Category Archives: plate tectonics

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.

Return to the Earthquake Reports page.

Earthquake Report: Phillipines

I put these together earlier this week for me classes and finally have a moment to write about these earthquakes. The Philippines region has been quite active lately, as it frequently is. I show below a series of earthquakes from the past ~30 days. These earthquakes occurred in 4 different regions and 3 different tectonic settings. These are probably unrelated to each other, but it is difficult to really know without further analyses. After I made these posters, there was an earthquake with a magnitude of M 5.8 on the island of Mindanao (I include the USGS link below), possibly associated with the Davao River fault (the closest fault mapped in this region).

Here are the USGS websites for these earthquakes.

I took a look at the seismicity from the past century. Here are Google Earth kml files from the USGS website for earthquakes from 1917-2017 with magnitudes M ≥ 7.0 and M ≥ 7.5.

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 the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 7.5.

  • 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. The
  • 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.

    I include some inset figures in the poster.

  • In the upper left corner is a figure from Hall, 2011. This shows the plate tectonic configuration in the equatorial Pacific. Note how the upper panel shows a west dipping slab on the east side of the Philippines. Note the contrast in the center panel (Halmahera), where the eastern fault is dipping to the east (westward vergent) and the western fault is dipping to the west (eastward vergent). This region near Halmahera forms the Molucca Strait, one of the most tectonically active areas in this region.
  • In the upper right corner is a map showing the regional faults from Noda (2013). The Philippine and
  • In the lower left corner I include a map showing the seismicity and tectonic plate boundary faults for this region (Smoczyk et al., 2013). Earthquakes are plotted with color representing depth and diameter representing magnitude (see legend).


  • I choose 4 earthquakes for which I plot the MMI intensity.
  • The easternmost two are earthquakes related to the subduction of the Philippine Sea plate.
  • The northwesternmost earthquake series (largest M = 5.9) is interesting and mysterious. There is a left-lateral forearc sliver fault (Philippine fault) that parallels the Philippine trench. Internal deformation in the upper plate is accommodated by a complicated series of other strike-slip faults. In the region near Manila, there is a north-south striking right-lateral Mirikina Valley fault system. The MV fault trends south from Manila into the region of the M 5.9 earthquake. This fault seems to terminate in a northeast-southwest trending zone of “extension and young volcanism” (Nelson et al., 2000). Further to the south is an east-west striking left-lateral Lubang fault. This may be an extension of the Sibuyan Sea fault (Noda, 2013), a splay of the Philippine fault. If we look at the moment tensor for the M 5.5 and 5.9 earthquakes, the nodal planes suggest either NE striking left-lateral or NW striking right-lateral motion. This does not fit the orientation nor sense of motion for any of the mapped faults in the region. It is possible that these earthquakes are related to the extensional rifting instead. The moment tensor for the M 5.1 earthquake here shows an extensional beach ball. The orientation is not completely correct based upon the Nelson et al. (2000) figure, but there are no faults shown on their map.
  • The southwestern earthquakes from March (both M 5.6) are also interesting. The epicenters show locations on north Sulawesi, part of Indonesia. There is a NW striking left-lateral strike-slip faults mapped to the west and east of these earthquakes. However, the moment tensor shows that this would be a right-lateral fault in that orientation. The NW striking reverse fault is also equally challenging to interpret. Needless to say, this is a tectonically complicated region.

Below is my interpretive poster for the M 5.9 earthquake.


  • This is the low-angle oblique view of the 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 fault map for the region on the Island of Luzon (where Manila is located) from Nelson et al. (2000). The panel on the right (B) shows the Marikina Valley fault system (MV) and the Lubang fault. The MV is a right-lateral strike slip fault and the Lubang fault is a left-lateral strike-slip fault.

  • Tectonic setting of the Marikina Valley fault system (MV) in central Luzon, the Philippines. Diagram A shows subduction zone trenches by barbed lines, other faults with high rates of Quaternary activity by heavy black lines. White dots show locations of recent earthquakes on the Philippine fault in Luzon (M 7.8; 1990) and the Aglubang River fault in Mindoro (M 7.1; 1994). Diagram B shows how the Marikina Valley pull-apart basin (MV) may have been formed through extension caused by clockwise rotation (dashed circle) and shearing of central Luzon, which is caught between two active left-lateral strike-slip faults—the Philippine fault (Nakata et al., 1977; Barrier et al., 1991; Ringenbach et al., 1993; Aurelio et al., 1993) and the Lubang fault. A zone of extension and young volcanism south of the fault system has also influenced the structural development of the valley (Fo¨rster et al., 1990; Defant et al., 1988).

  • Here is the map from Noda (2013) that shows various strike slip faults associated with subduction zones.

  • Modern examples of trench-linked strike-slip faults. (A) The Median Tectonic Line (MTL) active fault system in southwestern Japan, related to oblique subduction of the Philippine Sea Plate (PS) along the Nankai Trough (NT). (B) The Great Sumatra Fault system (GSF) along the Java–Sumatra Trench (JST). (C) Strike-slip faults in Alaska. Fault names: DF, Denali; BRF, Boarder Ranges; CSEF, Chugach St. Elias; FF, Fairweather; TF, Transition. (D) The Philippine Fault system (PF). Abbreviations: SSF, Sibuyan Sea Fault; MT, Manila Trench; PT, Philippine Trench; ELT, East Luzon Trough. Plate names: AM, Amur; OK, Okhotsk; PS, Philippine Sea; AU, Australian; SU, Sundaland; NA, North American; PA, Pacific; YMC, Yukutat microcontinent. Black and purple lines are subduction zones and trench-linked strike-slip faults, respectively. All maps were drawn using SRTM and GEBCO with plate boundary data [30]. Blue arrows indicate the direction and velocity of relative plate motion (mm yr-1) based on [31].

The USGS Maps and Cross-Sections

  • Here is the map from Smocyk et al., 2013, followed by the legend. The entire poster is here (92 MB pdf).


  • Below are two cross sections that show the subduction zone seismicity, followed by the legend. The location of these cross sections are labeled on the map above.



  • This map shows the seismic hazard for this region. The color represents the likelihood of any region experiencing ground shaking of a particular magnitude. The scale is “Peak Ground Acceleration.” Units are m/s^2. Purple represents gravitational acceleration of 1 g, gravity at Earth’s surface. Note how most of the earthquakes were in the region of higher likely ground shaking, except for the Sulawesi earthquakes.

  • In January of this year, there was an M 7.3 earthquake in the Celebes Sea south of the Philippines. Below is my interpretive map for that earthquake. I also present the same poster with 1917-2017 seismicity for earthquakes M ≥ 6.5. Here is my earthquake report for this M 7.3 earthquake. I include more background information for the Molucca Strait region on this page.


References:

  • Bock et al., 2003. Crustal motion in Indonesia from Global Positioning System measurements in JGR, v./ 108, no. B8, 2367, doi:10.1029/2001JB000324
  • Hall, R., 2011. Australia–SE Asia collision: plate tectonics and crustal flow in 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, 75–109.
  • 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
  • McCaffrey, R., Silver, E.A., and Raitt, R.W., 1980. Crustal Structure of the Molucca Sea Collision Zone, Indonesia in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands-Geophysical Monograph 23, p. 161-177.
  • Nelson, A.R., Personius, S.F., Rimando, R.E>, Punongbayan, R.S., Tungol, N, Mirabueno, H., and Rasdas, A., 2000. Multiple Large Earthquakes in the Past 1500 Years on a Fault in Metropolitan Manila, the Philippines in BSSA vol. 90, p. 73-85.
  • Noda, A., 2013. Strike-Slip Basin – Its Configuration and Sedimentary Facies in Mechanism of Sedimentary Basin Formation – Multidisciplinary Approach on Active Plate Margins http://www.intechopen.com/books/mechanism-of-sedimentarybasin-formation-multidisciplinary-approach-on-active-plate-margins http://dx.doi.org/10.5772/56593
  • Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013. Seismicity of the Earth 1900–2012 Philippine Sea plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, 1 sheet, scale 1:10,000,000.
  • Waltham et al., 2008. Basin formation by volcanic arc loading in GSA Special Papers 2008, v. 436, p. 11-26.
  • Zahirovic et al., 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: Botswana!

This is a very interesting M 6.5 earthquake, which was preceded by a probably unrelated M 5.2 earthquake.

    Here are the USGS web pages for these earthquakes


  • 2017.04.03 M 5.2
  • 2017.04.03 M 6.5

Below is my interpretive poster for this earthquake.

I plot the seismicity from the past century, with color representing depth and diameter representing magnitude (see legend). I include the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 2.5. The M 5.2 earthquake happened in a region that is seismically active and this preceded the M 6.5 earthquake. They are at a large distance and are unlikely related to each other.

I also include the generalized location of the East Africa Rift (EAR) in this region as yellow bands with white dashed lines. These are the Eastern Branch and Southwestern Branch of the EAR.

  • 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. This earthquake is a normal fault event that strikes to the northwest and dips to the northeast or southwest. Due to the paucity of seismic data and mapped faults here, it is difficult to tell which is the principal fault plane. Someone online suggested that this may not be an earthquake, but an alien invasion force. Just joking. However, it is possible that this might have been an underground explosion. But, at 25 km, this seems highly unlikely.
  • 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 poster.

  • In the lower right corner I include a geologic map for southern Africa and Botswana (Leseane et al., 2015). Note that the Southwestern Branch of the EAR extends into northwestern Botswana (the white fault lines in the red colored geologic unit). I place a red star in the general location of the M 6.5 earthquake. This earthquake happened in the Kaapvaal Craton, a region of low historical seismicity.
  • In the upper right corner I include a map that shows the seismic hazard for the EAR region of Africa (Hayes et al., 2014). Today’s M 6.5 earthquake happened in the lower left corner of the map. I place a red star in the general location of the M 6.5 earthquake. Note that this earthquake happened in a region of low seismic hazard.
  • In the upper left corner I include two maps and their associated plots. The map on the left is the shaking intensity map as modeled by the USGS, which uses the MMI scale. The plot below shows the results from the Ground Motion Prediction Equation (GMPE) used to generate the map. This plot shows how ground shaking (MMI) attenuates (diminishes as the seismic waves are absorbed by the earth) with distance from the earthquake. The map on the right is the “Did You Feel It?” map generated by the online responses from people who observed the ground shaking. This also uses the MMI scale. The plot below that shows how the reported observations match the GMPE relations for this earthquake. The GMPE relations are plotted as orange and green lines, which represent GMPE models developed for different geologic settings (e.g. hard rock like granite vs. soft rock like accreted terrane).
  • In the lower left corner I include the Rapid Assessment of an Earthquake’s Impact (PAGER) report. More on the PAGER program can be found here. An explanation of a PAGER report can be found here. PAGER reports are modeled estimates of damage. On the top left is a histogram showing estimated casualties and on the top right is an estimate of possible economic losses. There is a list of cities in the lower right corner which shows their populations and the MMI that they were likely exposed to.


  • This is the geologic map from the poster (Leseane et al., 2015). I include their caption below in blockquote.

  • Precambrian tectonic map of (a) southern Africa and (b) Botswana outlining the spatial extent of Archean cratons and Proterozoic orogenic belts. White lines represent the fault system of the Okavango Rift Zone. Modified after Singletary et al. [2003] and Begg et al. [2009].

  • Here is the USGS “Seismicity of the Earth” poster for this region (Hayes et al., 2014).

  • This is the latest geologic maps of Africa (Thieblemont, D., 2016). Click on the map for a 67 MB pdf version.

References:

  • Hayes, G.P., Jones, E.S., Stadler, T.J., Barnhart, W.D., McNamara, D.E., Benz, H.M., Furlong, K.P., and Villaseñor, Antonio, 2014, Seismicity of the Earth 1900–2013 East African Rift: U.S. Geological Survey Open-File Report 2010–1083-P, 1 sheet, scale 1:8,500,000 http://dx.doi.org/10.3133/of20101083P
  • Leseane, K., Atekwana, E.A., Mickus, K.L., Abdelsalam, M.G., Shemanq, E.M., and Atekwana, E.A., 2015. Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana in JGR: Solid Earth, v. 120, doi:10.1002/2014JB011029.
  • Thieblemont, D. (ed.), 2016. Geological Map of Africa et 1:10M scale, CGMW-BRGM 2016

Earthquake Report: Gulf California!

There was an earthquake yesterday in the Gulf of California nearby a series of earthquakes that happened in 2015 and earlier in 2013. The 2017 and 2013 earthquakes are happening along a fault that forms the Carmen Basin and the 2015 earthquakes are rupturing a fault that appears to be in the middle of the Farallon Basin. Here is my Earthquake Report for the 2013 earthquake (an early report, so it is rather basic). Here is my Earthquake Report for the 2015 earthquake sequence. This is an update to the initial 2015 report.

    Here are the USGS web pages for these earthquakes


    2013

  • 2013.10.19 M 6.4

  • 2015

  • 2015.09.13 M 6.6
  • 2015.09.13 M 5.3
  • 2015.09.13 M 4.9
  • 2015.09.13 M 5.2

  • 2017

  • 2017.03.29 M 5.7

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 the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 6.5.

  • 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 suspect that the fault that ruptured is eastward vergent (dipping to the west), so the west dipping nodal plane is probably the primary fault plane. However, this region of Kamchatka has numerous upper plate thrust and reverse faults (so the primary fault plane could be the other one, dipping to the east).
  • 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 poster.

  • In the upper left corner I include a map that shows the tectonic setting of this region, with the geological units colored relative to their age and type (marine or continental).This is from a paper that discusses the interaction between spreading ridges and subduction trenches (Fletcher et al., 2007).
  • In the upper right corner I include a larger saled version of this map with the same seismicity plotted, but without the MMI contours. One can see the shape of the seafloor and how this is formed due to plate tectonics. Note the spreading ridge in the lower right corner and the parallel ridges formed as the plates extend from the ridge. The magnitude scale is slightly different than the main map.
  • In the lower left corner I include a map from the 2015 earthquake series. I include this because I labeled the Basins formed by the enechelon steps in this plate boundary. In my 2015 report, I provide more maps that include the names of some of these fracture zones (the transform plate boundary strike-slip faults).


  • Here is a great diagram showing the major faults in the region. I include their figure caption below.

  • (A) Simplified map of the Gulf of California region and Baja California peninsula showing the present plate boundary and some major tectonic features related to the plate-tectonic history since 12 Ma. The Gulf extensional province in gray is bounded by the Main Gulf Escarpment (bold dashed lines), which runs through the Loreto area and is shown in Figure 3. The Salton trough in southern California is merely the northern part of the Gulf extensional province. (B) Map of part of the southern Gulf of California and Baja California peninsula showing bathymetry (in meters), the transform–spreading-ridge plate boundary, and the location of subsequent figures with maps. The bathymetry is after a map in Ness and Lyle (1991) and the transform–spreading-ridge plate boundary is from Lonsdale (1989). The lines with double arrows are the three proposed rift segments modified here after Axen (1995); MS—Mulege´ segment, LS—Loreto segment, TS—Timbabichi
    segment.

  • This map shows the magnetic anomalies and the geologic map for the land and the youngest oceanic crust.

  • (A) Tectonic map of the southern Baja California microplate (BCM) and Gulf of California extensional province (GEP). The Magdalena fan is deposited on oceanic crust of the Farallon-derived Magdalena microplate located west of Baja California. Deep Sea Drilling Project Site 471 is shown as black dot on the Magdalena fan. Abbreviations: BCT—Baja California trench, BM—Bahia Magdalena, LC—Los Cabos block, T—Trinidad block, LP—La Paz, PV—Puerto Vallarta, SMSLF—Santa Margarita–San Lazaro fault, TAF—Tosco-Abreojos fault, TS—Todos Santos, V—Vizcaino peninsula. Geology is simplifi ed from Muehlberger (1996). Interpretation of marine magnetic anomalies, with numbers denoting the chron of positively magnetized stripes, is from Severinghaus and Atwater (1989) and Lonsdale (1991).

  • This map shows a more broad view of the magnetic anomalies through time.

  • Map-view time slices showing the widely accepted model for the two-phase kinematic evolution of plate margin shearing around the Baja California microplate. (A) Configuration of active ridge segments (pink) west of Baja California just before they became largely abandoned ca. 12.3 Ma. (B) It is thought that plate motion from 12.3 to 6 Ma was kinematically partitioned into dextral strike slip (325 km) on faults west of Baja California and orthogonal rifting in the Gulf of California (90 km). This is known as the protogulf phase of rifting. (C) From 6 to 0 Ma faults west of Baja California are thought to have died and all plate motion was localized in the Gulf of California, which accommodated ~345 km of integrated transtensional shearing. Despite its wide acceptance, our data preclude this kinematic model. In all frames, the modern coastline is blue. Continental crust that accommodated post–12.3 Ma shearing is dark brown. Unfaulted microplates of continental crust are light tan. Farallon-derived microplates are light green. Middle Miocene trench-filling deposits like the Magdalena fan are colored dark green. Deep Sea Drilling Project Site 471 is the black dot on the southern Magdalena microplate. Yellow line (296 km) in the northern Gulf of California connects correlated terranes of Oskin and Stock (2003). Maps have Universal Transverse Mercator zone 12 projection with mainland Mexico fixed in present position.

  • Here is the Earthquake Report Poster from the 2015 sequence.

This is a nice simple figure, from the University of Sydney here, showing the terminology of strike slip faulting. It may help with the following figures.

Here is a fault block diagram showing how strike-slip step overs can create localized compression (positive flower) or extension (negative flower). More on strike-slip tectonics (and the source of this image) here.


Here is another great figure showing how sedimentary basins can be developed as a result of step overs in strike slip fault systems (source: Becky Dorsey, University of Oregon, Dept. of Geological Sciences).


I also put together an animation of seismicity from 1065 – 2015. First, here is a map that shows the spatial extent of this animation.


Here is the animation link (2 MB mp4 file) if you cannot view the embedded video below. Note how the animation begins in 1965, but has the recent seismicity plotted for reference.

    There have been two large magnitude earthquakes in this region over the past 50 years.

  • 2007.09.01 M 6.1
  • 2010.10.21 M 6.7

This is an animation from Tanya Atwater. Click on this link to take you to yt (if the embedded video below does not work).

Here is an animation from IRIS. This link takes you to yt (if you cannot view the embedded version below). Here is a link to download the 21 MB mp4 vile file.

This is a link to a tectonic summary map from the USGS. Click on the map below to download the 20 MB pdf file.


References:

  • Fletcher, J.M., Grove, M., Kimbrough, D., Lovera, O., and Gehrels, G.E., 2007. Ridge-trench interactions and the Neogene tectonic evolution of the Magdalena shelf and southern Gulf of California: Insights from detrital zircon U-Pb ages from the Magdalena fan and adjacent areas in GSA Bulletin, v. 119, no. 11/12, p. 1313-1336.
  • Umhoefer, P.J., Mayer, L., and Dorse, R.J., 2002. Evolution of the margin of the Gulf of California near Loreto, Baja California Peninsula, Mexico in GSA Bulletin, v. 114, no. 7, p. 849-868.

Earthquake Report: Kamchatka!

This earthquake happened last night as I was preparing course materials for this morning. Initially it was a magnitude 6.9, but later modified to be M 6.9.

This earthquake happened in an interesting region of the world where there is a junction between two plate boundaries, the Kamchatka subduction zone with the Aleutian subduction zone / Bering-Kresla Shear Zone. The Kamchatka Trench (KT) is formed by the subduction (a convergent plate boundary) beneath the Okhtosk plate (part of North America). The Aleutian Trench (AT) and Bering-Kresla Shear Zone (BKSZ) are formed by the oblique subduction of the Pacific plate beneath the Pacific plate. There is a deflection in the Kamchatka subduction zone north of the BKSZ, where the subduction trench is offset to the west. Some papers suggest the subduction zone to the north is a fossil (inactive) plate boundary fault system. There are also several strike-slip faults subparallel to the BKSZ to the north of the BKSZ. These are shown in two of the inset maps below.

    Here is the USGS website for this earthquake.

  • 2017.03.29 M 6.6

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 the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 7.0 (the search window is limited to the region west of the Amlia fracture zone).

  • I also include moment tensors for earthquakes associated with the Kamchatka-Kuril subduction zone. There are some interesting earthquakes plotted here:
    • The pair of earthquakes 2008.07.05 M 7.7 and 2013.05.24 M 8.3 are very deep earthquakes (the M 8.3 is one of the largest and deepest earthquake ever recorded by modern seismometers) and may be due to bending of the downgoing slab. Here is my report for the M 8.3 (it was an early report of mine).
    • The pair of earthquakes 2006.11.15 M 833 and 2007.01.13 M 8.1 are directly related to each other. Lay et al. (2009) discussed this earthquake sequence and how the 2006 subduction zone earthquake led to the 2007 outer-rise earthquake in the Pacific plate. Dr. Erica Emry studied this earthquake pair for her Ph.D. research. I also wrote a little about these earthquakes in my earthquake report here.
    • The largest earthquake plotted for this region is the 1952 M 9.0 earthquake (the large epicenter between teh 1993 and 1997 earthquakes. This earthquake is the 5th largest earthquake recorded by modern seismometers. However, there is no USGS moment tensor (I couldn’t find a focal mechanism either). This earthquake generated a tsunami that traveled across the Pacific.
  • 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 suspect that the fault that ruptured is eastward vergent (dipping to the west), so the west dipping nodal plane is probably the primary fault plane. However, this region of Kamchatka has numerous upper plate thrust and reverse faults (so the primary fault plane could be the other one, dipping to the east).
  • 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. The hypocentral depth of the M 5.5 plots this close to the location of the fault as mapped by Hayes et al. (2012).

    I include some inset figures in the poster.

  • In the upper left corner I include a map that shows the tectonic setting of this region, with the major plate boundary faults and volcanic arc designated by triangles (Bindeman et al., 2002). I placed an orange circle in the general location of the M 6.6 earthquake (sized relative to the magnitude range in the main map). Note the reverse fault mapped to the northeast of the epicenter. This fault dips to the southeast, supporting the east dipping solution for the M 6.6 moment tensor. I post this figure and their figure caption below.
  • To the right of this figure, I include a figure from Portnyagin and Manea (2008 ) that shows a low angle oblique view of the downgoing Pacific plate slab. I post this figure and their figure caption below.
  • In the lower left corner I include a map from the USGS Open File report (Rhea et al., 2010) that explains the historic seismicity for this region. I also plot the epicenter (orange dot).
  • In the upper right corner I include a map that shows more details about the faulting in the region. I place the epicenter for the M 6.6 as an orange circle. The location of the cross section I-I’ (plotted in the lower right corner) is designated by a dashed purple line.


  • Here is the tectonic map from Bindeman et al., 2002. The original figure caption is below in blockquote.

  • Tectonic setting of the Sredinny and Ganal Massifs in Kamchatka. Kamchatka/Aleutian junction is modified after Gaedicke et al. (2000). Onland geology is after Bogdanov and Khain (2000). 1, Active volcanoes (a) and Holocene monogenic vents (b). 2, Trench (a) and pull-apart basin in the Aleutian transform zone (b). 3, Thrust (a) and normal (b) faults. 4, Strike-slip faults. 5–6, Sredinny Massif. 5, Amphibolite-grade felsic paragneisses of the Kolpakovskaya series. 6, Allochthonous metasedimentary and metavolcanic rocks of the Malkinskaya series. 7, The Kvakhona arc. 8, Amphibolites and gabbro (solid circle) of the Ganal Massif. Lower inset shows the global position of Kamchatka. Upper inset shows main Cretaceous-Eocene tectonic units (Bogdanov and Khain 2000): Western Kamchatka (WK) composite unit including the Sredinny Massif, the Kvakhona arc, and the thick pile of Upper Cretaceous marine clastic rocks; Eastern Kamchatka (EK) arc, and Eastern Peninsulas terranes (EPT). Eastern Kamchatka is also known as the Olyutorka-Kamchatka arc (Nokleberg et al. 1998) or the Achaivayam-Valaginskaya arc (Konstantinovskaya 2000), while Eastern Peninsulas terranes are also called Kronotskaya arc (Levashova et al. 2000).

  • This map shows the configuration of the subducting slab. The original figure caption is below in blockquote.

  • Kamchatka subduction zone. A: Major geologic structures at the Kamchatka–Aleutian Arc junction. Thin dashed lines show isodepths to subducting Pacific plate (Gorbatov et al., 1997). Inset illustrates major volcanic zones in Kamchatka: EVB—Eastern Volcanic Belt; CKD—Central
    Kamchatka Depression (rift-like tectonic structure, which accommodates the northern end of EVB); SR—Sredinny Range. Distribution of Quaternary volcanic rocks in EVB and SR is shown in orange and green, respectively. Small dots are active vol canoes. Large circles denote CKD volcanoes: T—Tolbachik; K l — K l y u c h e v s k o y ; Z—Zarechny; Kh—Kharchinsky; Sh—Shiveluch; Shs—Shisheisky Complex; N—Nachikinsky. Location of profiles shown in Figures 2 and 3 is indicated. B: Three dimensional visualization of the Kamchatka subduction zone from the north. Surface relief is shown as semi-transparent layer. Labeled dashed lines and color (blue to red) gradation of subducting plate denote depths to the plate from the earth surface (in km). Bold arrow shows direction of Pacific Plate movement.

  • Here is the more detailed tectonic map from Konstantinovskaia et al. (2001).


  • This is the cross section associated with the above map.


  • Here is the Rhea et al. (2010) poster.

References:

  • Bindeman, I.N., Vinogradov, V.I., Valley, J.W., Wooden, J.L., and Natal’in, B.A., 2002. Archean Protolith and Accretion of Crust in Kamchatka: SHRIMP Dating of Zircons from Sredinny and Ganal Massifs in The Journal of Geology, v. 110, p. 271-289.
  • 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.
  • Konstantinovskaia, E.A., 2001. Arc-continent collision and subduction reversal in the Cenozoic evolution of the Northwest Pacific: and example from Kamchatka (NE Russia) in Tectonophysics, v. 333, p. 75-94.
  • Koulakov, I.Y., Dobretsov, N.L., Bushenkova, N.A., and Yakovlev, A.V., 2011. Slab shape in subduction zones beneath the Kurile–Kamchatka and Aleutian arcs based on regional tomography results in Russian Geology and Geophysics, v. 52, p. 650-667.
  • Krutikov, L., et al., 2008. Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179, doi:10.1029/179GM07
  • 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.
  • Portnyagin, M. and Manea, V.C., 2008. Mantle temperature control on composition of arc magmas along the Central Kamchatka Depression in Geology, v. 36, no. 7, p. 519-522.
  • Rhea, Susan, Tarr, A.C., Hayes, Gavin, Villaseñor, Antonio, 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, scale 1:5,000,000.

Good Friday Earthquake 27 March 1964

In March of 1964, plate tectonics was still a hotly debated topic at scientific meetings worldwide. Some people still do not accept this theory (some Russian geologists favor alternative hypotheses; Shevchenko et al., 2006). At the time, there was some debate about whether the M 9.2 earthquake (the 2nd largest earthquake recorded with modern seismometers) was from a strike-slip or from a revers/thrust earthquake. Plafker and his colleagues found the evidence to put that debate to rest (see USGS video below).

I have prepared a new map showing the 1964 earthquake in context to the plate boundary using the same methods I have been using for my other earthquake reports. I also found a focal mechanism for this M 9.2 earthquake and included this on the map (Stauder and Bollinger, 1966).

    Here is the USGS website for this earthquake.

  • 1964.03.27 M 9.2

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 a focal mechanism for the M 9.2 earthquake determined by Stauder and Bollinger (1966). I include the USGS epicenters for earthquakes with magnitudes M ≥ 7.0.

  • 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. The hypocentral depth of the M 5.5 plots this close to the location of the fault as mapped by Hayes et al. (2012).

    I include some inset figures in the poster.

  • In the upper left corner I include two maps from the USGS, both using the MMI scale of shaking intensity mentioned above. The map on the left is the USGS Shakemap. This is a map that shows an estimate of how strongly the ground would shake during this earthquake. This is based upon a numerical model using Ground Motion Prediction Equations (GMPE), which are empirical relations between fault types, earthquake magnitude, distance from the fault, and shaking intensity. The map on the right is based upon peoples’ direct observations. Below each map are plots that show how these models demonstrate that the MMI attenuates (diminishes) with distance. The lines are the empirical relations. The dots are the data points.
  • To the right of those maps and figures is a map produced by Dr. Peter Haeussler from the USGS Alaska Science Center (pheuslr at usgs.gov) that shows the historic earthquakes along the Aleutian-Alaska subduction zone.
  • In the lower right corner I include an inset map from the USGS Seismicity History poster for this region (Benz et al., 2011). There is one seismicity cross section with its locations plotted on the map. The USGS plot these hypocenters along this cross section E-E’ (in green).
  • In the upper right corner, I include a figure that shows the measurements of uplift and subsidence observed by Plafker and his colleagues following the earthquake (Plafker, 1969). This is shown in map view and as a cross section.


Below is an educational video from the USGS that presents material about subduction zones and the 1964 earthquake and tsunami in particular.
Youtube Source IRIS

mp4 file for downloading.

    Credits:

  • Animation & graphics by Jenda Johnson, geologist
  • Directed by Robert F. Butler, University of Portland
  • U.S. Geological Survey consultants: Robert C. Witter, Alaska Science Center Peter J. Haeussler, Alaska Science Center
  • Narrated by Roger Groom, Mount Tabor Middle School

This is a map from Haeussler et al. (2014). The region in red shows the area that subsided and the area in blue shows the region that uplifted during the earthquake. These regions were originally measured in the field by George Plafker and published in several documents, including this USGS Professional Paper (Plafker, 1969).


Here is a cross section showing the differences of vertical deformation between the coseismic (during the earthquake) and interseismic (between earthquakes).


This figure, from Atwater et al. (2005) shows the earthquake deformation cycle and includes the aspect that the uplift deformation of the seafloor can cause a tsunami.


Here is a figure recently published in the 5th International Conference of IGCP 588 by the Division of Geological and Geophysical Surveys, Dept. of Natural Resources, State of Alaska (State of Alaska, 2015). This is derived from a figure published originally by Plafker (1969). There is a cross section included that shows how the slip was distributed along upper plate faults (e.g. the Patton Bay and Middleton Island faults).


Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia, but the analogy works for Alaska also. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.


This is a photo that I took along the Seward HWY 1, that runs east of Anchorage along the Turnagain Arm. I attended the 2014 Seismological Society of America Meeting that was located in Anchorage to commemorate the anniversary of the Good Friday Earthquake. This is a ghost forest of trees that perished as a result of coseismic subsidence during the earthquake. Copyright Jason R. Patton (2014). This region subsided coseismically during the 1964 earthquake. Here are some photos from the paleoseismology field trip. (Please contact me for a higher resolution version of this image: quakejay at gmail.com)


Here is the USGS shakemap for this earthquake. The USGS used a fault model, delineated as black rectangles, to model ground shaking at the surface. The color scale refers to the Modified Mercalli Intensity scale, shown at the bottom.


http://earthjay.com/earthquakes/19640327_alaska/intensity.jpg

There is a great USGS Open File Report that summarizes the tectonics of Alaska and the Aleutian Islands (Benz et al., 2011). I include a section of their poster here. Below is the map legend.




Most recently, there was an earthquake along the Alaska Peninsula, a M 7.1 on 2016.01.24. Here is my earthquake report for this earthquake. Here is a map for the earthquakes of magnitude greater than or equal to M 7.0 between 1900 and today. This is the USGS query that I used to make this map. One may locate the USGS web pages for all the earthquakes on this map by following that link.


Here is an interesting map from Atwater et al., 2001. This figure shows how the estuarine setting in Portage, Alaska (along Turnagain Arm, southeast of Anchorage) had recovered its ground surface elevation in a short time following the earthquake. Within a decade, the region that had coseismically subsided was supporting a meadow with shrubs. By 1980, a spruce tree was growing here. This recovery was largely due to sedimentation, but an unreconciled amount of postseismic tectonic uplift contributed also. I include their figure caption as a blockquote.


(A and B) Tectonic setting of the 1964 Alaska earthquake. Subsidence from Plafker (1969). (C) Postearthquake deposits and their geologic setting in the early 1970s. (D–F) Area around Portage outlined in C, showing the landscape two years before the earthquake (D), two years after the earthquake (E), and nine years after the earthquake (F). In F, location of benchmark P 73 is from http://www.ngs.noaa.gov/cgi-bin/ds2.prl and the Seward (D-6) SE 7.5-minute quadrangle, provisional edition of 1984.

    Here is the tsunami forecast animation from the National Tsunami Warning Center. Below the animation, I include their caption as a blockquote. This includes information about the earthquake and the formation of the warning center.

  • Here is a link to the file for the embedded video below (22 MB 720 mp4)
  • Here is a link to the higher resolution file for the embedded video below (44 MB 1080 mp4)
  • At 5:36 pm on Friday, March 27, 1964 (28 March, 03:36Z UTC) the largest earthquake ever measured in North America, and the second-largest recorded anywhere, struck 40 miles west of Valdez, Alaska in Prince William Sound with a moment magnitude we now know to be 9.2. Almost an hour and a half later the Honolulu Magnetic and Seismic Observatory (later renamed the Pacific Tsunami Warning Center, or PTWC) was able to issue its first “tidal wave advisory” that noted that a tsunami was possible and that it could arrive in the Hawaiian Islands five hours later. Upon learning of a tsunami observation in Kodiak Island, Alaska, an hour and a half later the Honolulu Observatory issued a formal “tidal wave/seismic sea-wave warning” cautioning that damage was possible in Hawaii and throughout the Pacific Ocean but that it was not possible to predict the intensity of the tsunami. The earthquake did in fact generate a tsunami that killed 124 people (106 in Alaska, 13 in California, and 5 in Oregon) and caused about $2.3 billion (2016 dollars) in property loss all along the Pacific coast of North America from Alaska to southern California and in Hawaii. The greatest wave heights were in Alaska at over 67 m or 220 ft. and waves almost 10 m or 32 ft high struck British Columbia, Canada. In the “lower 48” waves as high as 4.5 m or 15 ft. struck Washington, as high as 3.7 m or 12 ft. struck Oregon, and as high as 4.8 m or over 15 ft. struck California. Waves of similar size struck Hawaii at nearly 5 m or over 16 ft. high. Waves over 1 m or 3 ft. high also struck Mexico, Chile, and even New Zealand.
  • As part of its response to this event the United States government created a second tsunami warning facility in 1967 at the Palmer Observatory, Alaska–now called the National Tsunami Warning Center (NTWC, http://ntwc.arh.noaa.gov/ )–to help mitigate future tsunami threats to Alaska, Canada, and the U.S. Mainland.
  • Today, more than 50 years since the Great Alaska Earthquake, PTWC and NTWC issue tsunami warnings in minutes, not hours, after a major earthquake occurs, and will also forecast how large any resulting tsunami will be as it is still crossing the ocean. PTWC can also create an animation of a historical tsunami with the same tool that it uses to determine tsunami hazards in real time for any tsunami today: the Real-Time Forecasting of Tsunamis (RIFT) forecast model. The RIFT model takes earthquake information as input and calculates how the waves move through the world’s oceans, predicting their speed, wavelength, and amplitude. This animation shows these values through the simulated motion of the waves and as they travel through the world’s oceans one can also see the distance between successive wave crests (wavelength) as well as their height (half-amplitude) indicated by their color. More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore. The color scheme is based on PTWC’s warning criteria, with blue-to-green representing no hazard (less than 30 cm or ~1 ft.), yellow-to-orange indicating low hazard with a stay-off-the-beach recommendation (30 to 100 cm or ~1 to 3 ft.), light red-to-bright red indicating significant hazard requiring evacuation (1 to 3 m or ~3 to 10 ft.), and dark red indicating a severe hazard possibly requiring a second-tier evacuation (greater than 3 m or ~10 ft.).
  • Toward the end of this simulated 24 hours of activity the wave animation will transition to the “energy map” of a mathematical surface representing the maximum rise in sea-level on the open ocean caused by the tsunami, a pattern that indicates that the kinetic energy of the tsunami was not distributed evenly across the oceans but instead forms a highly directional “beam” such that the tsunami was far more severe in the middle of the “beam” of energy than on its sides. This pattern also generally correlates to the coastal impacts; note how those coastlines directly in the “beam” are hit by larger waves than those to either side of it.

References:

Earthquake Report: Bougainville and Solomons

In the past couple of weeks, there were some earthquakes in the equatorial western Pacific.

    Here are the USGS websites for the two largest earthquakes in the two regions.

  • 2017.03.04 M 6.1
  • 2017.03.19 M 6.0

The M 6.1 was a well behaved subduction zone earthquake associated with subduction of the Solomon Sea plate beneath the Solomon Islands, an island arc formed between opposing subduction zones. The M 6.0 earthquake and related earthquakes are more interesting. Baldwin et al. (2012) and Holm et al. (2016) both consider the North Solomon Trench (NST) to be inactive subduction zone, while the South Solomon Trench (SST) is an active subduction zone fault. There were 9 earthquakes larger than magnitude 2.5. The depths ranged between 4.2 and 53 km. There does not appear to be any direction that favors a deepening trend (i.e. getting deeper in the direction of the downgoing subduction zone fault). It is possible that these depths are not well constrained (the M 6.0 has a 4.2 km depth). These NNT earthquakes are either related to the NST, related to crustal faults, or reactivation of NST fault structures. Regardless, these are some interesting and mysterious earthquakes.

Below is my interpretive poster for this earthquake.


I plot the seismicity from the past year, with color representing depth and diameter representing magnitude (see legend).

  • 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. The moment tensor shows northeast-southwest compression, perpendicular to the convergence at these plate boundaries.
  • 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.
  • I include some inset figures.

  • In the upper right corner is a map from Holm et al. (2016) that shows various key elements of the plate configuration in this region. Colors representing the age of the crust shows how the Woodlark Basin has an active spreading ridge system with ridges striking East-West. Large Igneous Provinces are shown here in dark gray. A relevant cross section is C-D, with the location highlighted in magenta on the map. The cross section shows how the North Solomon Trench is configured.
  • In the lower left corner is a generalized tectonic map of the region from Holm et al., 2016. This map shows the major plate boundary faults. Active subduction zones have shaded triangle fault symbols, while inactive subduction zones have un-shaded triangle fault line symbols.
  • In the upper left corner a figure from Oregon State University, which are based upon Hamilton (1979). “Tectonic microplates of the Melanesian region. Arrows show net plate motion relative to the Australian Plate.” This is from Johnson, 1976.
  • In the lower right corner is a figure from Baldwin et al. (2012). This figure shows a series of cross sections along this convergent plate boundary from the Solomon Islands in the east to Papua New Guinea in the west. Cross sections ‘A’ and ‘B’ are the most representative for the earthquakes today. I placed the general location of these cross sections on the main map as dashed orange lines. I present the map and this figure again below, with their original captions.


  • The M 6.1 is probably related to a series of earthquakes, including two M 7.9 megathrust events, further north along the San Crostobal Trench. Below are two earthquake interpretive posters for these earthquakes. This first poster is from my earthquake report on 2017.01.22.

  • In earlier earthquake reports, I discussed seismicity from 2000-2015 here. The seismicity on the west of this region appears aligned with north-south shortening along the New Britain trench, while seismicity on the east of this region appears aligned with more east-west shortening. Here is a map that I put together where I show these two tectonic domains with the seismicity from this time period (today’s earthquakes are not plotted on this map, but one may see where they might plot).


Background Figures

  • This figure shows an interpretation of the regional tectonics (Holm et al., 2016). I include the figure caption below as a blockquote.

  • Tectonic setting of Papua New Guinea and Solomon Islands. a) Regional plate boundaries and tectonic elements. Light grey shading illustrates bathymetry b 2000 m below sea level indicative of continental or arc crust, and oceanic plateaus; 1000 m depth contour is also shown. Adelbert Terrane (AT); Bismarck Sea fault (BSF); Bundi fault zone (BFZ); Feni Deep (FD); Finisterre Terrane (FT); Gazelle Peninsula (GP); Kia-Kaipito-Korigole fault zone (KKKF); Lagaip fault zone (LFZ); Mamberamo thrust belt (MTB); Manus Island (MI); New Britain (NB); New Ireland (NI); North Sepik arc (NSA); Ramu-Markham fault (RMF); Weitin Fault (WF);West Bismarck fault (WBF); Willaumez-Manus Rise (WMR).

  • This figure shows details of the regional tectonics (Holm et al., 2016). I include the figure caption below as a blockquote.

  • a) Present day tectonic features of the Papua New Guinea and Solomon Islands region as shown in plate reconstructions. Sea floor magnetic anomalies are shown for the Caroline plate (Gaina and Müller, 2007), Solomon Sea plate (Gaina and Müller, 2007) and Coral Sea (Weissel and Watts, 1979). Outline of the reconstructed Solomon Sea slab (SSP) and Vanuatu slab (VS)models are as indicated. b) Cross-sections related to the present day tectonic setting. Section locations are as indicated. Bismarck Sea fault (BSF); Feni Deep (FD); Louisiade Plateau
    (LP); Manus Basin (MB); New Britain trench (NBT); North Bismarck microplate (NBP); North Solomon trench (NST); Ontong Java Plateau (OJP); Ramu-Markham fault (RMF); San Cristobal trench (SCT); Solomon Sea plate (SSP); South Bismarck microplate (SBP); Trobriand trough (TT); projected Vanuatu slab (VS); West Bismarck fault (WBF); West Torres Plateau (WTP); Woodlark Basin (WB).

  • 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.


Background Videos

  • I put together an animation that shows the seismicity for this region from 1900-2016 for earthquakes with magnitude of M ≥ 6.5. Here is the USGS query I used to search for these data used in this animation. I show the location of the Benz et al. (2011) cross section E-E’ as a yellow line on the main map.
  • In this animation, I include some figures from the interpretive poster above. I also include a tectonic map based upon Hamilton (1979). Music is from copyright free music online and is entitled “Sub Strut.” Above the video I present a map showing all the earthquakes presented in the video.
  • Here is a link to the embedded video below (5 MB mp4)


References:

Earthquake Report: Alaska

We had an earthquake a few days ago along the Cook Strait west of Anchorage, Alaska. This earthquake happened nearby a couple earthquakes from the past 2 years that have similar senses of motion along faults that seem to be oriented the same. Here is my report for the 2016 M 7.1 earthquake.

    The USGS websites for the three large earthquakes with moment tensors plotted on the poster are here

  • 2015.07.29 M 6.3
  • 2016.01.24 M 7.1
  • 2017.03.02 M 5.5

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 also include seismicity from 2015-2017 for earthquakes with magnitudes M ≥ 4.0.

  • 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. The moment tensors for all three of these earthquakes are very similar.
  • 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. The hypocentral depth plots this close to the location of the fault as mapped by Hayes et al. (2012). All three earthquakes listed above plot clearly within the downgoing Pacific plate slab.

    I include some inset figures in the poster.

  • In the upper right corner is a map produced by Dr. Peter Haeussler from the USGS Alaska Science Center (pheuslr at usgs.gov) that shows the historic earthquakes along the Aleutian-Alaska subduction zone.
  • In the upper left corner I include an inset map from the USGS Seismicity History poster for this region (Benz et al., 2010). There is one seismicity cross section with its locations plotted on the map. The USGS plot these hypocenters along this cross section and I include that below (with the legend). I placed orange circles on the map and cross section showing the general location of the M 7.1 and M 5.5 earthquakes. The M 6.3 earthquake would plot almost in the same location as the M 7.1.
  • To the right of the Benz et al. (2010) figures is a map showing seismicity plotted as dots colored vs. depth. This map is from the Alaska Earthquake Center as presented by IRIS.
  • In the lower right corner are the MMI intensity maps for the two earthquakes listed above: 2015 M 6.3, 2016 M 7.1, and 2017 M 5.5. These figures were created by the USGS, but were made at different sizes, so they don’t match perfectly (don’t ask me why they keep changing the sizes of their figures and maps, I don’t know the answer). Below each map are plotted the reports from the Did You Feel It? USGS website for each earthquake. These reports are plotted as green dots with intensity on the vertical axes and distance on the horizontal axes. There are comparisons with Ground Motion Prediction Equation (attenuation relations) results (the orange model uses empirical data from central and eastern US earthquakes; the green model uses empirical data from earthquakes in California). Neither model seems appropriate given the DYFI results, though the California model works slightly better for the M 5.5 earthquake.


Here is the same poster with only seismicity from the past 30 days plotted.


  • Dr. Peter Haeussler produced a cross section for this region, as prepared for the 2016 M 7.1 earthquake. Below is his description of this figure.

  • I made up a quick diagram (thanks Alaska Earthquake Center tools) showing the tectonic setting of the earthquake. This was a “Benioff zone” event, which means that the earthquake is related to bending of the subducting Pacific Plate as it slides into the mantle.

  • Here is a map for the earthquakes of magnitude greater than or equal to M 7.0 between 1900 and 2016. This is the USGS query that I used to make this map. One may locate the USGS web pages for all the earthquakes on this map by following that link.

    • Here is a cross section showing the differences of vertical deformation between the coseismic (during the earthquake) and interseismic (between earthquakes).

    • Here is a figure recently published in the 5th International Conference of IGCP 588 by the Division of Geological and Geophysical Surveys, Dept. of Natural Resources, State of Alaska (State of Alaska, 2015). This is derived from a figure published originally by Plafker (1969). There is a cross section included that shows how the slip was distributed along upper plate faults (e.g. the Patton Bay and Middleton Island faults).

    References:

  • Earthquake Report: EOTD Chile M 8.8 2010.02.27

    Earthquake of the Day: 2010.02.27 M 8.8 Maule, Chile.

    There was an earthquake with a magnitude of M 8.8 on this day in 2010. I have prepared an interpretive poster that shows the extent of ground shaking modeled for this earthquake. The attenuation relations (how the ground shaking diminishes with distance from the rutpure) generally match the ground shaking reports on the USGS “Did You Feel It?” web page.

    I also include other material on the poster, including information about the 1960 M 9.5 Chile earthquake, which is the largest that we have ever recorded on modern seismologic instruments. Below are the USGS web pages for these two earthquakes.

    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 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. The moment tensor shows northeast-southwest compression, perpendicular to the convergence at this plate boundary. Most of the recent seismicity in this region is associated with convergence along the New Britain trench or the South Solomon trench.
    • 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. The hypocentral depth plots this close to the location of the fault as mapped by Hayes et al. (2012).

      I include some inset figures in the poster.

    • In the upper right corner, I include a time-space diagram from Moernaut et al. (2010).
    • In the upper left corner I include an inset map from the USGS Seismicity History poster for this region (Rhea et al., 2010). There is one seismicity cross section with its locations plotted on the map. The USGS plot these hypocenters along this cross section and I include that below (with the legend).
    • In the lower right corner are the MMI intensity maps for the two earthquakes listed above: 1960 M 9.5 & 2010 M 8.8. Note these are at different map scales.


    Here is a version that includes the MMI contours for the 1960 earthquake as well.


    • Here is a great figure from Lin et al. (2013) that shows the tectonic context of the 2010 Maule earthquake. On the map are plotted extents of historic earthquakes along this convergent plate margin. On the right is a large scale map showing the active magmatic arc volcanoes associated with this subduction zone. Finally, there is a cross section showing where the coseismic slip and postseismic slip occurred. I include the figure captions as blockquote.

    • (a) Regional tectonic map showing slab isodepth contours (blue lines) [Cahill and Isacks, 1992], M>=4 earthquakes from the National Earthquake Information Center catalog between 1976 and 2011 (yellow circles for depths less than 50 km, and blue circles for depths greater than 50 km), active volcanoes (red triangles), and the approximate extent of large megathrust earthquakes during the past hundred years (red ellipses) modified from Campos et al. [2002]. The large white vector represents the direction of Nazca Plate with respect to stable South America [Kendrick et al., 2003]. (b) Simplified seismo-tectonic map of the study area. Major Quaternary faults are modified after Melnick et al. [2009] (black lines). The Neogene Deformation Front is modified from Folguera et al. [2004]. The west-vergent thrust fault that bounds the west of the Andes between 32 and 38S is modified from Melnick et al. [2009]. (c) Schematic cross-section along line A–A0 (Figure 1b), modified from Folguera and Ramos [2009]. The upper bound of the coseismic slip coincides with the boundary between the frontal accretionary prism and the paleo-accretionary prism [Contreras-Reyes et al., 2010], whereas the contact between the coseismic and postseismic patch is from this study. The thick solid red line and dashed red line on top of the slab represent the approximate coseismic and postseismic plus interseismic slip section of the subduction interface. The thin red and grey lines within the overriding plate are active and inactive structures in the retroarc, modified from Folguera and Ramos [2009]. The red dashed line underneath the Andean Block represents the regional décollement. Background seismicity is from the TIPTEQ catalog, recorded between November 2004 and October 2005 [Rietbrock et al., 2005; Haberland et al., 2009].

    • Below are some figures from Moreno et al. (2011) that show estimates of locking along the plate interface in this region. I include the figure captions as blockquote.
    • The first figure shows how the region of today’s earthquake is in an area of higher locking.

    • a) Optimal distribution of locking rate in the plate interface. Predicted interseismic velocities and GPS vectors corrected by the postseismic signals are shown by green and blue arrows, respectively. b) Tradeoff curve for a broad range of the smoothing parameter (β). The optimal value for β is 0.0095 located at the inflection of the curve.

    • This second figure shows the moment released during historic earthquakes and the moment accumulated due to seismogenic locking along the megathrust.

    • a) Latitudinal distribution of the coseismic moment (Mc) released by the 1960 Valdivia (Moreno et al., 2009) (red line) and 2010 Maule (Tong et al., 2010) (blue line) earthquakes, and of accumulated deficit of moment (Md) due to interseismic locking of the plate interface 50 (orange line) and 300 (gray line) years after the 1960 earthquake, respectively. The range of errors of the Md rate is depicted by dashed lines. High rate of Md was found in the earthquake rupture boundary, where slip deficit accumulated since 1835 seems to be not completely released by the 2010 Maule earthquake. b) Schematic map showing the deformation processes that control the observed deformation in the southern Andes and the similarity between coseismic and locking patches. Blue and red contours denote the coseismic slip for the 2010 Maule (Tong et al., 2010) and 1960 Valdivia (Moreno et al., 2009) earthquakes, respectively. Patches with locking degree over 0.75 are shown by brown shaded areas. The 1960 earthquake (red star) nucleated in the segment boundary, area that appears to be highly locked at present. The 2011 Mw 7.1 aftershock (gray) may indicate that stress has been transmitted to the southern limit of the Arauco peninsula.

    • Here is a figure from Moreno et al. (2010) that shows the seismogenic locking for the region that includes the 2010 earthquake (shown with a focal mechanism from the M 8.8 earthquake. The figure caption is included below in blockquote.

    • Tectonic setting of the study area, data, observations and results. a, Shaded relief map of the Andean subduction zone in South- Central Chile. Earthquake segmentation along the margin is indicated by ellipses that enclose the approximate rupture areas of historic earthquakes (updated from refs 4–6). The inset shows the location of panel a (rectangle) relative to the South American continent. b, Compilation of GPS-observed surface velocities (1996–2008) with respect to stable South America before the 2010 Maule earthquake (for references see online-only Methods). Ellipses attached to the arrows represent 95% confidence limits. c, GPS 1 FEM modelled interface locking (fraction of plate convergence) distribution along the Andean subduction zone megathrust in the decade before the 2010 Maule earthquake. The epicentre (white star, USGS NEIC) and focal mechanism (beach ball, GCMT, http://www.globalcmt.org) of the 2010 Maule earthquake are shown in panels a and c.

    • This is also from Moreno et al. (2010) and shows the relations between different parts of the earthquake cycle. Recall these parts are the interseismic (between earthquakes), coseismic (during the earthquake), preseismic (before the earthquake), and postseismic (after the earthquake). The postseismic phase can last days to decades.

    • Relationship [sic] between pre, co- and postseismic deformation patterns. a, Coseismic slip distribution during the 2010 (blue contours; USGS slip model26) and 1960 (green contours; from ref. 30) earthquakes overlain onto pre-seismic locking pattern (red shading $0.75), as well as early (during the first 48 h post-shock) M$5 aftershock locations (the grey circle sizes scale with magnitude; GEOFON data29). b, Histograms of early (first 48 h; total number of events, 80) and late (first 3 months; total number of events, 168) aftershock density along a north–south profile (GEOFON data29, M$5). c, Residual slip deficits since 1835 as observed after the 2010 earthquake along a north–south profile (left column, based on the USGS slip model26). The middle and right columns show the effects on slip deficit of overlapping twentieth-century earthquakes (the black lines are polynomial fits to the data). Coloured data points and dates indicate earthquakes by year of occurrence.

    • This figure shows the results of analyses from Lin et al. (2013) where they estimate the spatial variation in postseismic slip associated with the 201 M 8.8 Maule earthquake. They used GPS observations along the upper plate to estimate how the fault continued to slip after the main earthquake.

    • Comparison of the postseismic slip model between the 1st and 488th day constrained by (a) horizontal GPS observations only, (b) all three components of GPS observations, and (c) three component GPS observations plus InSAR data. The coseismic slip model is of 2.5 m contour intervals (gray lines). (d) The same afterslip model as Figure 9c. Red dots are aftershocks [Rietbrock et al., 2012]. Black triangles represent the location of GPS stations. A is the afterslip downdip of the coseismic slip patch, with the black arrows indicating the along-strike extent. B and C correspond to two regions of afterslip that bound the southern and northern end of the coseismic slip patch. D is a deep slip patch that may reflect some tropospheric errors in the Andes.

    • Here is the space-time diagram from Moernaut et al., 2010. I include their figure caption below in blockquote.

    • Fig.: Setting and historical earthquakes in South-Central Chile. Data derived from Barrientos (2007); Campos et al. (2002); Melnick et al.(2009)

    • Here is the cross section of the subduction zone just to the south of the Sept/Nov 2015 swarm (Melnick et al., 2006). Below I include the text from the Melnick et al. (2006) figure caption as block text.

    • (A) Seismotectonic segments, rupture zones of historical subduction earthquakes, and main tectonic features of the south-central Andean convergent margin. Earthquakes were compiled from Lomnitz (1970, 2004), Kelleher (1972), Comte et al. (1986), Cifuentes (1989), Beck et al. (1998 ), and Campos et al. (2002). Nazca plate and trench are from Bangs and Cande (1997) and Tebbens and Cande (1997). Maximum extension of glaciers is from Rabassa and Clapperton (1990). F.Z.—fracture zone. (B) Regional morphotectonic units, Quaternary faults, and location of the study area. Trench and slope have been interpreted from multibeam bathymetry and seismic-reflection profiles (Reichert et al., 2002). (C) Profile of the offshore Chile margin at ~37°S, indicated by thick stippled line on the map and based on seismic-reflection profiles SO161-24 and ENAP-017. Integrated Seismological experiment in the Southern Andes (ISSA) local network seismicity (Bohm et al., 2002) is shown by dots; focal mechanism is from Bruhn (2003). Updip limit of seismogenic coupling zone from heat-fl ow measurements (Grevemeyer et al., 2003). Basal accretion of trench sediments from sandbox models (Lohrmann, 2002; Glodny et al., 2005). Convergence parameters from Somoza (1998 ).

    • In September through November of 2015, there was a M 8.3 earthquake further to the north. Below is my interpretive poster for that earthquake and here is my report, where I discuss the relations between the 2010, 2015, and other historic earthquakes in this region. Here is my report from September.

    • Here is a space time diagram from Beck et al. (1998 ). The 2015 earthquake occurs in the region of the 1943 and 1880 earthquakes. I updated this figure to show the latitudinal extent of the 2010 and 2015 earthquakes.

    References:

    Earthquake Report: 1700 Cascadia subduction zone 317 year commemoration

    Today (possibly tonight at about 9 PM) is the birthday of the last known Cascadia subduction zone (CSZ) earthquake. There is some evidence that there have been more recent CSZ earthquakes (e.g. late 19th century in southern OR / northern CA), but they were not near full margin ruptures (where the entire fault, or most of it, slipped during the earthquake).

    I have been posting material about the CSZ for the past couple of years here and below are some prior Anniversary posts, as well as Earthquake Reports sorted according to their region along the CSZ. Below I present some of the material included in those prior reports (to help bring it all together), but I have prepared a new map for today’s report as well.


    On this evening, 317 years ago, the Cascadia subduction zone fault ruptured as a margin wide earthquake. I here commemorate this birthday with some figures that are in two USGS open source professional papers. The Atwater et al. (2005) paper discusses how we came to the conclusion that this last full margin earthquake happened on January 26, 1700 at about 9 PM (there may have been other large magnitude earthquakes in Cascadia in the 19th century). The Goldfinger et al. (2012) paper discusses how we have concluded that the records from terrestrial paleoseismology are correlable and how we think that the margin may have ruptured in the past (rupture patch sizes and timing). The reference list is extensive and this is but a tiny snapshot of what we have learned about Cascadia subduction zone earthquakes. Brian Atwater and his colleagues have updated the Orphan Tsunami and produced a second edition available here for download and here for hard copy purchase (I have a hard copy).

    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).


    Today I prepared this new map showing the results of shakemap scenario model prepared by the USGS. I prepared this map using data that can be downloaded from the USGS website here. Shakemaps show what we think might happen during an earthquake, specifically showing how strongly the ground might shake. There are different measures of this, which include Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV), and Modified Mercalli Intensity (MMI). More background information about the shakemap program at the USGS can be found here. One thing that all of these measures share is that they show that there is a diminishing of ground shaking with distance from the earthquake. This means that the further from the earthquake, the less strongly the shaking will be felt. This can be seen on the maps below. The USGS prepares shakemaps for all earthquakes with sufficiently large magnitudes (i.e. we don’t need shakemaps for earthquakes of magnitude M = 1.5). An archive of these USGS shakemaps can be found here. All the scenario USGS shakemaps can be found here.

    I chose to use the MMI representation of ground shaking because it is most easily comparable for people to understand. This is because MMI scale is designed based upon relations between ground shaking intensity and observations that people are able to make (e.g. how strongly they felt the earthquake, how much objects in their residences or places of business responded, how much buildings were damaged, etc.).

    The MMI ground motion model 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. More on the MMI scale can be found here and here.


    Here is the USGS version of this map. The outline of the fault that was used to generate the ground motions that these maps are based upon is outlined in black.


    I prepared an end of the year summary for earthquakes along the CSZ. Below is my map from this Earthquake Report.

    • Here is the map where I show the epicenters as circles with colors designating the age. I also plot the USGS moment tensors for each earthquake, with arrows showing the sense of motion for each earthquake.
    • I placed a moment tensor / focal mechanism legend in the lower left corner of the map. 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.
    • In some cases, I am able to interpret the sense of motion for strike-slip earthquakes. In other cases, I do not know enough to be able to make this interpretation (so I plot both solutions).

      I include some inset figures in the poster.

    • In the upper left 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)
    • Below the CSZ map 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.
    • To the lower right of the Cascadia map and cross section is a map showing the latest version of the Uniform California Earthquake Rupture Forecast (UCERF). Let it be known that this is not really a forecast, and this name was poorly chosen. People cannot forecast earthquakes. However, it is still useful. The faults are colored vs. their likelihood of rupturing. More can be found about UCERF here. Note that the San Andreas fault, and her two sister faults (Maacama and Bartlett Springs), are orange-red.
    • To the upper right of the Cascadia map and cross section is a map showing the shaking intensities based upon the USGS Shakemap model. Earthquake Scenarios describe the expected ground motions and effects of specific hypothetical large earthquakes. The color scale is the same as found on many of my #EarthquakeReport interpretive posters, the Modified Mercalli Intensity Scale (MMI). The latest version of this map is here.
    • In the upper right corner I include generalized fault map of northern California from Wallace (1990).
    • To the left of the Wallace (1990) map is a figure that shows the evolution of the San Andreas fault system since 30 million years ago (Ma). This is a figure from the USGS here.
    • In the lower right corner I include the Earthquake Shaking Potential map from the state of California. This is a probabilistic seismic hazard map, basically a map that shows the likelihood that there will be shaking of a given amount over a period of time. More can be found from the California Geological Survey here. I place a yellow star in the approximate location of today’s earthquake.


    This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.


    Here is a version of the CSZ cross section alone (Plafker, 1972).


    Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.

    Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.


    Here is a photo of the ghost forest, created from coseismic subsidence during the Jan. 26, 1700 Cascadia subduction zone earthquake. Atwater et al., 2005.


    Here is a photo I took in Alaska, where there was a subduction zone earthquake in 1964. These tree snags were living trees prior to the earthquake and remain to remind us of the earthquake hazards along subduction zones.


    This shows how a tsunami deposit may be preserved in the sediment stratigraphy following a subduction zone earthquake, like in Cascadia. Atwater et al., 2005. If there is a source of sediment to be transported by a tsunami, it will come along for the ride and possibly be deposited upon the pre-earthquake ground surface. Following the earthquake, tidal sediment is deposited above the tsunami transported sediment. Sometimes plants that were growing prior to the earthquake get entombed within the tsunami deposit.


    The NOAA/NWS/Pacific Tsunami Warning Center has updated their animation of the simulation of the 1700 “Orphan Tsunami.”

    Source: Nathan C. Becker, Ph.D. nathan.becker at noaa.gov


    Below are some links and embedded videos.

    • Here is the yt link for the embedded video below.
    • Here is the mp4 link for the embedded video below. (2160p 145 mb mp4)
    • Here is the mp4 link for the embedded video below. (1080p 145 mb mp4)
    • Here is the text associated with this animation:

      Just before midnight on January 27, 1700 a tsunami struck the coasts of Japan without warning since no one in Japan felt the earthquake that must have caused it. Nearly 300 years later scientists and historians in Japan and the United States solved the mystery of what caused this “orphan tsunami” through careful analysis of historical records in Japan as well as oral histories of Native Americans, sediment deposits, and ghost forests of drowned trees in the Pacific Northwest of North America, a region also known as Cascadia. They learned that this geologically active region, the Cascadia Subduction Zone, not only hosts erupting volcanoes but also produces megathrust earthquakes capable of generating devastating, ocean-crossing tsunamis. By comparing the tree rings of dead trees with those still living they could tell when the last of these great earthquakes struck the region. The trees all died in the winter of 1699-1700 when the coasts of northern California, Oregon, and Washington suddenly dropped 1-2 m (3-6 ft.), flooding them with seawater. That much motion over such a large area requires a very large earthquake to explain it—perhaps as large as 9.2 magnitude, comparable to the Great Alaska Earthquake of 1964. Such an earthquake would have ruptured the earth along the entire length of the 1000 km (600 mi) -long fault of the Cascadia Subduction Zone and severe shaking could have lasted for 5 minutes or longer. Its tsunami would cross the Pacific Ocean and reach Japan in about 9 hours, so the earthquake must have occurred around 9 o’clock at night in Cascadia on January 26, 1700 (05:00 January 27 UTC).

      The Pacific Tsunami Warning Center (PTWC) can create an animation of a historical tsunami like this one using the same too that they use for determining tsunami hazard in real time for any tsunami today: the Real-Time Forecasting of Tsunamis (RIFT) forecast model. The RIFT model takes earthquake information as input and calculates how the waves move through the world’s oceans, predicting their speed, wavelength, and amplitude. This animation shows these values through the simulated motion of the waves and as they race around the globe one can also see the distance between successive wave crests (wavelength) as well as their height (half-amplitude) indicated by their color. More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore. The color scheme is based on PTWC’s warning criteria, with blue-to-green representing no hazard (less than 30 cm or ~1 ft.), yellow-to-orange indicating low hazard with a stay-off-the-beach recommendation (30 to 100 cm or ~1 to 3 ft.), light red-to-bright red indicating significant hazard requiring evacuation (1 to 3 m or ~3 to 10 ft.), and dark red indicating a severe hazard possibly requiring a second-tier evacuation (greater than 3 m or ~10 ft.).

      Toward the end of this simulated 24-hours of activity the wave animation will transition to the “energy map” of a mathematical surface representing the maximum rise in sea-level on the open ocean caused by the tsunami, a pattern that indicates that the kinetic energy of the tsunami was not distributed evenly across the oceans but instead forms a highly directional “beam” such that the tsunami was far more severe in the middle of the “beam” of energy than on its sides. This pattern also generally correlates to the coastal impacts; note how those coastlines directly in the “beam” have a much higher impact than those to either side of it.

      Offshore, Goldfinger and others (from the 1960’s into the 21st Century, see references in Goldfinger et al., 2012) collected cores in the deep sea. These cores contain submarine landslide deposits (called turbidites). These turbidites are thought to have been deposited as a result of strong ground shaking from large magnitude earthquakes. Goldfinger et al. (2012) compile their research in the USGS professional paper. This map shows where the cores are located.


      Here is an example of how these “seismoturbidites” have been correlated. The correlations are the basis for the interpretation that these submarine landslides were triggered by Cascadia subduction zone earthquakes. This correlation figure demonstrates how well these turbidites have been correlated. Goldfinger et al., 2012.


      This map shows the various possible prehistoric earthquake rupture regions (patches) for the past 10,000 years. Goldfinger et al., 2012. These rupture scenarios have been adopted by the USGS hazards team that determines the seismic hazards for the USA.


      Here is an update of this plot given new correlations from recent work (Goldfinger et al., 2016).


      Here is a plot showing the earthquakes in a linear timescale.


      I combined the plot above into another figure that includes all the recurrence intervals and segment lengths in a single figure. This is modified from Goldfinger et al. (2012).