Center opened. Pretty cool.
  • Here is a view of the lake as it was in May, 2014. Note the dead trees. The landslide is the bare looking mountainside in the distance on the left. We are looking to the West.

  • Here is a view of the landslide from my truck.

  • Here are all the people waiting to go into the visitor center on opening day.

  • Here is another cool view of the ghost forest.

  • Here is an educational display near the lake. Click on the image and one may zoom in within their browser, or save the image and zoom in that way. The text is readable if one wants to follow along.

  • This is from the poster and shows the landslide dam after it formed.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    References:

  • Chapman et al., 2009. Development of Methodologies for the Identification of Volcanic and Tectonic Hazards to Potential HLW Repository Sites in Japan –The Kyushu Case Study-, NUMO-TR-09-02, NOv. 2009, 192 pp.
  • Fan, X., et al., 2012. Transient water and sediment storage of the decaying landslide dams induced by the 2008 Wenchuan earthquake, China in Geomorphology, v. 171-172, p. 58-68, doi:10.1016/j.geomorph.2012.05.003
  • Fujiwara, H., Morikawa, N., Okumura, T., Ishikawa, Y., and Nojima, N., 2012. Revision of Probabilistic Seismic Hazard Assessment for Japan after the 2011 Tohoku-oki Mega-thrust Earthquake (M9.0) in Proceedings of the 15th World Conference on Earthquake Engineering, 15th World Conference on Earthquake Engineering, Lisbon.
  • Gorum, T., Fan, X., van Westen, C.J., Huang, R., Xu, Q., Tang, C., Wang, G., 2011. Distribution pattern of earthquake-induced landslides triggered by the 12 May 2008 Wenchuan earthquake in Geomorphology, v. 133, p. 152-167, doi:10.1016/j.geomorph.2010.12.030
  • Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
  • Itoh, Y., Ishiuyama, T., and Nagasaki, Y., 2005. Deformation mode in the frontal edge of an arc–arc collision zone: subsurface geology, active faults and paleomagnetism in southern central Hokkaido, Japan in Tectonophysics, v. 395, p. 81-97 doi:10.1016/j.tecto.2004.09.003
  • Iwasaki, T., et al., 2004. Upper and middle crustal deformation of an arc–arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experiments in Tectonophysics, v. 388, p. 59-73, doi:10.1016/j.tecto.2004.03.025
  • Keefer, D.K., 1984. Landslides caused by earthquakes in Geological Society of America Bulletin, v. 95, p. 406-421, doi: 10.1130/0016-7606(1984)95<406:LCBE>2.0.CO;2
  • Kurikami et al., 2009. Study on strategy and methodology for repository concept development for the Japanese geological disposal project, NUMO-TR-09-04, Sept. 20-09, 101 pp.
  • Lay, T., and Kanamori, H., 1980, Earthquake doublets in the Solomon Islands: Physics of the Earth and Planetary Interiors, v. 21, p. 283-304.
  • Lay, T., Ammon, C.J., Kanamori, H., Kim, M.J., and Xue, L., 2011. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 713-718.
  • Lay, T., H. Kanamori, C. J. Ammon, A. R. Hutko, K. Furlong, and L. Rivera, 2009. The 2006 – 2007 Kuril Islands great earthquake sequence in J. Geophys. Res., 114, B11308, doi:10.1029/2008JB006280.
  • Liu, X., Zhao, D., and Li, DS., 2013. Seismic heterogeneity and anisotropy of the southern Kuril arc: insight into megathrust earthquakes in Geophysical Journal International, Volume 194, Issue 2, 1 August 2013, Pages 1069–1090, https://doi.org/10.1093/gji/ggt150
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • Rhea, S., Tarr, A.C., Hayes, G., Villaseñor, A., Furlong, K.P., and Benz, H.M., 2010. Seismicity of the Earth 1900-2007, Kuril-Kamchatka arc and vicinity: U.S. Geological Survey Open-File Report 2010-1083-C, 1 map sheet, scale 1:5,000,000.
  • Van Horne, A., Sato, H., Ishiyama, T., 2017. Evolution of the Sea of Japan back-arc and some unsolved issues in Tectonophysics, v. 710-711, p. 6-20, http://dx.doi.org/10.1016/j.tecto.2016.08.020

Return to the Earthquake Reports page.

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Earthquake Report: Deep in Peru

Busy week!

Still

Got to this intermittently today and it is fine that it took me a while to get this report together. One might ask “why?”. Well, this earthquake, while having a large magnitude, was quite deep. Because earthquake intensity decreases with distance from the earthquake source, the shaking intensity from this earthquake was so low that nobody submitted a single report to the USGS “Did You Feel It?” website for this earthquake.

This report let me spend some time thinking about the historic earthquakes in this region. In other reports (e.g. the M 8.2 Fiji earthquake from a few days ago) I discuss various reasons why there are earthquakes at these great depths. I will mention some of that below, but in general, we think that there are various physical and chemical changes to earth materials at these great depths that lead to changes in stress and strain, leading to earthquakes.

While doing my lit review, I found the Okal and Bina (1994) paper where they use various methods to determine focal mechanisms for the some deep earthquakes in northern Peru. More about focal mechanisms below. These authors created focal mechanisms for the 1921 and 1922 deep earthquakes so they could lean more about the 1970 deep earthquake. Their seminal work here forms an important record of deep earthquakes globally. These three earthquakes are all extensional earthquakes, similar to the other deep earthquakes in this region. I label the 1921 and 1922 earthquakes a couplet on the poster.

There was also a pair of earthquakes that happened in November, 2015. These two earthquakes happened about 5 minutes apart. They have many similar characteristics, suggest that they slipped similar faults, if not the same fault. I label these as doublets also.

So, there may be a doublet companion to today’s M 7.1 earthquake. However, there may be not. There are examples of both (single and doublet) and it might not really matter for 99.99% of the people on Earth since the seimsic hazard from these deep earthquakes is very low.

Other examples of doublets include the 2006 | 2007 Kuril Doublets (Ammon et al., 2008) and the 2011 Kermadec Doublets (Todd and Lay, 2013).

In January, there was a M 7.1 subduction zone earthquake and I present material about that earthquake here.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 7.0 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes. I include the focal mechanisms from Okal and Bini (1994).

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I label the main MMI contour as MMI 2.5. This is very low. Using the legend on the poster, we read that the shaking is “light” and the damage is “none.”
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. However, the slab contours are only in the southwestern portion of this map.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Australia plate.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper right corner is a section of the map from Rhea et al. (2010), which is a USGS map documenting the seismicity of the earth in this region. The cross section B-B’ is shown to the left. The cross section plots the earthquake depths along the profile shown on the map. The B-B’ profile crosses the subduction zone very close to where this earthquake happened. I place a blue star in the general location of today’s M 7.1 earthquake.
  • In the lower right corner is a low-angle oblique view of the megathrust fault as it dips beneath the South America plate. Today’s earthquake is deeper than this figure represents (max depth = 200 km; today’s M 7.1 depth = 610 km). I place a blue star in the general location as the epicenter in the upper part of the figure (that represents topography, vertically exaggerated) and the lower part of the figure (a depth slice at 200 km, so this is kind of like a 200 km deep epicenter; not really an epicenter or an hypocenter).
  • In the upper left corner is a map showing the location of some cross sections that Scire et al. (2017) prepared. These authors used seismic tomography to look into the subsurface geometry of the plates and mantle. Seismic tomography is like a CT-scan, a 3-D X-Ray, into the earth using seismic waves instead of X-Rays.
  • In the lower left corner are some of these tomographic slices into the Earth. The M 7.1 earthquake happened between sections A-A’ and B-B.’ In these images, the color the velocity (speed) of seismic waves in that material. Blue= fast, red= slow. Generally, oceanic crust is old and cold (more dense, etc. and sinking) and mantle is warmer and has a lower seismic velocity. We may think that the blue bleb dipping to the east is the subducting Nazca (Farallon) plate.
  • Here is the map with a month’s seismicity plotted.


  • Here is the map with a century’s seismicity plotted, along with USGS earthquakes M ≥ 7.0.


Other Report Pages

Some Relevant Discussion and Figures

  • Here is an animation from IRIS that reviews the tectonics of the Peru-Chile subduction zone. For the animation, first is a screen shot and below that is the embedded video. This animation is from IRIS. Written and directed by Robert F. Butler, University of Portland. Animation and Graphics: Jenda Johnson, geologist. Consultant: Susan Beck, University or Arizona. Narration: Elayne Shapiro, University of Portland.

  • Here is a download link for the embedded video below (34 MB mp4)
  • The Rhea et al. (2016) document is excellent and can be downloaded here. The USGS prepared another cool poster that shows the seismicity for this region (though there does not seem to be a reference for this).

  • This is a great visualization from Dr. Laura Wagner. This shows how the downgoing Nazca plate is shaped, based upon their modeling. Today’s M 7.1 earthquake is almost due south of Nazca, Peru labeled on the map.

  • Below are all figures from Scire et al. (2017).
  • This first one shows the location of (1) their cross sections (see below), (2) the locations of the seismometers and other equipment used in this study, and (3) historic seismicity used in their analyses.

  • Map showing seismic station locations (squares—broadband; inverted triangles—short period) for individual networks used in the study and topography of the central Andes. Slab contours (gray) are from the Slab1.0 global subduction zone model (Hayes et al., 2012). Earthquake data (circles) for deep earthquakes (depth >375 km) are from 1973 to 2012 (magnitude >4.0) and were obtained from the U.S. Geological Survey National Earthquake Information Center (NEIC) catalog (https://earthquake.usgs.gov/earthquakes/). Red triangles mark the location of Holocene volcanoes (Global Volcanism Program, 2013). Plate motion vector is from Somoza and Ghidella (2012). Cross section lines (yellow) are shown for cross sections in Figures 5 and 8.

  • Here are all the tomographic cross sections.


  • Trench-perpendicular cross sections through the tomography model. Velocity anomalies are shown in blue for fast anomalies, red for slow anomalies. Cross section locations are as shown in Figure 1. Dashed lines are the same as in Figure 6. Yellow dots are earthquake locations from the EHB catalog (Engdahl et al., 1998). Solid black line marks the top of the Nazca slab from the Slab1.0 model (Hayes et al., 2012).

  • This figure that shows an estimate of the geometery of the slab (scire et al., 2016). This surface is based on a contrast between material properties of the slab and the overlying material (mantle). Note the north arrow. These authors were interested in many things, including how the Nazca Ridge changes the geometry of ht emegathrust fault. Today’s M 7.1 happened in a place where the fault is steeply dipping. Use the latitude and longitude to findthe location of today’s earthquake relativ to this figure. 11° South and 70.8° East, with a depth of 610 km.

  • 3-D diagram of the resolved subducting Nazca slab and prominent mantle low-velocity anomalies inferred from our tomographic models. The isosurfaces for this diagram are obtained by tracing the most coherent low-velocity anomalies (less than negative 3 per cent) and slab-related (greater than positive 3 per cent) coherent fast anomalies in the tomographic model. Geomorphic provinces (fine dashed lines) are the same as in Fig. 1(a). Heavy black outline marks the projection of the subducted Nazca Ridge from Hampel (2002). Anomalies A, C, D and E labelled as in previous figures. Downloaded from http://gji.oxfordjournals.org/ at Yale University on December 1, 2015

  • These next 3 figures are from Kumar et al. (2016) and reveal the shape of the plate boundary based upon seismicity.
  • This map shows the earthquakes used in their study (color = depth, use this legend for the other map). The thin black lines show their estimate of where the slab is (the megathrust, where the Nazca plate meets the South America plate), depth in km. The NR is the grayed out polygon in the lower left part of the figure (see next map).

  • Map of first motion focal mechanisms plotted in lower hemisphere projec-tion. Mechanisms are color coded by earthquake depth and mainly show normal faulting across the study area. Solid lines are slab contours from Antonijevic et al.(2015). See Figs. S4 and S5 of the supplementary material for zoom-in map of focal mechanism for events inside the red and blue box respectively.

  • This map shows where the cross section profiles are located (Kumar et al., 2016). Today’s M 7.1 earthquake plots almost exactly at the southwestern tip of the P3 profile line.

  • Map showing locations of (a) trench-parallel (BB) and trench-perpendicular (P1, P2, P3, and P7) transects used to plot seismicity cross-sections. Red tick marks on BBrepresents distance interval of 100 km.

  • Here are the earthquake hypocenters plotted for the 4 cross sections plotted in the map above (Kumar et al., 2016). Today’s M 7.1 earthquake plots near the westernmost limit of profile P3. Given a hypocentral depth of ~40 km, this plots in the upper plate. So, perhaps this earthquake is not on the megathrust, but along the decollement. While plotted at a different scale, the same is true when looking at the seismicity cross section from Rhea et al. (2010). Of course, these are just models and could be wrong.

  • Seismicity cross-sections (P1, P2, P3, and P7) perpendicular to the trench. Earthquakes within ±35 km are projected onto each cross-section. The solid line in each cross section is the slab contour from Antonijevic et al.(2015). Red star in each trench-perpendicular cross section marks the intersection with BBcross section. See Figs. S2 and S3 of the supplementary material for the remaining set of trench-parallel and trench-perpendicular seismicity cross-sections.

  • Here is a map and a cross section showing earthquake locations for the 2015 doublet sequence (Ruiz et a., 2017).

  • Regional seismic data used to study the 2015 doublet in Peru. A) Inverted triangles denote the regional broad band instruments of the Peruvian and Brazilian seismic networks. The blue inverted triangles were used in the kinematic inversion and all of them were used to compute the localization of aftershocks. Dots are the aftershocks localized in this work. B) Vertical cross section along profile AA shown in panelA. Dots are the aftershocks of the Peru deep doublet, stars the hypocen-ter of the two main-shocks. The continuous line is the slab modeled by Hayes et al.(2012). The focal mechanisms are those of the two events in the 2015 doublet (USGS, National Earthquake Information Center, PDE). (For interpretation of the ref-erences to color in this figure legend, the reader is referred to the web version of this article.)

  • Here are some figures from Ammon et al. (2008) that shows the earthquake doublet in the Kuril subduction zone. The doublets in Peru are deep and occur for a different reason than do the shallow earthquakes in the Kuril example. For the earthquakes in 2006 | 2007, there was an earthquake along the megathrust fault and an earthquake along the outer rise (within the Pacific plate).
  • This map shows earthquake mechanisms for historic earthquakes in this region. Color represents time.

  • Great doublet rupture region. Central Kuril islands earthquake locations (circles) from the USGS National Earthquake Information Center (NEIC) catalogue and lower hemisphere GCMT solutions (http://www.globalcmt.org/CMTsearch.html). Epicentres are colour-coded to show activity before 15 November 2006 (grey), between the doublet events (yellow), and after the 13 January 2007 event (orange). Focal mechanisms of foreshocks of the 15 November 2006 event are grey, subsequent events are red. Focal mechanisms are plotted at the NEIC epicentres; the stars are the GCMT centroid locations for the doublet events. The arrow indicates the direction of motion of the Pacific plate at about 80 mm/yr.

  • This figure shows how energy was released during earthquakes through time and space during his earthquake sequence preceding and following the 2006 M 8.3 earthquake.

  • Seismicity pattern. Space–time seismicity pattern for the 2006–2007 Kuril islands earthquake sequence, as a function of time relative to the 15 November 2006 event and a function of distance along the trench relative to that event’s epicentre. The foreshock sequence 45 days before the November event (Fig. 1) and the two main-shock sequences are distinct in time, although many of the early aftershocks of the November event are located in the outer rise (Fig. 1) where the normal fault ruptured 60 days later.

  • This figure shows their estimate of slip distribution (how much the faults slipped and where) for these doublet earthquakes.

  • Coseismic slip distributions. Surface projection of coseismic slip for the 15 November 2006 (average slip 4.6 m) and 13 January 2007 (average slip 9.6 m) events (NEIC epicentres shown by yellow circles, GCMT centroid epicentres shown by stars). GCMT mechanisms (centred on NEIC epicentres) for large events between June 2006 and May 2007 are shown;
    enlarged mechanisms are shown for the doublet events. Grey mechanisms indicate events before the 15 November 2006 event, red mechanisms indicate events after that rupture. The focal mechanism and epicentre of the 16 March 1963 compressional outer-rise event (yellow hexagon) are included. The arrow indicates the direction of the Pacific plate motion at 80 mm/yr.

    Geologic Fundamentals

    • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
    • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

    • Here is another way to look at these beach balls.
    • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
    • Strike Slip:

      Compressional:

      Extensional:

    • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

    • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

    • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

    • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

      Social Media

      References:

      • Ammon, C.J., Kanamori, H., and Lay, T., 2008. A great earthquake doublet and seismic stress transfer cycle in the central Kuril islands in Nature, v. 451, doi:10.1038/nature06521
      • Antonijevic, S.K., et a;l., 2015. The role of ridges in the formation and longevity of flat slabs in Nature, v. 524, p. 212-215, doi:10.1038/nature14648
      • Bishop, B.T., Beck, S.L., Zandt, G., Wagner, L., Long, M., Knezevic Antonijevic, S., Kumar, A., and Tavera, H., 2017, Causes and consequences of flat-slab subduction in southern Peru: Geosphere, v. 13, no. 5, p. 1392–1407, doi:10.1130/GES01440.1.
      • Chlieh, M., et al., 2011. Interseismic coupling and seismic potential along the Central Andes subduction zone in JGR, v. 116, B12405, doi:10.1029/2010JB008166
      • Espurt, N., Baby, P., Brusset, S., Roddaz, M., Hermoza, W., Regard, V., Antoine, P.-O., Salas-Gismodi, R., and Bolaños, R., 2007. How does the Nazca Ridge subduction influence the modern Amazonian foreland basin? in Geology, v. 35, no. 6, p. 515-518.
      • Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
      • Kumar, A., et al., 2016. Seismicity and state of stress in the central and southern Peruvian flat slab in EPSL, v. 441, p. 71-80. http://dx.doi.org/10.1016/j.epsl.2016.02.023
      • Ray., J.S., et al., 2012. Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ~30 Myr Hotspot Record in Journal of Petrology, v. 53., no. 7, p. 1417-1448.
      • Rhea, S., Tarr, A.C., Hayes, G., Villaseñor, A., Furlong, K.P., and Benz, H.M., 2010. Seismicity of the Earth 1900-2007, Nazca plate and South America: U.S. Geological Survey Open-File Report 2010-1083-E, 1 map sheet, scale 1:12,000,000.
      • Ruiz, S., Tavera, H., Poli, p., Herrera, C., Flores, C., Rivera, E., and Madariaga, R., 2017. The deep Peru 2015 doublet earthquakes in EPSL, v. 471, p. 102-109
      • Scire, A., Zandt, G., Beck, S., Long, M., and Wagner, L., 2017. The deforming Nazca slab in the mantle transition zone and lower mantle: Constraints from teleseismic tomography on the deeply subducted slab between 6°S and 32°S: Geosphere, v. 13, no. 3, p. 665–680, doi:10.1130/GES01436.1.
      • Scire, A., Zandt, G., Beck, S., Long, M., Wagner, L., Minaya, E., and Tavera, H., 2016. Imaging the transition from flat to normal subduction: variations in the structure of the Nazca slab and upper mantle under southern Peru and northwestern Bolivia ion Geophysical Journal International, Volume 204, Issue 1, 1 January 2016, Pages 457–479, https://doi.org/10.1093/gji/ggv452
      • Villegas-Lanza, J.C., et al., 2016. Active tectonics of Peru: Heterogeneous interseismic coupling along the Nazca Megathrust, rigid motion of the Peruvian Sliver and Subandean shortening accommodation in JGR, doi: 10.1002/2016JB013080

      Return to the Earthquake Reports page.

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Earthquake Report: Blanco fracture zone

As I was getting ready for school today, I noticed the M 6.2 notification from the USGS Earthquake Notification Service. People can sign up for the USGS ENS so that they can get emails when the USGS broadcasts this information. Most all apps that people install on their devices use the USGS feed as a basis for the sources for those apps. So, it is rather ironic when people make claims that they use these apps because they don’t trust the USGS. When I read statements like that, I just roll my eyes. People love ways to promote their conspiratorial views of the world. Here is the USGS ENS web page.

The most recent earthquake on the Blanco fracture zone was less than a month ago. Here is my report on that earthquake.

The BFZ is a transform plate boundary that connects the Juan de Fuca ridge with the Gorda rise spreading centers.

As for all individual earthquakes along the BFZ, there are no direct implications for earthquake or tsunami hazards along the Cascadia subduction zone (CSZ) as a result of these BFZ earthquakes. Even though people felt this M 6.2 along the coast of Oregon, as well as in the Willamette Valley and Portland, the earthquake is just too far away from the CSZ to change the static stresses within the CSZ megathrust fault, or within the North America, Juan de Fuca, or Gorda plates.

Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • Note that along the Gorda rise, the magnetic anomaly is red, showing that the spreading ridge has a normal polarity, like that of today. Prior to about 780,000 years ago, the polarity was reversed. During the Bruhnes-Matuyama magnetic polarity reversal, the polarity flipped to the way it is today. Note how as one goes away from the Gorda rise (east or west), the magnetic anomaly changes color to blue. At the boundary between red and blue is the Bruhnes-Matuyama magnetic polarity reversal.
  • The structures in the Gorda, Juad de Fuca, and Pacific plates in this region are largely inherited from the extensional tectonic and volcanic processes at the Gorda rise and Juan de Fuca Ridge. However, the Gorda plate is being pulverized by the surrounding tectonic plates. There are several interpretations about how the plate is deforming and some debate about whether the Gorda plate is even behaving like a plate.
  • Note how some of the magnetic anomalies appear to be offset along lines that are sub-parallel to the BFZ. This is because they are.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I one version, I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.0.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include some inset figures.

  • In the upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004). I placed a blue stars in the general location of today’s earthquake (as in other inset figures in this poster).
  • In the lower right corner is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. Today’s earthquake did not occur along the CSZ, so did not produce crustal deformation like this. However, it is useful to know this when studying the CSZ. Today’s earthquakes happened in the lower Gorda plate
  • In the upper left corner is a map showing the details for the faulting along the BFZ (Braunmiller and Nabelek (2008). Note that this zone is quite complicated and includes several normal fault bounded pull-apart basins.
  • In the lower left corner is a map from Dziak et al. (2000) that shows the topography (in the upper panel) and the faulting (in the lower panel) along the BFZ. Blue = lower elevation, deeper oceanic depths; Red = shallower oceanic depth, higher elevation. I placed orange arrows to help one locate the normal faults (perpendicular to the strike-slip faults) in this map. Compare this inset map with the Google Earth bathymetry in the main map. Can you see the BFZ perpendicular ridges?
  • I include two main interpretive posters for this earthquake. One includes information from this earthquake, including the MMI contours and USGS “Did You Feel It?” colored polygons. This way we can compare the modeled estimate of intensity (MMI contours) and the reports from real people (DYFI data). There are some good matches and some mismatches (in western Oregon). Check this out and try to think about why there may be mismatches.

  • The second poster includes earthquake information for earthquakes with M ≥ 6.0. I place fault mechanisms for all existing USGS mechanisms from the Blanco fracture zone and I include some examples from the rest of the region. These other mechanisms show how different areas have different tectonic regimes. Earthquakes within the Gorda plate are largely responding to being deformed in a tectonic die between the surrounding stronger plates (northeast striking (oriented) left-lateral strike-slip earthquakes). I include one earthquake along the Mendocino fracture zone, a right-lateral (dextral) strike-slip earthquake from 1994. I include one of the more memorable thrust earthquakes, the 1992 Cape Mendocino earthquake. I also include an extensional earthquake from central Oregon that may represent extension (basin and range?) in the northwestern region of the basin and range.

Some Relevant Discussion and Figures

  • Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).

  • Here is a version of the CSZ cross section alone (Plafker, 1972). This shows two parts of the earthquake cycle: the interseismic part (between earthquakes) and the coseismic part (during earthquakes). Regions that experience uplift during the interseismic period tend to experience subsidence during the coseismic period.

  • This is the figure from Dziak et al. (2000) for us to evaluate. I include their long figure caption below.

  • (Top) Sea Beam bathymetric map of the Cascadia Depression, Blanco Ridge, and Gorda Depression, eastern Blanco Transform Fault Zone (BTFZ).Multibeam bathymetry was collected by the NOAA R/V’s Surveyor and Discoverer and the R/V Laney Chouest during 12 cruises in the 1980’s and 90’s. Bathymetry displayed using a 500 m grid interval. Numbers with arrows show look directions of three-dimensional diagrams in Figures 2 and 3. (Bottom) Structure map, interpreted from bathymetry, showing active faults and major geologic features of the region. Solid lines represent faults, dashed lines are fracture zones, and dotted lines show course of turbidite channels. When possible to estimate sense of motion on a fault, a filled circle shows the down-thrown side. Inset maps show location and generalized geologic structure of the BTFZ. Location of seismic reflection and gravity/magnetics profiles indicated by opposing brackets. D-D’ and E-E’ are the seismic reflection profiles shown in Figures 8a and 8b, and G-G’ is the gravity and magnetics profile shown in Figure 13. Submersible dive tracklines from sites 1 through 4 are highlighted in red. L1 and L2 are two lineations seen in three-dimensional bathymetry shown in Figures 2 and 3. Location of two Blanco Ridge slump scars indicated by half-rectangles, inferred direction of slump shown by arrow, and debris location (when identified) designated by an ‘S’. CD stands for Cascadia Depression, BR is Blanco Ridge, GD is Gorda Depression, and GR is Gorda Ridge. Numbers on north and south side of transform represent Juan de Fuca and Pacific plate crustal ages inferred from magnetic anomalies. Long-term plate motion rate between the Pacific and southern Juan de Fuca plates from Wilson (1989).

BFZ Historic Seismicity

  • There were two Mw 4.2 earthquakes associated with this plate boundary fault system in mid 2015. I plot the moment tensors for these earthquakes (USGS pages: 4/7/15 and 4/11/15) in this map below. I also have placed the relative plate motions as arrows, labeled the plates, and placed a transparent focal mechanism plot above the BFZ showing the general sense of motion across this plate boundary. There have been several earthquakes along the Mendocino fault recently and I write about them 1/2015 here and 4/2015 here.

  • There was also seismic activity along the BFZ later in 2015. Here are my report and report update.
  • Here is a map showing these earthquakes, with moment tensors plotted for the M 5.8 and M 5.5 earthquakes. I include an inset map showing the plate configuration based upon the Nelson et al. (2004) and Chaytor et al. (2004) papers (I modified it). I also include a cross section of the subduction zone, as it is configured in-between earthquakes (interseismic) and during earthquakes (coseismic), modified from Plafker (1972).

  • I put together an animation that includes the seismicity from 1/1/2000 until 6/1/2015 for the region near the Blanco fracture zone, with earthquake magnitudes greater than or equal to M = 5.0. The map here shows all these epicenters, with the moment tensors for earthquakes of M = 6 or more (plus the two largest earthquakes from today’s swarm). Here is the page that I posted regarding the beginning of this swarm. Here is a post from some earthquakes earlier this year along the BFZ.
  • Earthquake epicenters are plotted with the depth designated by color and the magnitude depicted by the size of the circle. These are all fairly shallow earthquakes at depths suitable for oceanic lithosphere.

    Here is the list of the earthquakes with moment tensors plotted in the above maps (with links to the USGS websites for those earthquakes):

  • 2000/06/02 M 6.0
  • 2003/01/16 M 6.3
  • 2008/01/10 M 6.3
  • 2012/04/12 M 6.0
  • 2015/06/01 M 5.8
  • 2015/06/01 M 5.9
    Here are some files that are outputs from that USGS search above.

  • csv file
  • kml file (not animated)
  • kml file (animated)

VIDEOS

    Here are links to the video files (it might be easier to download them and view them remotely as the files are large).

  • First Animation (20 mb mp4 file)
  • Second Animation (10 mb mp4 file)

Here is the first animation that first adds the epicenters through time (beginning with the oldest earthquakes), then removes them through time (beginning with the oldest earthquakes).

Here is the second animation that uses a one-year moving window. This way, one year after an earthquake is plotted, it is removed from the plot. This animation is good to see the spatiotemporal variation of seismicity along the BFZ.

Here is a map with all the fore- and after-shocks plotted to date.

Gorda Plate Seismicity

  • Here is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. The moment tensor (at the moment i write this) shows a north-south striking fault with a reverse or thrust faulting mechanism. While this region of faulting is dominated by strike slip faults (and most all prior earthquake moment tensors showed strike slip earthquakes), when strike slip faults bend, they can create compression (transpression) and extension (transtension). This transpressive or transtentional deformation may produce thrust/reverse earthquakes or normal fault earthquakes, respectively. The transverse ranges north of Los Angeles are an example of uplift/transpression due to the bend in the San Andreas fault in that region.

  • A: Mapped faults and fault-related ridges within Gorda plate based on basement structure and surface morphology, overlain on bathymetric contours (gray lines—250 m interval). Approximate boundaries of three structural segments are also shown. Black arrows indicated approximate location of possible northwest- trending large-scale folds. B, C: uninterpreted and interpreted enlargements of center of plate showing location of interpreted second-generation strike-slip faults and features that they appear to offset. OSC—overlapping spreading center.

  • These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.
  • Mw = 5 Trinidad Chaytor

    Models of brittle deformation for Gorda plate overlain on magnetic anomalies modified from Raff and Mason (1961). Models A–F were proposed prior to collection and analysis of full-plate multibeam data. Deformation model of Gulick et al. (2001) is included in model A. Model G represents modification of Stoddard’s (1987) flexural-slip model proposed in this paper.

  • Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).

  • Tectonic configuration of the Gorda deformation zone and locations and source models for 1976–2010 M ≥ 5.9 earthquakes. Letters designate chronological order of earthquakes (Table 1 and Appendix A). Plate motion vectors relative to the Pacific Plate (gray arrows in main diagram) are from Wilson [1989], with Cande and Kent’s [1995] timescale correction.

  • In this map below, I label a number of other significant earthquakes in this Mendocino triple junction region. Another historic right-lateral earthquake on the Mendocino fault system was in 1994. There was a series of earthquakes possibly along the easternmost section of the Mendocino fault system in late January 2015, here is my post about that earthquake series.

The Gorda and Juan de Fuca plates subduct beneath the North America plate to form the Cascadia subduction zone fault system. In 1992 there was a swarm of earthquakes with the magnitude Mw 7.2 Mainshock on 4/25. Initially this earthquake was interpreted to have been on the Cascadia subduction zone (CSZ). The moment tensor shows a compressional mechanism. However the two largest aftershocks on 4/26/1992 (Mw 6.5 and Mw 6.7), had strike-slip moment tensors. These two aftershocks align on what may be the eastern extension of the Mendocino fault.

There have been several series of intra-plate earthquakes in the Gorda plate. Two main shocks that I plot of this type of earthquake are the 1980 (Mw 7.2) and 2005 (Mw 7.2) earthquakes. I place orange lines approximately where the faults are that ruptured in 1980 and 2005. These are also plotted in the Rollins and Stein (2010) figure above. The Gorda plate is being deformed due to compression between the Pacific plate to the south and the Juan de Fuca plate to the north. Due to this north-south compression, the plate is deforming internally so that normal faults that formed at the spreading center (the Gorda Rise) are reactivated as left-lateral strike-slip faults. In 2014, there was another swarm of left-lateral earthquakes in the Gorda plate. I posted some material about the Gorda plate setting on this page.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.


    Social Media

    References:

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • Dziak, R.P., Fox, C.G., Embleey, R.W., Nabelek, J.L., Braunmiller, J., and Koski, R.A., 2000. Recent tectonics of the Blanco Ridge, eastern blanco transform fault zone in Marine Geophysical Researches, vol. 21, p. 423-450
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • Lin, J., R. S. Stein, M. Meghraoui, S. Toda, A. Ayadi, C. Dorbath, and S. Belabbes (2011), Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003 Mw = 6.9 Zemmouri, Algeria, earthquake, J. Geophys. Res., 116, B03305, doi:10.1029/2010JB007654.
  • McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Yue, H., Zhang, Z., Chen, Y.J., 2008. Interaction between adjacent left-lateral strike-slip faults and thrust faults: the 1976 Songpan earthquake sequence in Chinese Science Bulletin, v. 53, no. 16, p. 2520-2526
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

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Earthquake Report: Venezuela

Busy week!

We just had a M 7.3 earthquake in northern Venezuela. Sadly, this large earthquake has the potential to be quite damaging to people and their belongings (buildings, infrastructure).

The northeastern part of Venezuela lies a large strike-slip plate boundary fault, the El Pilar fault. This fault is rather complicated as it strikes through the region. There are thrust faults and normal faults forming ocean basins and mountains along strike.

Many of the earthquakes along this fault system are strike-slip earthquakes (e.g. the 1997.07.09 M 7.0 earthquake which is just to the southwest of today’s temblor. However, today’s earthquake broke my immediate expectations for strike-slip tectonics. There is a south vergent (dipping to the north) thrust fault system that strikes (is oriented) east-west along the Península de Paria, just north of highway 9, east of Carupano, Venezuela. Audenard et al. (2000, 2006) compiled a Quaternary Fault database for Venezuela, which helps us interpret today’s earthquake. I suspect that this earthquake occurred on this thrust fault system. I bet those that work in this area even know the name of this fault. However, looking at the epicenter and the location of the thrust fault, this is probably not on this thrust fault. When I initially wrote this report, the depth was much shallower. Currently, the hypocentral (3-D location) depth is 123 km, so cannot be on that thrust fault.

The best alternative might be the subduction zone associated with the Lesser Antilles.

GPS data support the hypothesis that the El Pilar fault is accumulating strike-slip strain, but there is a paucity of evidence that there is active convergence across the thrust fault. However, there does appear to be some small amount of contraction (Reinoza, et a.,. 2015).

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.0 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. However, the slab contours are only in the southwestern portion of this map.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a map from Pindel and Kennan (2009) that shows the plate tectonic boundaries of the Caribbean and northern South America.
  • In the lower right corner is another map that shows teh regional tectonics (Levander et al., 2006).
  • In the upper right corner is a larger scale map showing the faulting in the region surrounding today’s M 7.3 earthquake (Audenard et al., 2000).
  • Here is the map with a month’s seismicity plotted, along with USGS earthquakes M ≥ 6.0.


Other Report Pages

Some Relevant Discussion and Figures

  • Here is the Pindell and Kennan (2009) map.

  • Present day tectonic map of the Caribbean region.

  • Here is the Pindell and Kennan (2009) figure that shows how the large strike-slip plate boundary on teh north side of Venezuela grew from the west over time.

  • Motion histories of: North (NA) and South America (SA) relative to Indo-Atlantic hot spot (IAHS) Mu¨ller et al. (1993) reference frame (grey lines; NA wrt IAHS and SA wrt IAHS); hot spots relative to North America (dashed black line; IAHS wrt NA); Caribbean relative to North America (heaviest black line; Car wrt NA), as summarized from former relative positions of the Caribbean Trench (lighter black lines). Also shown: Cayman Trough (grey outline); Cenozoic convergence between the Americas (inset upper right; P88 ¼ Pindell et al. 1988; M99 ¼ Mu¨ller et al. 1999); seismic tomographic profile of van der Hilst (1990) (inset, lower right).

  • Here is the Audemard (2000) map showing the many faults in this region.

  • This is a map from Reinoza et al. (2015) where they present their geodetic analysis (analysis of the deformation of the earth). These authors use GPS data to evaluate the potential activity of the El Pilar fault.

  • Location map of the active faults in northeastern Venezuela [Audemard et al., 2000] showing distribution of the GNSS stations: yellow squares, green circles, and red triangles are GNSS sites on which the acquisition campaigns were carried out in 2003, 2005, and 2013 respectively; the blue star corresponds to the cGNSS CUMA station of REMOS-IGVSB Network. We show the epicenter location of 1929 and 1997 events with their respective proposed ruptures (orange lines) [Audemard, 2007]. (top right) The inset box shows a schematic geodynamic map of the southeastern Caribbean [Audemard, 1999b; Audemard et al., 2000; Weber et al., 2001]. Legend: BF = Boconó Fault, EPF = El Pilar Fault, OAF = Oca Ancón fault, SMBF = Santa Marta Bucaramanga Fault, and SSF = San Sebastian Fault.

  • Here are the GPS data. The white arrows (vectors) show the observed velocities (motion rate) for the GPS sites shown on the previous map. The black arrows (vectors) show how their model results compare with the observational data.

  • Observed velocities (white arrows) with error ellipses for 66% confidence level and simulated velocities (black arrows) according to the upgrade of displacement-simulation method. All displacements are based on the South America reference frame.

  • Here are some cross sections showing the El Pilar fault, along with some of the thrust faults in the region. Section B is just to the west of where this M 7.3 earthquake happened.

  • Simplified sections across the southeastern Caribbean margin (based on maps and sections by Bellizzia et al. (1976), Stéphan et al. (1980), Campos (1981), Beck (1986), Chevalier (1987); locations in Fig. 1).

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    References:

  • Audemard, F.A., Machette, M.N., Cox, J.W., Dart, R.L., and Haller, K.M., 2000. Map and Database of Quaternary Faults in Venezuela and its Offshore Regions, USGS Open File Report 00-018
  • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
  • Jouanne, F., Audemard, F.A., Beckm, C., Van Welden, A., Ollarves, R., and Reinoz, C., 2011. Present-day deformation along the El Pilar Fault in eastern Venezuela: Evidence of creep along a major transform boundary in Journal of Geodynamics, v. 51., p. 398-410, doi:10.1016/j.jog.2010.11.003
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • Pindell, J.L. and Kennan, L., 2009. Tectonic evolution of the Gulf of Mexico, Caribbean and northern South America in the mantle reference frame: an update in JAMES, K. H., LORENTE, M. A. & PINDELL, J. L. (eds) The Origin and Evolution of the Caribbean Plate. Geological Society, London, Special Publications, 328, 1–55.
    DOI: 10.1144/SP328.1
  • Reinoza, C., F. Jouanne, F. A. Audemard, M. Schmitz, and C. Beck (2015), Geodetic exploration of strain along the El Pilar Fault in northeastern Venezuela, J. Geophys. Res. Solid Earth, 120, 1993– 2013, doi:10.1002/2014JB011483.

°

Earthquake Report: Lombok, Indonesia

Well well.

After a pretty seismically quiet first half of 2018, we have been catching up rapidly. The ultra deep Great Earthquake in Fiji. And now the Lombok sequence continues.

The inhabitants and tourists in the Lombok, Indonesia region have been experiencing quite a few deadly and damaging earthquakes.

This ongoing sequence began in late July with a Mw 6.4 earthquake. Followed less than 2 weeks later with a Mw 6.9 earthquake.

Today there was an M 6.3 soon followed by an M 6.9 earthquake (and a couple M 5.X quakes).

These earthquakes have been occurring along a thrust fault system along the northern portion of Lombok, Indonesia, an island in the magamatic arc related to the Sunda subduction zone. The Flores thrust fault is a backthrust to the subduction zone. The tectonics are complicated in this region of the world and there are lots of varying views on the tectonic history. However, there has been several decades of work on the Flores thrust (e.g. Silver et al., 1986). The Flores thrust is an east-west striking (oriented) north vergent (dipping to the south) thrust fault that extends from eastern Java towards the Islands of Flores and Timor. Above the main thrust fault are a series of imbricate (overlapping) thrust faults. These imbricate thrust faults are shallower in depth than the main Flores thrust.

The earthquakes that have been happening appear to be on these shallower thrust faults, but there is a possibility that they are activating the Flores thrust itself. Perhaps further research will illuminate the relations between these shallower faults and the main player, the Flores thrust.

There are 2 main ways that earthquakes may be triggered by a previous earthquake.

  1. Dynamic Triggering is when seismic waves are traveling through the crust from an earlier earthquake and these seismic waves increase the stress on a second or the same fault, causing a second (or more) earthquake.
  2. Static Triggering is when an earlier earthquake slips and deforms the crust/lithosphere surrounding the earthquake. These changes can impart changes in static coulomb stress in the adjacent crust. These changes can lead to increases or decreases of stress along faults in that adjacent part of the crust (e.g. Lin & Stein, 2004).

Both types of triggering impart a very very small amount of increased stress on a given fault or fault system. This means that the way for an earthquake to be triggered in this manner, the potentially triggered fault will need to be on the verge of rupturing on its own. The stresses released by earthquakes are much larger than those stresses imparted by dynamic or static triggering, so the faults need to be “ready to go” if they are to be triggered.

I presented this on my earlier earthquake report, but this still holds true. People had been asking me if we might expect another large or larger earthquake in this region. So, here is what I have told them:

  • It is difficult to say if there will be a larger or another large earthquake or not.
  • Based upon historic seismicity, the M 6.9 is probably the mainshock in this sequence. But the historic record is short (100 yrs +-), so may not be a perfect sample of what could happen.
  • The M 6.9 probably ruptured the Flores thrust fault, a back thrust to the subduction zone.
  • There is probably a small chance that the Flores thrust fault (east west fault dipping to the south) to the east and west of the M 6.9 has an increased amount of stress imparted upon it from the M 6.9 (small amount, so if the fault was almost ready to go, this change might make it go). but this is a small possibility (but still possible). (i.e. Bali). (Today’s M 6.9 is evidence that the fault saw an increase in stress from the earlier earthquakes.)
  • There is also a small chance that the subduction zone (south of the islands, dipping to the north) also has an increased amount of stress from this M 6.9 earthquake. but this is probably less likely than the other example (due to the distance between the M .6.9 and the subduction zone fault.
  • Though there will probably be earthquakes up to M 5 or mid M 5 as aftershocks… and as time passes, the chance of a larger earthquake diminish to the background risk of such an earthquake. by the time it is Sept through Dec, we will probably have passed the increased risk due to the M 6.9 sequence. (Though today we saw two M > 6 earthquakes)
  • But we must always remember, we cannot absolutely know what will happen. our observational history is only a few centuries and seismometers are only a century old (and modern ones, with a global network, maybe 50 years). so it is challenging to think that we know about how this region (or any region) behaves tectonically.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.0 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes. The focal mechanism from the 1977.08.19 M 8.3 earthquake came from Lynnes and Lay, 1988.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Australia plate.

    I include some inset figures.

  • In the upper right corner is a low angle oblique view of the Sunda subduction zone beneath Java, Bali, Lombok, and Sumbawa (from Earth Observatory Singapore). I place a blue star in the general location of today’s earthquake’s epicenter (as for all figures here). The India-Australia plate is subducting northwards beneath the Sunda plate (part of the Eurasia plate). I include a blue star in the general location of today’s M 6.9 earthquake (as in other inset figures).
  • In the upper left corner is a plate tectonic map showing the major fault systems, volcanic arc islands, and oceanic plateaus and basins of the region (Darman, 2012). The map shows the Flores thrust extending as far west as Lombok. Compare the complicated tectonics in the eastern portion of this region compared to the western portion of this region.
  • In the lower right corner is a cross section showing earthquake hypocenters (3-D locations) from Darman et a. (2012).
  • To the right of the Darman et al. (2012) map is a figure from Lin and Stein (2004). Their paper discusses changes in static coulomb stress imparted by earthquakes in different configurations. The upper panel is a map view of how a thrust fault earthquake imparts changes in stress in the adjacent crust. Warm colors = increases in stress. Cool colors indicate decreases in stress. The fault in this example is an east-west thrust fault. Note how the region above the fault sees a decrease in stress while the region surrounding the ruptured fault sees an increase in stress. This is probably what is happening in Lombok right now. The 7/28 M 6.4 earthquake likely increased the stress in the adjacent crust, leading to the 8/5 M 6.9 earthquake. This (and the other earthquakes) also led to increased stress in the adjacent crust, probably triggering the M 6.9 earthquake from today. There is no reason to think that this won’t continue to the east or to the west. But we cannot really know if there will be a continued “unzipping” of these Flores thrust related faults. While the historic seismologic records are incredibly short (a century or less), there are no good analogues to this happening in this region (there are in other regions of the Earth).
  • In the lower right corner is a surface deformation map prepared by Dr. Eric Fielding, from NASA JPL. This map shows the result of modeling surface deformation using interferrometric Radar (InSAR). Basically, using Radar data from time periods before and after an earthquake, one can subtract the two data sets from each other to estimate how much the ground deformed during the earthquake. Note that there is approximately 40 cm of overall deformation from this 8/5 M 6.9 earthquake. Here is the technical definition of what this interferrogram is (from Dr. Fielding): an “interferogram is [a] measurement of displacement in the radar line-of-sight (LOS) direction, not a model. The LOS for this measurement is up and east.”
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a centuries’ seismicity plotted.

  • Here is an updated local scale (large scale) map showing the earthquake fault mechanisms for the current sequence. I label them with yellow numbers according to the sequence timing. I outlined the general areas that have had earthquakes into two zones (phases). Phase I includes the earthquakes up until today and Phase II includes the earthquakes from today. There is some overlap, but only for a few earthquakes. In general, it appears that the earthquakes have slipped in two areas of the Flores fault (or maybe two shallower thrust faults).

  • Here is the 8/5 M 6.9 earthquake map with a month’s seismicity plotted.

  • Here is the 8/5 M 6.9 earthquake map with a centuries’ seismicity plotted.

  • Here is the interpretive posted from the M 6.4 7/28 earthquake, with historic seismicity and earthquake mechanisms.

Other Report Pages

Some Relevant Discussion and Figures

  • Below is a map showing historic seismicity (Jones et al., 2014). Cross sections B-B’ and C-C’ are shown. The seismicity for the cross sections below are sourced from within each respective rectangle.

  • Here are the seismcity cross sections.

  • Below are the maps and cross sections from Darman et al., 2012.
  • Here is the map in the interpretive poster above.

  • Tectonic map of the Lesser Sunda Islands, showing the main tectonic units, main faults, bathymetry and location of seismic sections discussed in this paper.

  • Here is the seismicity cross section in the interpretive poster above.

  • This plot shows the earthquake localizations on a South-North cross section for the lat -14°/-4° long 114°/124° quadrant corresponding to the Lesser Sunda Islands region. The localizations are extracted from the USGS database and corresponds to magnitude greater than 4.5 in the 1973-2004 time period (shallow earthquakes with undetermined depth have been omitted.

  • Here is their interpretations of seismic data used to interpret the tectonics of the subduction zone and Flores thrust.

  • Six 15 km deep seismic sections acquired by BGR from west to east traversing oceanic crust, deep sea trench, accretionary prism, outer arc high and fore-arc basin, derived from Kirchoff prestack depth migration (PreSDM) with a frequency range of 4-60 Hz. Profile BGR06-313 shows exemplarily a velocity-depth model according to refraction/wide-angle
    seismic tomography on coincident profile P31 (modified after Lüschen et al, 2011).

  • Here is the map from McCaffrey and Nabelek (1987). They used seismic reflection profiles, gravity modeling along these profiles, seismicity, and earthquake source mechanism analyses to support their interpretations of the structures in this region.

  • Tectonic and geographic map of the eastern Sunda arc and vicinity. Active volcanoes are represented by triangles, and bathymetric contours are in kilometers. Thrust faults are shown with teeth on the upper plate. The dashed box encloses the study area.

  • Here is the Audley (2011) cross section showing how the backthrust relates to the subduction zone beneath Timor. I include their figure caption in blockquote below.

  • Cartoon cross section of Timor today, (cf. Richardson & Blundell 1996, their BIRPS figs 3b, 4b & 7; and their fig. 6 gravity model 2 after Woodside et al. 1989; and Snyder et al. 1996 their fig. 6a). Dimensions of the filled 40 km deep present-day Timor Tectonic Collision Zone are based on BIRPS seismic, earthquake seismicity and gravity data all re-interpreted here from Richardson & Blundell (1996) and from Snyder et al. (1996). NB. The Bobonaro Melange, its broken formation and other facies are not indicated, but they are included with the Gondwana mega-sequence. Note defunct Banda Trench, now the Timor TCZ, filled with Australian continental crust and Asian nappes that occupy all space between Wetar Suture and the 2–3 km deep deformation front north of the axis of the Timor Trough. Note the much younger decollement D5 used exactly the same part of the Jurassic lithology of the Gondwana mega-sequence in the older D1 decollement that produced what appears to be much stronger deformation.

  • This are the seismicity cross sections from Hangesh and Whitney (2016). These are shown to compare the subduction zone offshore of Java and the collision zone in the Timor region.

  • Comparison of hypocentral profiles across the (a) Java subduction zone and (b) Timor collision zone (paleo-Banda trench). Catalog compiled from multiple reporting agencies listed in Table 1. Events of Mw>4.0 are shown for period 1815 to 2015.

  • Here is a map of the same general area from Silver et al. (1986), used here to locate the following large scale map.

  • Location of SeaMARC II survey (Plate 1 and Figures 2) and geographic features discussed in text. Triangles on upper plates of thrust zones.

  • This is the large scale map showing the detailed thrust fault mapping (Silver et al., 1986).


  • Bathymetry, faults, and mud diapirs of the central Flores thrust zone, based on interpretation of SeaMARC II data and seismic reflection profiles. Shown also are locations (circled numbers) of all seismic profiles. Mud diapirs are solid black. Triangles on upper plates of thrust faults.

  • Here is the tectonic map from Hangesh and Whitney (2016).

  • Illustration of major tectonic elements in triple junction geometry: tectonic features labeled per Figure 1; seismicity from ISC-GEM catalog [Storchak et al., 2013]; faults in Savu basin from Rigg and Hall [2011] and Harris et al. [2009]. Purple line is edge of Australian continental basement and fore arc [Rigg and Hall, 2011]. Abbreviations: AR = Ashmore Reef; SR = Scott Reef; RS = Rowley Shoals; TCZ = Timor Collision Zone; ST = Savu thrust; SB = Savu Basin; TT = Timor thrust; WT =Wetar thrust; WASZ = Western Australia Shear Zone. Open arrows indicate relative direction of motion; solid arrows direction of vergence.

  • Here are some focal mechanisms from earthquakes in the region from Hangesh and Whitney (2016). Symbol color represents depth.

  • (a) Focal mechanism solutions for the study region. The focal mechanisms are classified based on depth intervals to illustrate the style of faulting within the different structural domains. Note (b) sinistral reverse motion along Timor trough, (c) subduction related pattern along Java trench, and dextral solutions along the western Australia extended margin (Figure 4a) north of 20°S. Centroid moment tensor (CMT) solutions [Dziewonski et al., 1981] are from the CMT project [Ekström et al., 2012; http://www.globalcmt.org/CMTcite.html] for events of Mw>5.0 for the period 1976 onward.

  • This map from Hangesh and Whitney (2016) shows the GPS velocities in this region. Note the termination of the Flores thrust and the north-northeast striking (oriented) cross fault between Lombok and Sumbawa.

  • GPS velocities of Sunda and Banda arc region. Large black and grey arrow shows motion of Australia relative to Eurasia [DeMets et al., 1994]. Thin black arrows show GPS velocities of Sunda and Banda arc regions relative to Australia [Nugroho et al., 2009]. Seismicity from ISC-GEM catalog [Storchak et al., 2013]. Note reduction of station velocities from west to east indicating progressive coupling of the Banda arc to the Australian plate compared to the area along the Sunda arc.

  • Below are the 4 figures from Koulani et al., 2016. First is the plate tectonic map. I include their figure captions in block quote.

  • Seismotectonic setting of the Sunda-Banda arc-continent collision, East Indonesia. Major faults (thick black lines) [Hamilton, 1979]. Topography and bathymetry are from Shuttle Radar Topography Mission (http://topex.ucsd.edu/www_html/srtm30_plus.html). Focal mechanisms are from the Global Centroid Moment Tensor. Blue mechanisms correspond to earthquakes with Mw>7 (brown transparent ellipses are the corresponding rupture areas for Flores 1992 and Alor 2004 earthquakes), while the green focal mechanism shows the highest magnitude recorded in Sumbawa. Red dots indicate the locations of major historical earthquakes [Musson, 2012].

  • This figure shows their estimates for plate motion relative velocities as derived from GPS data, constrained by the fault geometry in their block modeling.

  • GPS velocities determined in this study with respect to Sunda Block. Uncertainty ellipses represent 95% confidence level. The inset figure corresponds to the area of the dashed rectangle in the map. Light blue arrows show the velocities for East and West Makassar Blocks.

  • This figure shows their estimates of slip rate deficit along all the plate boundary faults in this region.

  • Relative slip vectors across block boundaries, derived from our best fit model. Arrows show motion of the hanging wall (moving block) relative to the footwall (fixed block) with 95% confidence ellipses. The tails of arrows is located within the “moving” block. Black thick lines show well-defined boundaries we use as active faults in our model and dashed lines show less well-defined boundaries (green : free-slipping boundaries and black: fixed locked faults) . Principal axes of the horizontal strain tensor estimated for the SUMB, EMAK, and EJAV are shown in pink. The thick pink arrow shows the relative motion of Australia with respect to Sunda (AUST/SUND). Abbreviations are Sumba Block (SUMB), West Makassar Block (WMAK), East Makassar Block (EMAK), East Java Block (EJAV), and Timor Block (TIMO). The background seismicity is from the International Seismological Centre catalog with magnitudes ≥5.5 and depths <40 km.

  • Here is their figure that shows the slip deficit along the plate boundary faults.

  • Fault slip rate components: (a) fault normal (extension positive) and (b) fault parallel (right-lateral positive).

  • Here are some figures from Lin and Stein (2004).
  • This first figure shows the geometry of thrust faults and also shows the range of parameters for earthquakes used in their analysis.

  • Depth of burial as a function of fault length/width (L/W) ratio for some well-studied thrust faults. Burial depth is normalized by the vertical extent of the fault, as shown in the inset. Large subduction earthquakes tend to locate in the upper right; moderate size continental thrust faults tend to locate to the left. Sources are listed in the paper.

  • This second figure shows the results from a cross section view for earthquakes used in their analysis. Increase in stress is represented by warm color.

  • Cross sections (left) through the center and (right) beyond the end of the fault of a 45°-dipping thrust source fault. Optimally oriented receiver thrust planes are shown in areas of increased Coulomb stress. Both the 1971 San Fernando and 1994 Northridge faults dip about 45°. (a) The surface cutting thrust (Mw = 7.0) drops the stress in the upper crust, (b) whereas a blind thrust (Mw = 6.8) increases the stress over much of the upper crust, despite its smaller magnitude. Near-surface regions of stress increase are sometimes relieved by secondary surface faulting, as occurred in the Northridge shock. (c) Stress changes caused by blind and surface fault slip. (d–f ) Beyond the ends of the faults the stress distribution is relatively insensitive to whether the thrust is surface-cutting or blind, where the along strike projection of faults is dotted.

  • This first figure shows the geometry of thrust faults and also shows the range of parameters for earthquakes used in their analysis.

  • Stress change caused by the 1 October 1987 Mw = 6.0 Whittier Narrows earthquake. (a) Map view of maximum stress change for depth range of 10.0–14.4 km, with seismicity (1 October 1987 to 31 December 1994, M ≥ 1.0, horizontal error <0.5 km) from Shearer [1997] for the same depth range. The source fault model, shown by the black inscribed line, has tapered thrust slip on a 4.5 X 4.5 km fault with strike 270°, dip 25°, and rake 90°, following Lin and Stein [1989]; receiver faults are assumed to have the same parameters. (b) Coulomb stress change in cross section cutting the center of the fault. The resulting stress component is shown in the top left-hand corner. (c) Normal stress change. Unclamping is positive. There were no earthquakes recorded during 1975–1987 at the minimum catalog magnitude of M ≥ 0.8 [Richards-Dinger and Shearer, 2000], and so the aftershock pattern is more likely a response to the stress changes imparted by the main shock than a continuation of the background seismicity.

  • This is another cross sectional view of changes in stress imparted by a thrust fault earthquake. The upper panel shows the results for thrust “receiver faults.” Receiver faults are the faults that potentially are triggered through this process.

  • Cross-sectional areas across the midpoint of a thrust fault, showing stresses imparted by a 30°-dipping blind thrust source fault on nearby (a, b) reverse and (c, d) strike-slip receiver faults. The pattern of stress change on strike-slip receiver faults differs markedly for long (Figure 5c) and short (Figure 5d) source faults. Strike-slip faulting is also enhanced above a blind thrust fault (Figure 5d). These cross sections can be compared with the map view for the same cases in Figure 4.

  • Here is the InSAR result from Eric Fielding at NASA, the files are available here.
  • These data are from a change in position between 2018.07.30 and 2018.08.05, so they compare the ground motion of only the M 6.9 earthquake (generally speaking).

  • From Dr. Fielding
  • Deformation of Lombok Island, Indonesia due to 5 August 2018 earthquake shows uplift of northwest corner due to fault slip at depth, measured with #InSAR of Copernicus Sentinel-1 radar images processed by Caltech-JPL ARIA project. Data at https://go.nasa.gov/2OlbxY6

    Black contours are 5 cm (2 inches). Copernicus Sentinel-1 data acquired on 30 July and 5 August 2018. White areas where measurement not possible, largely due to dense forests.

    Measurements with #InSAR are in direction towards satellite, so not purely vertical or horizontal. Mostly vertical in this case.

    My preliminary interpretation is that uplift is due to a north-dipping blind thrust fault that would project to the surface near the “zero” level of the interferogram, but a south-dipping thrust fault is also possible with down-dip end of rupture beneath the “zero” line

  • These two InSAR images allow us to compare ground deformation from these two earthquakes. Rusi P presents these results on twitter here. This tweet is also posted below in the Social Media section.
  • This is the analysis for the M 6.4 earthquake. This interferogram is made from SAR data collected on 7/18 and 7/30.

  • This is the analysis for the M 6.9 earthquake. This interferogram is made from SAR data collected on 7/30 and 8/05.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    References:

  • Audley-Charles, M.G., 1986. Rates of Neogene and Quaternary tectonic movements in the Southern Banda Arc based on micropalaeontology in: Journal of fhe Geological Society, London, Vol. 143, 1986, pp. 161-175.
  • Audley-Charles, M.G., 2011. Tectonic post-collision processes in Timor, Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 241–266.
  • Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region in Annu. Rev. Earth Planet. Sci., v. 41, p. 485-520.
  • Benz, H.M., Herman, Matthew, Tarr, A.C., Hayes, G.P., Furlong, K.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 New Guinea and vicinity: U.S. Geological Survey Open-File Report 2010–1083-H, scale 1:8,000,000.
  • Darman, H., 2012. Seismic Expression of Tectonic Features in the Lesser Sunda Islands, Indonesia in Berita Sedimentologi, Indonesian Journal of Sedimentary Geology, no. 25, po. 16-25.
  • Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
  • Hangesh, J. and Whitney, B., 2014. Quaternary Reactivation of Australia’s Western Passive Margin: Inception of a New Plate Boundary? in: 5th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 21-27 September 2014, Busan, Korea, 4 pp.
  • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
  • Jones, E.S., Hayes, G.P., Bernardino, Melissa, Dannemann, F.K., Furlong, K.P., Benz, H.M., and Villaseñor, Antonio, 2014. Seismicity of the Earth 1900–2012 Java and vicinity: U.S. Geological Survey Open-File Report 2010–1083-N, 1 sheet, scale 1:5,000,000, https://dx.doi.org/10.3133/ofr20101083N.
  • Koulali, A., S. Susilo, S. McClusky, I. Meilano, P. Cummins, P. Tregoning, G. Lister, J. Efendi, and M. A. Syafi’i, 2016. Crustal strain partitioning and the associated earthquake hazard in the eastern Sunda-Banda Arc in Geophys. Res. Lett., 43, 1943–1949, doi:10.1002/2016GL067941
  • Lin, J., and R. S. Stein (2004), Stress triggering in thrust and subduction earthquakes and stress interaction between the southern San Andreas and nearby thrust and strike-slip faults, J. Geophys. Res., 109, B02303, doi:10.1029/2003JB002607.
  • Lüschen, E., Müller, C., Kopp, H., Engels, M., Lutz, R., Planert, L., Shulgin, A., Djajadihardja, Y. S., 2011. Structure, evolution and tectonic activity of the eastern Sunda forearc,Indonesia from marine seismic investigations, Tectonophysics, 508, p. 6-21
  • McCaffrey, R., and Nabelek, J.L., 1984. The geometry of back arc thrusting along the Eastern Sunda Arc, Indonesia: Constraints from earthquake and gravity data in JGR, Atm., vol., 925, no. B1, p. 441-4620, DOI: 10.1029/JB089iB07p06171
  • Okal, E. A., & Reymond, D., 2003. The mechanism of great Banda Sea earthquake of 1 February 1938: applying the method of preliminary determination of focal mechanism to a historical event in EPSL, v. 216, p. 1-15.
  • Silver, E.A., Breen, N.A., and Prastyo, H., 1986. Multibeam Study of the Flores Backarc Thrust Belt, Indonesia, in JGR., vol. 91, no. B3, p. 3489-3500
  • Zahirovic, S., Seton, M., and Müller, R.D., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014

°

Earthquake Report: Fiji

WOW

We just had a Great Earthquake in the region of the Fiji Islands, in the central-western Pacific. Great Earthquakes are earthquakes with magnitudes M ≥ 8.0.

This earthquake is one of the largest earthquakes recorded historically in this region. I include the other Large and Great Earthquakes in the posters below for some comparisons.

Today’s earthquake has a Moment Magnitude of M = 8.2. The depth is over 550 km, so is very very deep. This region has an historic record of having deep earthquakes here. Here is the USGS website for this M 8.2 earthquake. While I was writing this, there was an M 6.8 deep earthquake to the northeast of the M 8.2. The M 6.8 is much shallower (about 420 km deep) and also a compressional earthquake, in contrast to the extensional M 8.2.

This M 8.2 earthquake occurred along the Tonga subduction zone, which is a convergent plate boundary where the Pacific plate on the east subducts to the west, beneath the Australia plate. This subduction zone forms the Tonga trench.

The subduction zone megathrust fault dips downwards to the west and the location of this “slab” has been evaluated by Hayes et al. (2012). These USGS geologists have updated the global slab model and I will incorporate these new data in upcoming reports. Today’s earthquake hypocenter (the 3-dimensional location of the earthquake) is at 563 km and the slab depth is about 520 km in this location (pretty good match given the range of depths for earthquakes relative to the fault location.

Due to the large depth, this earthquake did not shake very strongly at Earth’s surface. In addition, due to the large depth, a large tsunami is not expected. I checked the UNESCO IOC Sea Level Monitoring Facility, which posts a global set of tide gage data online. Here is their online map interface.

In 1994 there was a deep Great Earthquake (M 8.0) very close to today’s M 8.2 earthquake. One interesting thing is that the 2002 earthquake was compressional (a thrust or reverse fault earthquake) and today’s M 8.2 earthquake is extensional (a normal fault earthquake).

We are still unsure what causes an earthquake at such great a depth. The majority of earthquakes happen at shallower depths, caused largely by the frictional between differently moving plates or crustal blocks (where earth materials like the crust behave with brittle behavior and not elastic behavior). Some of these shallow earthquakes are also due to internal deformation within plates or crustal blocks.

As plates dive into the Earth at subduction zones, they undergo a variety of changes (temperature, pressure, stress). However, because people cannot directly observe what is happening at these depths, we must rely on inferences, laboratory analogs, and other indirect methods to estimate what is going on.

Below is a review of possible explanations as provided by Thorne Lay (UC Santa Cruz) in an interview in response to the 2013 M 8.3 Okhtosk Earthquake.

One option could be “fluid-assisted faulting,” in which water is released from minerals as they change phases during faulting, thus lubricating the plates, Lay says.

But although this is a common mechanism for earthquakes between 70 and 400 kilometers deep, it’s unlikely to be the cause of this quake because the plate is significantly dewatered by the time it reaches 400 kilometers deep. Minerals releasing carbon dioxide as they are compacted could provide an alternative fluid to lubricate the fault, he says, much like water does at shallower depths.

And another possibility is that a transition in mineral form from low-pressure polymorphs (the form in which a mineral is stable at the surface) to high-pressure polymorphs (a denser form of a mineral that is stable at greater depths), gives the fault a start. According to this model, the plate subducts too quickly for the mineral to slowly transition to its denser form. The mineral will reach depths greater than where it is normally stable, and thus the transformation may be a catastrophic process, causing a jolt at 600 kilometers, which would allow for movement along the fault, Lay says.

There have been a number of deep earthquakes globally in the past several years. These include the 2013 M 8.3 in the Sea of Okhtosk, the 2015 M 7.8 along the Izu-Bonin Arc, and several along the central Andes. I present some interpretive posters for these earthquakes below.

In early 2017 there was an M 6.9 earthquake in this region near Fiji. Here is the report for this earthquake.

There are many interesting earthquakes on this map and I will attempt to fill in this report with discussion and figures for some of these earthquakes. For example the 2009 Samoa earthquake, the 2009 Vanuatu doublet earthquakes, and the 1995 : 1998 earthquakes at the southern New Hebrides Trench.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 7.50 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • Note the magnetic anomalies (alternating bands of red and blue), parallel to the spreading ridges (the green lines with diverging orange arrows in the North Fiji Basin).

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the lower left corner is a portion of the map from Benz et al. (2011). This map shows earthquake epicenters (2-D locations) for seismicity from the past century or so. Depth is represented by color and earthquake magnitude is represented by the size of the circle symbols. Seismicity cross sections are located along the green (H-H’) and blue (I-I’) lines. I place a blue star in the general location of today’s M 8.2 earthquake on this map, the H-H’ cross section, as well as the other inset figures.
  • Cross sections showing earthquake hypocenters along the two profiles (H-H’ and I-I’) are presented above the Benz et al. (2011) map. These seismicity cross section locations are also shown on the main map.
  • The lower right corner includes a map from de Alteriis et al. (1993) that shows some details of the plate boundaries in this region. Note the subduction zones (New Hebrides Trench and Tonga Trench). Also not some strike-slip fault systems (e.g. the Hunter fracture zone and the North Fiji fracture zone). There is a good example of a strike-slip earthquake along the Hunter fracturezone from 1990.
  • In the upper right corner is a figure that shows the tectonic development of the region surrounding Fiji (Begg and Gray, 2002). These authors worked on the volcanic and tectonic history of the Fiji Plateau.
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a centuries seismicity plotted with M ≥ 7.5.

Other Report Pages

Some Relevant Discussion and Figures

  • This is the plate tectonic map from de Alteriis et al. (1993) that shows the major fault systems in the region.

  • Location map of North Fiji Basin ridge; box indicates full multibeam covered area of Figure 2. Heavy lines denote north-south, N15°, and N160° main segments of ridge axis; dashed lines are pseudofaults indicating double propagation. F. Z.— fracture zone.

  • Here is a figure from Schellart et al. (2002) that shows their model of tectonic development of the North Fiji Basin. Schellart et al. (2002) include a long list of references for the tectonics in this region here. Below I include the text from the original figure caption in blockquote.

  • Tectonic reconstruction of the New Hebrides – Tonga region (modified and interpreted from Auzende et al. [1988], Pelletier et al. [1993], Hathway [1993] and Schellart et al.(2002a)) at (a) ~ 13 Ma, (b) ~ 9 Ma, (c) 5 Ma and (d) Present. The Indo-Australian plate is fixed. DER = d’Entrcasteaux Ridge, HFZ = Hunter Fracture Zone, NHT = New Hebrides Trench, TT = Tonga Trench, WTP = West Torres Plateau. Arrows indicate direction of arc migration. During opening of the North Fiji Basin, the New Hebrides block has rotated some 40-50° clockwise [Musgrave and Firth 1999], while the Fiji Plateau has rotated some 70-115° anticlockwise [Malahoff et al. 1982]. During opening of the Lau Basin, the Tonga Ridge has rotated ~ 20° clockwise [Sager et al. 1994]. (Click for enlargement)

  • This is the plate tectonic history map from Begg and Gray (1993) that shows how they interpret the Fiji Plateau to have formed.

  • Tectonic setting (Figures 1a–1c) and tectonic reconstructions (Figures 1d and 1e) of the Outer Melanesian region (adapted from Hathway [1993]; reprinted with permission from the Geological Society of London).

  • (a) Map of the Fiji platform and north end of the Lau Ridge showing the major islands in the Fiji area, the major early Pliocene volcanoes of Viti Levu, the major seafloor fracture zones, and part of the spreading center of the Fiji Basin (adapted from Gill and Whelan [1989]). Shoshonitic volcanoes, including the Tavua Volcano (T), are shown by squares and calc-alkaline volcanoes by circles.
  • (b) Tectonic features of the northeastern segment of the plate boundary between the Australian and Pacific plates showing the Outer Melanesian Arc of the southwest Pacific, trenches and ridge systems, and oceanic plateaus (adapted from Kroenke [1984]). Fiji, as part of the Fiji Platform, consists of a series of islands at the north end of the Lau Ridge, with the North Fiji Basin formed as part of a spreading center.
  • (c) Present plate configuration.
  • (d) Reconstruction at 5.5 Ma.
  • (e) Reconstruction at 10 Ma. In Figures 1a–1e the Australian plate is fixed and the east-west convergence rate between plates was assumed to be 9–10 cm yr-1. Shading represents submarine depths <2000 m.
  • Abbreviations are as follows: VT, Vitiaz trench; VAT, Vanuatu trench; LR, Lau Ridge; LB, Lau Basin; TR, Tonga Ridge; FFZ, Fiji Fracture Zone; LL, Lomaiviti lineament; V-BL, Vatulele-Beqa lineament. Long dashes denote southern margin of the Melanesian Border Plateau (MBP). The open square (Figures 1b and 1c) denotes the location of the Tavua Volcano.
  • Okal (1997) conducted an analysis of seismological records from a deep earthquake that happened in the region of the 2017.01.03 M 6.3 earthquake. This earthquake occurred on 26 May 1932, long before modern seismometers made it to the scene. Okal estimated the magnitude to be similar in size to earthquakes in the mid M 7 range. Here is a figure from Okal (1997) that shows some focal mechanisms for the earthquakes from 1932. Compare the mainshock (the largest focal mechanism) with the moment tensor for the 2016.01.02 M 6.3 earthquake. Below I include the text from the original figure caption in blockquote.
  • 1932.05.26 M 7.6 (USGS)

  • Focal mechanism of the 1932 earthquake, as determined in this study. We also show CMT solutions in the immediate vicinity of the event, as available from Dziewonski et al. (1983, and subsequent quarterly updates) and Huang et al. (1997). Their spatial distribution is shown in map view. The background map at the upper right sets the study area (shaded) into the familiar bathymetry of the Fiji-Tonga-Kermadec region. The separation of isobaths is 1000 m.

  • Interestingly, deep focus earthquakes take up ~66% of the deep earthquakes globally. From Yu and Wen (2012), we can see some moment tensors for deep earthquakes in this region. The 1994.07.30 earthquake is just west of the 2017 M 6.3 earthquake and also has a similar moment tensor to the 2017 M 6.3 earthquake.

  • Regional map of deep-focus similar earthquake pairs and seismicity near the Tonga–Fiji subduction zone. Deep similar earthquake pairs (black stars) and their available Global Centroid Moment Tensor (CMT) (Dziewonski et al., 1981; Ekstrom et al., 2003) are labeled with event date and doublet/cluster ID where applicable. Source parameters of the doublets/clusters are listed in Tables 1, 2. Background deep seismicity is shown as gray dots. Black lines indicate the slab contours below 300 km depth (Gudmundsson and Sambridge, 1998), with an interval of 100 km. Regional map of the Tonga–Fiji–Kermadec subduction zone is shown in the inset, with gray dotted box indicating the region blow-up in the main figure. Black lines are the slab contours below 300 km depth and the Tonga–Kermadec trench (Bird, 2003). The color version of this figure is available only in the electronic edition.

  • Green (2007) presents a great review about what may control the mechanics of deep earthquakes. I present his abstract in its entirety because it is so well written. Below are a couple supporting figures. Read the paper for more insight.
  • Abstract: Deep earthquakes have been a paradox since their discovery in the 1920s. The combined increase of pressure and temperature with depth precludes brittle failure or frictional sliding beyond a few tens of kilometers, yet earthquakes occur continually in subduction zones to ≈700 km. The expected healing effects of pressure and temperature and growing amounts of seismic and experimental data suggest that earthquakes at depth probably represent self-organized failure analogous to, but different from, brittle failure. The only high-pressure shearing instabilities identified by experiment require generation in situ of a small fraction of very weak material differing significantly in density from the parent material. This “fluid” spontaneously forms mode I microcracks or microanticracks that self-organize via the elastic strain fields at their tips, leading to shear failure. Growing evidence suggests that the great majority of subduction zone earthquakes shallower than 400 km are initiated by breakdown of hydrous phases and that deeper ones probably initiate as a shearing instability associated with breakdown of metastable olivine to its higher-pressure polymorphs. In either case, fault propagation could be enhanced by shear heating, just as is sometimes the case with frictional sliding in the crust. Extensive seismological interrogation of the region of the Tonga subduction zone in the southwest Pacific Ocean provides evidence suggesting significant metastable olivine, with implication for its presence in other regions of deep seismicity. If metastable olivine is confirmed, either current thermal models of subducting slabs are too warm or published kinetics of olivine breakdown reactions are too fast.

  • Here is a profile into the Earth that shows depths for various chemical – mechanical process that are thought to control seismicity in various ways (Green, 2007).

  • Earthquake depth distribution. (a) Semilog plot of global earthquake frequency per 10-km-thick depth interval, showing a bimodal distribution. All earthquakes below 50 km are in subduction zones, the coldest parts of the mantle. The boundary between the mantle transition zone and lower mantle in subduction zones is at 700 km. No earthquake has ever been detected in the lower mantle. Modified from ref. 35. (b) Cartoon of subduction zone and earthquake distribution. Lithosphere (speckled) at right, with uppermost layers altered to antigorite (serpentine), is subducting beneath lithosphere at left. Earthquakes in olivine-dominated upper mantle are shown as red dots in serpentine and white diamonds. In the mantle transition zone, olivine is hypothesized to remain present despite being no longer thermodynamically stable and to slowly react away to spinel (wadsleyite or ringwoodite) during descent, occasionally generating earthquakes (black dots) by the process discussed in the text. Note volume reductions accompanying phase transformations at 410 and 660 km. Modified from ref. 36.

  • Here is an illustration showing a visualization of the slab associated with the Tonga subduction zone (Green, 2007).

  • Cartoon showing active Tonga subduction zone and fossil slab floating above it. Original figure is modified after ref. 26. Yellow and orange stars and circles were added in ref. 28.

  • The Goes et al. (2017) paper presents an excellent review of the various forces and earthquake types along subduction zones globally. This paper is open source and free to download. Below are some summary figures.
  • This shows the general relations between various forces exerted on a subducting slab.

  • Schematic diagram showing the main forces that affect how slabs interact with the transition zone. The slab sinks driven by its negative thermal buoyancy (white filled arrows). Sinking is resisted by viscous drag in the mantle (black arrows) and the frictional/viscous coupling between the subducting and upper plate (pink arrows). To be able to sink, the slab must bend at the trench. This bending is resisted by slab strength (curved green arrow). The amount the slab needs to bend depends on whether the trench is able to retreat, a process driven by the downward force of the slab and resisted (double green arrow) by upper-plate strength and mantle drag (black arrows) below the upper plate. At the transition from ringwoodite to the postspinel phases of bridgmanite and magnesiowüstite (rg – bm + mw), which marks the interface between the upper and lower mantle, the slab’s further sinking is hampered by increased viscous resistance (thick black arrows) as well as the deepening of the endothermic phase transition in the cold slab, which adds positive buoyancy (open white arrow) to the slab.

    By contrast, the shallowing of exothermic phase transition from olivine to wadsleyite (ol-wd) adds an additional driving force (downward open white arrow), unless it is kinetically delayed in the cold core of the slab (dashed green line), in which case it diminishes the driving force. Phase transitions in the crustal part of the slab (not shown) will additionally affect slab buoyancy. Buckling of the slab in response to the increased sinking resistance at the upper-lower mantle boundary is again resisted by slab strength.

  • Here is a plot showing their summary of observations for various subduction zones globally.

  • Summary of morphologies of transition-zone slabs as imaged by tomographic studies and their Benioff stress state. Arrows on the map indicate the approximate locations of the cross sections shown around the map, with their points in downdip direction. Blue shapes are schematic representations of slab morphologies (based on the extent of fast seismic anomalies that were tomographically resolvable from the references listed). Horizontal black lines indicate the base of the transition zone (~660 km depth). For flattened slabs, the approximate length of the flat section is given in white text inside the shapes. For penetrating slabs, the approximate depth to which the slabs are continuous is given in black text next to the slabs. Circles inside the slabs indicate whether the mechanisms of earthquakes at intermediate (100–350 km) and deep (350–700 km) are predominantly downdip extensional (black) or compressional (white). Stress states are from the compilations of Isacks and Molnar (1971), Alpert et al. (2010), Bailey et al. (2012), complemented by Gorbatov et al. (1997) for Kamchatka, Stein et al. (1982) for the Antilles, McCrory et al. (2012) for Cascadia, Papazachos et al. (2000) for the Hellenic zone, and Forsyth (1975) for Scotia. The subduction zones considered are (from left to right and top to bottom): RYU—Ryukyu, IZU—Izu, HON—Honshu, KUR—Kuriles, KAM—Kamchatka, ALE—Aleutians, ALA—Alaska, CAL—Calabria, HEL—Hellenic, IND—India, MAR—Marianas, CAS—Cascadia, FAR—Farallon, SUM—Sumatra, JAV—Java, COC—Cocos, ANT—Antilles, TON—Tonga, KER—Kermadec, CHI—Chile, PER—Peru, SCO—Scotia. Numbers next to the red subduction zone codes refer to the tomographic studies used to define the slab shapes

    I include some inset figures in this interpretive poster for the 2017.01.03 M 6.9 Fiji Earthquake.

  • In the lower left corner I include map that shows the historic seismicity for this region (Martin, 2014). The color shows well how the earthquakes that happen along the Tonga Trench get deeper along with the subducting slab. Shallow earthquakes are generally subduction zone earthquakes and deeper earthquakes are related (generally) to processes happening withing the downgoing slab. The 2017.01.02 M 6.3 earthquake is one of these deep earthquakes. I will briefly compare this M 6.3 earthquake with an earthquake from the region that occurred in 1932 (Okal, 1997).
  • In the center top I include a figure that shows a small scale map of the southwestern Pacific (a) and a large scale map of the North Fiji Basin (b) from Martin, 2013. The various spreading ridges are indicated as double lines. I present this figure below.
  • In the upper right corner I include a figure from Schellart et al. (2002) that shows a conceptual model for the development of the North Fiji Basin formed by extension in the plate as the Basin rotated clockwise towards the New Hebrides Trench. I present this below.
  • In the lower right corner I include a figure from Richards et al. (2011) that shows their model of how the subducting slabs have interacted through time. These authors think that there is a stalled out and torn slab at depth below the North Fiji Basin. The M 7.2 earthquake occurred near the cross section c-c’.


Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    References:

  • Auzende, J-M., Pelletier, B., Lafoy, Y., 1994. Twin active spreading ridges in the North Fiji Basin (southwest Pacific) in Geology, v. 22, p. 63-66.
  • Begg, G. and Gray, D.R., 2002. Arc dynamics and tectonic history of Fiji based on stress and kinematic analysis of dikes and faults of the Tavua Volcano, Viti Levu Island, Fiji in Tectonics, v. 21, no. 4, DOI: 10.1029/2000TC001259
  • Benz, H.M., Herman, Matthew, Tarr, A.C., Furlong, K.P., Hayes, G.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 eastern margin of the Australia plate: U.S. Geological Survey Open-File Report 2010–1083-I, scale 1:8,000,000.
  • de Alterris, G. et al., 1993. Propagating rifts in the North Fiji Basin southwest Pacific in Geology, v. 21, p. 583-586.
  • Goes, S., Agrusta, R., van Hunen, J., and Garel, F., 2017. Subduction-transition zone interaction: A review: Geosphere, v. 13, no. 3, p. 1–21, doi:10.1130/GES01476.1.
  • Green, H.W., 2007. Shearing instabilities accompanying high-pressure phase transformations and the mechanics of deep earthquakes in PNAS, v. 104, no. 22, DOI: https://doi.org/10.1073/pnas.0608045104
  • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
  • Martin, A.K., 2013. Double-saloon-door tectonics in the North Fiji Basin in EPSL, v. 374, p. 191-203.
  • Martin, A.K., 2014. Concave slab out board of the Tonga subduction zone caused by opposite toroidal flows under the North Fiji Basin in Tectonophysics, v. 622, p. 56-61.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • Okal, 1997. A reassessment of the deep Fiji earthquake of 26 May 1932 in Tectonophysics v., 275, p. 313-329.
  • Richards, S., Holm., R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region, Geology, v. 39, pp. 787-790.
  • Schellart, W., Lister, G. and Jessell, M. 2002. Analogue modelling of asymmetrical back-arc extension. In: (Ed.) Wouter Schellart, and Cees W. Passchier, Analogue modelling of large-scale tectonic processes, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, volume 7, paper 3, doi:10.3809/jvirtex.2002.00046
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