Category Archives: College of the Redwoods

Earthquake Report: Lombok, Indonesia

Well well.

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

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

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

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

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

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

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

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

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

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

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

Below is my interpretive poster for this earthquake


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

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

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

    Magnetic Anomalies

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

    I include some inset figures.

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

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

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

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

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

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

Other Report Pages

Some Relevant Discussion and Figures

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

  • Here are the seismcity cross sections.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Geologic Fundamentals

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

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

    Compressional:

    Extensional:

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

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

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

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

    References:

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

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Earthquake Report: Fiji

WOW

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Below is my interpretive poster for this earthquake


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

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

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

    Magnetic Anomalies

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

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

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

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

Other Report Pages

Some Relevant Discussion and Figures

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


Geologic Fundamentals

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

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

    Compressional:

    Extensional:

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

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

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

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

    References:

  • Auzende, J-M., Pelletier, B., Lafoy, Y., 1994. Twin active spreading ridges in the North Fiji Basin (southwest Pacific) in Geology, v. 22, p. 63-66.
  • Begg, G. and Gray, D.R., 2002. Arc dynamics and tectonic history of Fiji based on stress and kinematic analysis of dikes and faults of the Tavua Volcano, Viti Levu Island, Fiji in Tectonics, v. 21, no. 4, DOI: 10.1029/2000TC001259
  • Benz, H.M., Herman, Matthew, Tarr, A.C., Furlong, K.P., Hayes, G.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 eastern margin of the Australia plate: U.S. Geological Survey Open-File Report 2010–1083-I, scale 1:8,000,000.
  • de Alterris, G. et al., 1993. Propagating rifts in the North Fiji Basin southwest Pacific in Geology, v. 21, p. 583-586.
  • Goes, S., Agrusta, R., van Hunen, J., and Garel, F., 2017. Subduction-transition zone interaction: A review: Geosphere, v. 13, no. 3, p. 1–21, doi:10.1130/GES01476.1.
  • Green, H.W., 2007. Shearing instabilities accompanying high-pressure phase transformations and the mechanics of deep earthquakes in PNAS, v. 104, no. 22, DOI: https://doi.org/10.1073/pnas.0608045104
  • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
  • Martin, A.K., 2013. Double-saloon-door tectonics in the North Fiji Basin in EPSL, v. 374, p. 191-203.
  • Martin, A.K., 2014. Concave slab out board of the Tonga subduction zone caused by opposite toroidal flows under the North Fiji Basin in Tectonophysics, v. 622, p. 56-61.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • Okal, 1997. A reassessment of the deep Fiji earthquake of 26 May 1932 in Tectonophysics v., 275, p. 313-329.
  • Richards, S., Holm., R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region, Geology, v. 39, pp. 787-790.
  • Schellart, W., Lister, G. and Jessell, M. 2002. Analogue modelling of asymmetrical back-arc extension. In: (Ed.) Wouter Schellart, and Cees W. Passchier, Analogue modelling of large-scale tectonic processes, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, volume 7, paper 3, doi:10.3809/jvirtex.2002.00046
  • Yu, W. and Wen, L., 2012. Deep-Focus Repeating Earthquakes in the Tonga–Fiji Subduction Zone, BSSA, v. 102, no. 4, pp. 1829-1849

°

Earthquake Report: Andreanof Islands, Aleutians

Well, yesterday while I was installing the final window in a reconstruction project, there was an earthquake along the Aleutian Island Arc (a subduction zone) in the region of the Andreanof Islands. Here is the USGS website for the M 6.6 earthquake. This earthquake is close to the depth of the megathrust fault, but maybe not close enough. So, this may be on the subduction zone, but may also be on an upper plate fault (I interpret this due to the compressive earthquake fault mechanism). The earthquake has a hypocentral depth of 20 km and the slab model (see Hayes et al., 2013 below and in the poster) is at 40 km at this location. There is uncertainty in both the slab model and the hypocentral depth.

The Andreanof Islands is one of the most active parts of the Aleutian Arc. There have been many historic earthquakes here, some of which have been tsunamigenic (in fact, the email that notified me of this earthquake was from the ITIC Tsunami Bulletin Board).

Possibly the most significant earthquake was the 1957 Andreanof Islands M 8.6 Great (M ≥ 8.0) earthquake, though the 1986 M 8.0 Great earthquake is also quite significant. As was the 1996 M 7.9 and 2003 M 7.8 earthquakes. Lest we forget smaller earthquakes, like the 2007 M 7.2. So many earthquakes, so little time.

I include some earthquakes along this plate boundary system that are also interesting as they reveal how the plate boundary changes along strike, and how the margins of the plate boundary (e.g. the western and eastern termini) behave.

The M 6.6 earthquake is the result of north-northwest compression from the subduction of the Pacific plate underneath the North America plate to the north.

The majority of the Aleutian Islands are volcanic arc islands formed as a result of the subduction of the Pacific plate beneath the North America plate. As the oceanic crust subducts, the water in the rock tends is released into the overlying mantle, leading to magma formation. This magma is less dense and rises to form volcanoes that comprise this magmatic arc.

This and other earthquakes have occurred in the region of the subduction zone west of where the Adak fracture zone is aligned. Further to the east is the Amlia fracture zone. The Amlia fracture zone is a left lateral strike slip oriented fracture zone, which displaces crust of unequal age, beneath the megathrust. The difference in age results in a variety of factors that may contribute to differences in fault stress across the fracture zone (buoyancy, thermal properties, etc). For example, older crust is colder and denser, so it sinks lower into the mantle and exerts a different tectonic force upon the overriding plate.

To the west, there is another subduction zone along the Kuril and Kamchatka volcanic arcs. These subduction zones form deep sea trenches (the deepest parts of the ocean are in subduction zone trenches). Between these 2 subduction zones is another linear trough, but this does not denote the location of a subduction zone. The plate boundary between the Kamchatka and Aleutian trenches is the Bering Kresla shear zone (BKSZ). Below I present some earthquake reports that help explain the western terminus of the Aleutian subduction zone.

This earthquake sequence is unrelated to the earthquakes in northern Alaska earlier this week. Here is my report for that sequence.

There was also a sequence (that is still experiencing aftershocks) in the Gulf of Alaska. Here is my main report (there were updates) for this Gulf of Alaska earthquake.

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 ≥ 3.0 in one version.

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

Mechanisms for historic earthquakes that come from publications other than the USGS fault plane solutions include the 1957 M 8.7 (Brown et al., 2013), the 1965 M 8.7 (Stauyder, 1968), and the 1965 M 7.6 earthquakes (Abe, 1972).

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

    Magnetic Anomalies

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

    I include some inset figures.

  • In the upper center is a map from IRIS that shows seismicity plotted relative to depth using color. One may observe that the earthquakes get deeper to the north, relative to the subduction zone fault (labeled Aleutain Trench in the posters below). I place a yellow star in the general location of this earthquake sequence (same for other figures here).
  • In the center right is a companion figure from IRIS that shows a low angle oblique view of this Pacific – North America plate boundary. Note how the downgoing Pacific plate subducts beneath the North America plate as a megathrust fault.
  • In the lower left corner is a figure from Torsvik et al. (2017) which shows the age progression for the seamounts along the Emperor and Hawai’i seamount chains. This age progression is a key evidence for plate tectonic theory and a foundation for our knowledge of plate motion rates globally.
  • In the lower right corner is a figure from Sykes et al. (1980) that includes a map and a space-time diagram (shows spatial extent and timing for historic earthquakes along various fault systems.
  • In the upper right corner is a figure that shows the historic earthquake ruptures along the Aleutian Megathrust (Peter Haeussler, USGS).
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a centuries seismicity plotted for earthquakes M ≥ 6.6.

Other Report Pages

Some Background about the North America – Pacific plate boundary

  • Here is a map that shows historic earthquake slip regions as pink polygons (Peter Haeussler, USGS). Dr. Haeussler also plotted the magnetic anomalies (grey regions), the arc volcanoes (black diamonds), and the plate motion vectors (mm/yr, NAP vs PP).

  • Speaking of the 1964 earthquake, here is a map that shows the regions of coseismic uplift and subsidence observed following that earthquake. The 27 March, 1964 M 9.2 earthquake is the second largest earthquake ever recorded on modern seismometers. This figure can be compared to the cross section below.

  • Here is the Plafker (1972)cross-section graphic on its own.

  • 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).

  • This figure shows a summary of the measured horizontal and vertical displacements from the Good Friday Earthquake. I include a figure caption from here below as a blockquote.

  • Profile and section of coseismic deformation associated with the 1964 Alaska earthquake across the Aleutian arc (oriented NW-SE through Middleton and Montague Islands). Profile of horizontal and vertical components of coseismic slip (above) and inferred slip partitioning between the megathrust and intraplate faults (below). From Plafker (1965, 1967; 1972)

  • 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). (Please contact me for a higher resolution version of this image: quakejay at gmail.com)

  • 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.

Some Relevant Discussion and Figures

  • In june 2017, there was an M 6.8 earthquake that happened in a region where the Pacific-North America plate boundary transitions from a subduction zone to a shear zone. To the east of this region, the Pacific plate subducts beneath the North America plate to form the Alaska-Aleutian subduction zone. As a result of this subduction, a deep oceanic trench is formed. To the west of this earthquake, the plate boundary is in the form of a shear zone composed of several strike-slip faults. The main fault that is positioned in the trench is the Bering-Kresla shear zone (BKSZ), a right-lateral strike-slip fault. In the oceanic basin to the north of the BKSZ there are a series of parallel fracture zones, also right-lateral strike-slip faults. Below are my thoughts, some from my Earthquake Report here.
  • My initial thought is that the entire Aleutian trench was a subduction zone prior to about 47 million years ago (Wilson, 1963; Torsvik et al., 2017). Prior to 47 Ma, the relative plate motion in the region of the BKSZ would have been more orthogonal (possibly leading to subduction there). After 47 Ma, the relative plate motion in the region of the BKSZ has been parallel to the plate boundary, owing to the strike-slip motion here. However, Konstantinovskaia (2001) used paleomagnetic data for a plate motion reconstruction through the Cenozoic and they have concluded that there is a much more complicated tectonic history here (with strike-slip faults in the region prior to 47 Ma and other faults extending much farther east into the plate boundary). When considering this, I was reminded that the relative plate motion in the central Aleutian subduction zone is oblique. This results in strain partitioning where the oblique motion is partitioned into fault-normal fault movement (subduction) and fault-parallel fault movement (strike-slip, along forearc sliver faults). The magmatic arc in the central Aleutian subduction zone has a forearc sliver fault, but also appears to have blocks that rotate in response to this shear (Krutikov, 2008).
  • There have been several other M ~6 earthquakes to the west that are good examples of this strike-slip faulting in this area. On 2003.12.05 there was a M 6.7 earthquake along the Bering fracture zone (the first major strike-slip fault northeast of the BKSZ). On 2016.09.05 there was a M 6.3 earthquake also on the Bering fracture zone. Here is my earthquake report for the 2016 M 6.3 earthquake. The next major strike-slip fault, moving away from the BKSZ, is the right-lateral Alpha fracture zone. The M 6.8 earthquake may be related to this northwest striking fracture zone. However, aftershocks instead suggest that this M 6.8 earthquake is on a fault oriented in the northeast direction. There is no northeast striking strike-slip fault mapped in this area and the Shirshov Ridge is mapped as a thrust fault (albeit inactive). There is a left-lateral strike-slip fault that splays off the northern boundary of Bowers Ridge. If this fault strikes a little more counter-slockwise than is currently mapped at, the orientation would match the fault plane solution for this M 6.8 earthquake (and also satisfies the left-lateral motion for this orientation). The bathymetry used in Google Earth does not reveal the orientation of this fault, but the aftershocks sure align nicely with this hypothesis.
  • I include some inset figures in the poster
    • In the upper right corner is a figure that shows the historic earthquake ruptures along the Aleutian Megathrust (Peter Haeussler, USGS). I place a yellow star in the general location of this earthquake sequence (same for other figures here).
    • In the upper left corner is a figure from Gaedicke et al. (2000) which shows some of the major tectonic faults in this region.
    • In the lower right corner is a figure from Konstantnovskaia et al. (2001) that shows a very detailed view of all the faults in this complicated region.


  • Here is the interpretive poster from the 2016.09.05 M 6.3 #EarthquakeReport.

  • Here are several figures from Gaedicke et al. (2000) showing their tectonic reconstructions. I include their figure captions below in blockquote. The first map shows the general tectonic setting as in the poster above.

  • Map of the Aleutian–Bering region and location of the study area (rectangle). Lines with barbs indicate subduction zones: (1) Kamchatka Trench and (2) Aleutian Trench; lines with sense of displacement mark fracture zones (FZs): (3) Steller, (4) Pikezh and (5) Bering FZs. Single arrows show relative direction of convergence of the Pacific (P) and North American (NA) plates. Bathymetric contours are in meters.

  • This figure shows the complicated intersection of the BKSZ and the Kuril-Kamchatka Trench (a subduction zone).

  • The main tectonic features of the Kamchatka–Aleutian junction area modified from Seliverstov (1983), Seliverstov et al. (1988) and Baranov et al. (1991). The eastern side of the Central Kamchatka depression is bounded by normal faults. Contour interval is 1000 m. Lines A and B indicate the locations of profiles shown in Fig. 3; the rectangle marks the location of the area shown in Fig. 4.

  • This figure shows a medium scale view of the faults here, along with the major historic earthquakes. In this figure the BKSZ is labeled the Aleutian fracture zone (AFZ).

  • Rupture zones of the major earthquakes in the Kamchatka–Aleutian junction area [according to Vikulin (1997)]. Earthquakes with a magnitude of Mw>7 are shown.

  • Here is a great illustration that shows how forearc sliver faults form due to oblique convergence at a subduction zone (Lange et al., 2008). Strain is partitioned into fault normal faults (the subduction zone) and fault parallel faults (the forearc sliver faults, which are strike-slip). This figure is for southern Chile, but is applicable globally.

  • Proposed tectonic model for southern Chile. Partitioning of the oblique convergence vector between the Nazca plate and South American plate results in a dextral strike-slip fault zone in the magmatic arc and a northward moving forearc sliver. Modified after Lavenu and Cembrano (1999).

  • Here is a figure from Krutikov (2008) showing the block rotation and forearc sliver faults associated with the oblique subduction in the central Aleutian subduction zone. Note that there are blocks that are rotating to accommodate the oblique convergence. There are also margin parallel strike slip faults that bound these blocks. These faults are in the upper plate, but may impart localized strain to the lower plate, resulting in strike slip motion on the lower plate (my arm waving part of this). Note how the upper plate strike-slip faults have the same sense of motion as these deeper earthquakes.

  • Here are several figures from Konstantnovskaia et al. (2001) showing their tectonic reconstructions. I include their figure captions below in blockquote. The first figure is the one included in the poster above.

  • Geodynamic setting of Kamchatka in framework of the Northwest Pacific. Modified after Nokleberg et al. (1994) and Kharakhinov (1996)). Simplified cross-section line I-I’ is shown in Fig. 2. The inset shows location of Sredinny and Eastern Ranges. [More figure caption text in the publication].

  • Here are 4 panels that show the details of their reconstructions. Panels shown are for 65 Ma, 55 Ma, 37 Ma, and Present.


  • The Cenozoic evolution in the Northwest Pacific. Plate kinematics is shown in hotspot reference frame after (Engebretson et al., 1985). Keys distinguish zones of active volcanism (thick black lines), inactive volcanic belts (thick gray lines), deformed arc terranes (hatched pattern), subduction zones: active (black triangles), inactive *(empty triangles). In letters: sa = Sikhote-aline, bs = Bering shelf belts; SH = Shirshov Ridge; V = Vitus arch; KA = Kuril; RA = Ryukyu’ LA = Luzon; IBMA = Izu-Bonin-Mariana arcs; WPB = Western Philippine, BB = Bowers basins.

  • On 2017.05.08 there was an earthquake further to the east, with a magnitude M 6.2. Here is my interpretive poster for this earthquake, which includes fault plane solutions for several historic earthquakes in the region. These fault plane solutions reveal the complicated intersection of these two different types of faulting along this plate boundary. Here is my earthquake report for this earthquake sequence.

  • Here is the figure from Bassett and Watts (2015) for the Aleutians. They use gravity profile data to characterize subduction zones globally.

  • Aleutian subduction zone. Symbols as in Figure 3. (a) Residual free-air gravity anomaly and seismicity. The outer-arc high, trench-parallel fore-arc ridge and block-bounding faults are dashed in blue, black, and red, respectively. Annotations are AP = Amchitka Pass; BHR = Black-Hills Ridge; SS = Sunday Sumit Basin; PD = Pratt Depression. (b) Published asperities and slip-distributions/aftershock areas for large magnitude earthquakes. (c) Cross sections showing residual bathymetry (green), residual free-air gravity anomaly (black), and the geometry of the seismogenic zone [Hayes et al., 2012].

  • Here is the schematic figure from Bassett and Watts (2015).

  • Schematic diagram summarizing the key spatial associations interpreted between the morphology of the fore-arc and variations in the seismogenic behavior of subduction megathrusts.

  • Here is a beautiful illustration for the Aleutian Trench from Alpha (1973) as posted on the David Rumsey Collection online.

  • Here is the figure from Sykes et al. (1980) that shows the space time relations for historic earthquakes in relation to the map.

  • Above: Rupture zones of earthquakes of magnitude M > 7.4 from 1925-1971 as delineated by their aftershocks along plate boundary in Aleutians, southern Alaska and offshore British Columbia [after Sykes, 1971]. Contours in fathoms. Various symbols denote individual aftershock sequences as follows: crosses, 1949, 1957 and 1964; squares, 1938, 1958 and 1965; open triangles, 1946; solid triangles, 1948; solid circles, 1929, 1972. Larger symbols denote more precise locations. C = Chirikof Island. Below: Space-time diagram showing lengths of rupture zones, magnitudes [Richter, 1958; Kanamori, 1977 b; Kondorskay and Shebalin, 1977; Kanamori and Abe, 1979; Perez and Jacob, 1980] and locations of mainshocks for known events of M > 7.4 from 1784 to 1980. Dashes denote uncertainties in size of rupture zones. Magnitudes pertain to surface wave scale, M unless otherwise indicated. M is ultra-long period magnitude of Kanamori 1977 b; Mt is tsunami magnitude of Abe[ 1979]. Large shocks 1929 and 1965 that involve normal faulting in trench and were not located along plate interface are omitted. Absence of shocks before 1898 along several portions of plate boundary reflects lack of an historic record of earthquakes for those areas.

  • This is a map from Sykes et al. (1980) that shows the regions of slip inferred for these historic earthquakes.

  • Aftershock areas of earthquakes of magnitude M > 7.4 in the Aleutians, southern Alaska and offshore British Columbia from 1938 to 1979, after Sykess [1971] and McCann et al. [1979]. Heavy arrows denote motion of Pacific plate with respect to North American plate as calculated by Chase [1978]. Two thousand fathom contour is shown for Aleutian trench. Ms and Mw denote magnitude scales described by Kanamori [1977b].

Geologic Fundamentals

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

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

    Compressional:

    Extensional:

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

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

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

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

    Social Media

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Earthquake Anniversary: M 7.8 Gorkha (Nepal) Earthquake

An anniversary is for the Gorkha (Nepal) Earthquake from 1 year ago. I have several Earthquake Reports listed below. These include a variety of observations and comparisons with historic and prehistoric earthquakes that I compiled.

USGS slip models: http://earthjay.com/?p=2478

First Report: http://earthjay.com/?p=2357
Comparison with historic EQs http://earthjay.com/?p=2361
USGS Intensity Reports http://earthjay.com/?p=2387
Surface Displacement and Ground Motion Models http://earthjay.com/?p=2391
More historic comparisons http://earthjay.com/?p=2396
Coseismic Surface Deformation Model http://earthjay.com/?p=2410
Aftershock Report http://earthjay.com/?p=2437
Mainshock & Aftershock Update #1 http://earthjay.com/?p=2439
Mainshock & Aftershock Update #2 http://earthjay.com/?p=2450
Mainshock & Aftershock Update #3 (and interview with Ian Pierce and Steven Angster) http://earthjay.com/?p=2466

Here is a summary of the observations:

Mw 7.8 Earthquake Finite Fault Plane Solution from the USGS.

Mw 7.3 Earthquake Finite Fault Plane Solution.

Here is the map that I put together. I have placed the USGS epicenters with two color schemes. The size of the yellow dots represents earthquake magnitude. The degree of redness designates the time (earlier-April = pink & later-May = red). Note how there are some pink colored epicenters in the region of the M 7.3 earthquake. These pink colored earthquakes all occurred in April. The red ones are from May. These epicenters may not be plotted with the greatest certainty, though any uncertainty is possibly shared between them. So, there relative positions are possibly good.


Here is an updated regional map that incorporates Hough and Bilham (2008 ) and today’s seismicity. The historic and prehistoric earthquake slip patches are also shown. The three other data sets now include Bilham (2004), Bettinelli et al (2006), and Berryman et al. (2009). I provide information about how I compiled these data sets on this page.


Here is the updated DYFI map. Note how broadly this earthquake was felt.


Here are two visualizations of the seismic waves as they propagate through the Earth. These are records from the USArray Transportable Array. Your tax dollars at work, unless congress defunds these projects. This first video shows vertical motion as red and blue.

This second video shows horizontal motion with magnitude and direction.

Earthquake Report: Burma!

Here is my preliminary earthquake report. There was an earthquake with a magnitude of M 6.9 in Burma. Here is the USGS web page for this earthquake. Based upon the modeling, this appears to likely be a very damaging earthquake to people and their belongings.

Here is the poster. I will update this later today.


There was an earthquake with a magnitude M 6.7 in January. Here is my report for that earthquake.
Below is the earthquake poster for that report. The M 6.7 earthquake (here is the USGS web page for this earthquake) possibly occurred along the Churachandpur-Mao fault (Wang et al., 2014). Based upon our knowledge of the regional tectonics I interpret this earthquake to have a right-lateral oblique sense of motion.


Here is the Curray (2005) plate tectonic map.


Here is a map from Maurin and Rangin (2009) that shows the regional tectonics at a larger scale. They show how the Burma and Sunda plates are configured, along with the major plate boundary faults and tectonic features (ninetyeast ridge). The plate motion vectors for India vs Sunda (I/S) and India vs Burma (I/B) are shown in the middle of the map. Note the Sunda trench is a subduction zone, and the IBW is also a zone of convergence. There is still some debate about the sense of motion of the plate boundary between these two systems. This map shows it as strike slip, though there is evidence that this region slipped as a subduction zone (not strike-slip) during the 2004 Sumatra-Andaman subduction zone earthquake. I include their figure caption as a blockquote below.


Structural fabric of the Bay of Bengal with its present kinematic setting. Shaded background is the gravity map from Sandwell and Smith [1997]. Fractures and magnetic anomalies in black color are from Desa et al.[2006]. Dashed black lines are inferred oceanic fracture zones which directions are deduced from Desa et al. in the Bay of Bengal and from the gravity map east of the 90E Ridge. We have flagged particularly the 90E and the 85E ridges (thick black lines). Gray arrow shows the Indo-Burmese Wedge (indicated as a white and blue hatched area) growth direction discussed in this paper. For kinematics, black arrows show the motion of the India Plate with respect to the Burma Plate and to the Sunda Plate (I/B and I/S, respectively). The Eurasia, Burma, and Sunda plates are represented in green, blue, and red, respectively.

Wang et al. (2014) also have a very detailed map showing historic earthquakes along the major fault systems in this region. They also interpret the plate boundary into different sections, with different ratios of convergence:shear. I include their figure caption as a blockquote below.


Simplified neotectonic map of the Myanmar region. Black lines encompass the six neotectonic domains that we have defined. Green and Yellow dots show epicenters of the major twentieth century earthquakes (source: Engdahl and Villasenor [2002]). Green and yellow beach balls are focal mechanisms of significant modern earthquakes (source: GCMT database since 1976). Pink arrows show the relative plate motion between the Indian and Burma plates modified from several plate motion models [Kreemer et al., 2003a; Socquet et al., 2006; DeMets et al., 2010]. The major faults west of the eastern Himalayan syntax are adapted from Leloup et al. [1995] and Tapponnier et al. [2001]. Yellow triangle shows the uncertainty of Indian-Burma plate-motion direction.

Here is a map from Wang et al. (2014) that shows even more details about the faulting in the IBW. Today’s fault occurred nearby the CMf label. I include their figure caption as a blockquote below. Wang et al. (2014) found evidence for active faulting in the form of shutter ridges and an offset alluvial fan. Shutter ridges are mountain ridges that get offset during a strike-slip earthquake and look like window shutters. This geologic evidence is consistent with the moment tensor from today’s earthquake. There is a cross section (C-C’) that is plotted at about 22 degrees North (we can compare this with the Maurin and Rangin (2009) cross section if we like).


Figure 6. (a) Active faults and anticlines of the Dhaka domain superimposed on SRTM topography. Most of the active anticlines lie within 120 km of the deformation front. Red lines are structures that we interpret to be active. Black lines are structures that we consider to be inactive. CT = Comilla Tract. White boxes contain the dates and magnitudes of earthquakes mentioned in the text. CMf = Churachandpur-Mao fault; SM = St. Martin’s island antilcline; Da = Dakshin Nila anticline; M= Maheshkhali anticline; J = Jaldi anticline; P = Patiya anticline; Si = Sitakund anticline; SW= Sandwip anticline; L = Lalmai anticline; H = Habiganj anticline; R = Rashidpur anticline; F = Fenchunganj anticline; Ha = Hararganj anticline; Pa = Patharia anticline. (b) Profile from SRTM topography of Sandwip Island.

Here is the Wang et al. (2014) cross section. I include their figure caption as a blockquote below.


Schematic cross sections through two domains of the northern Sunda megathrust show the geometry of the megathrust and hanging wall structures. Symbols as in Figure 18. (a) The megathrust along the Dhaka domain dips very shallowly and has secondary active thrust faults within 120 km of the deformation front. See Figures 2 and 6 for profile location.

Science on Tap Humboldt 2016.04.06

I present material about the Cascadia subduction zone at the Humboldt Science on Tap held on 4/6/16 at Blondies in Arcata, CA. This page has some supporting material from this presentation, including the digital presentation file.

    This is the digital presentation

  • Here is the digital presentation (100 MB pptx)

    This is a video of the presentation

  • Here is the digital file of the embedded video below (350 MB mp4)
  • Here is the yt link for the embedded video below

    Here are some sources of information about the Cascadia subduction zone

  • For the 315th anniversary of the most recent full rupture CSZ earthquake I put together a summary of our state of knowledge about the CSZ and that 1700 A.D. Jan. 26 earthquake. 2015.01.26
  • The USGS (and others) put together an educational video about the CSZ. I post this video and other supporting information online here: 2015.10.08

Here is an educational video about Cascadia subduction zone earthquakes and tsunamis.

Here is a tsunami hindcast for the Jan 26, 1700 Cascadia subduction zone megathrust earthquake that may have ruptured all the way south to Humboldt Bay. This is the download link for the embedded video below (35 MB mp4).

Here is an animation showing the Holocene record of earthquakes along the Cascadia subduction zone (Goldfinger et al., 2012).

Here is an animation of a Cascadia subduction zone earthquake generated tsunami. This is the download link for the embedded video below (15 MB mp4).

Volcano Report: Pavlof

Pavlof Volcano (PV) is erupting. PV is located near Sand Point Alaska, along the eastern Aleutian Magmatic Arc. The Alaska Volcano Observatory placed the PV into alert level “warning” and aviation color code red. Below is the description of the current conditions in blockquote:

Pavlof Volcano began erupting abruptly this afternoon, sending an ash cloud to 20,000 ft ASL as reported by a pilot. As of 4:18 pm AKDT (00:18 UTC), ash was reportedly moving northward from the volcano. Seismicity began to increase from background levels at about 3:53 pm (23:53 UTC) with quick onset of continuous tremor, which remains at high levels. AVO is raising the Aviation Color Code to RED and the Volcano Alert Level to WARNING.

Here is the AVO page for Pavlof.

This is a map that shows the Volcanoes in this region (Schaefer et al., 2014). Here is a link to a larger sized, higher resolution version of the map (34 MB pdf).


This is a screen shot showing the alert status and the definition of “red.” This page is dynamic, so if you click on the above link to the AVO page, it will have different content. I include images from the Forecast parts of the page below.


Pavlof Volcano Description


From the AVO:

Pavlof Volcano is a snow- and ice-covered stratovolcano located on the southwestern end of the Alaska Peninsula about 953 km (592 mi) southwest of Anchorage. The volcano is about 7 km (4.4 mi) in diameter and has active vents on the north and east sides close to the summit. With over 40 historic eruptions, it is one of the most consistently active volcanoes in the Aleutian arc. Eruptive activity is generally characterized by sporadic Strombolian lava fountaining continuing for a several-month period. Ash plumes as high as 49,000 ft ASL have been generated by past eruptions of Pavlof, and during the 2013 eruption, ash plumes as high as 27,000 feet above sea level extending as much as 500 km (310 mi) beyond the volcano were generated. The nearest community, Cold Bay, is located 60 km (37 miles) to the southwest of Pavlof.

    Forecasts

  • Ashfall Forecast
    • This and all following ashfall graphics is the output of a mathematical model of volcanic ash transport and deposition on the ground (Ash3D, USGS).
    • This model shows expected ashfall accumulation (deposit thickness) for actual or hypothetical eruptions.
    • AVO produces this graphic when a volcano is restless by assuming a reasonable hypothetical eruption, in order to provide a pre-eruptive forecast of areas likely to be affected. During an eruption, AVO updates the forecast with actual observations (eruption start time and duration, plume height) as they become available.
    • Colored contour lines represent points of equal ash thickness on the ground. Small accumulations of ash may occur beyond the “Trace” contour. Actual deposit thickness may vary from the forecast as the modelled points are based on our best estimates. Thickness terms are explained here.
    • This graphic does not show ash cloud movement in the atmosphere; please refer to the other graphics for ash cloud forecasts. Click here to return to other models output.


  • Ash Cloud Height Forecast
    • This model shows expected movement of an ash cloud in the atmosphere for actual or hypothetical eruptions.
    • AVO produces this graphic when a volcano is restless by assuming a reasonable hypothetical eruption, in order to provide a pre-eruptive forecast of airspace likely to be affected. During an eruption, AVO updates the forecast with actual observations (eruption start time and duration, plume height) as they become available.
    • Colors represent the height of the top of the ash cloud, in feet above sea level, as it drifts downwind.
    • This graphic does not show ashfall deposition on the ground; go here for ashfall graphic. Note that it is possible for ash clouds to move overhead with little or no fallout on the ground.
    • For more information about ASH3D, see USGS Open-File Report 2013-1122.


  • Ash Cloud Load Forecast
    • This model shows expected load (amount) of ash in the atmosphere for actual or hypothetical eruptions.
    • AVO produces this graphic when a volcano is restless by assuming a reasonable hypothetical eruption, in order to provide a pre-eruptive forecast of airspace likely to be affected. During an eruption, AVO updates the forecast with actual observations (eruption start time and duration, plume height) as they become available.
    • Colors represent amounts of ash in the atmosphere, summed from the bottom to the top of the cloud. Warmer colors represent areas of greater ash; colder colors mean less ash.
      This graphic does not show ashfall deposition on the ground; go here for ashfall graphic. Note that it is possible for ash clouds to move overhead with little or no fallout on the ground.


  • Puff Cloud Height Forecast
    • This model shows expected movement of an ash cloud in the atmosphere for actual or hypothetical eruptions.
    • AVO produces this graphic when a volcano is restless by assuming a reasonable hypothetical eruption, in order to provide a pre-eruptive forecast of airspace likely to be affected. During an eruption, AVO will update the forecast with actual observations (eruption start time and duration, plume height) as they become available.
    • Colored dots represent the estimated height of the top of the ash cloud, in feet above sea level, as it drifts downwind. [Change the color bar legend to “Height of top of ash cloud”]
    • This graphic does not show ashfall deposition on the ground; go here for ashfall graphic. Note that it is possible for ash clouds to move overhead with little or no fallout on the ground.
    • For more information about Puff, see http://pafc.arh.noaa.gov/puff/index.html.


  • Trajectory Forecast
    • This trajectory graphic is the output of a mathematical model showing wind direction and speed at different altitudes above sea level (HYSPLIT, NOAA). It does not contain information about ash emissions from the volcano.
    • Colored lines show the direction an ash cloud emanating from a point source (the volcano) would travel at different altitudes in feet above ground level. A given eruption cloud may not reach all altitudes shown.
    • Symbols are spaced one hour apart and reflect the forecast speed of the ash cloud.
    • This model is updated every 6 hours.
      • UTC to AKDT conversion (Alaska Daylight Time):

      • 0000 UTC = 4:00 PM AKDT on the previous day as UTC
      • 0600 UTC = 10:00 PM AKDT on the previous day as UTC
      • 1200 UTC = 4:00 AM AKDT on the same day as UTC
      • 1800 UTC= 10:00 AM AKDT on the same day as UTC
    • For more information about HYSPLIT see: http://www.arl.noaa.gov/ready/traj_alaska.html.


    Earthquakes Also

  • Interesting that to the west there have been a few earthquakes recently. Here is a map that shows those regions, along with the volcano locations. Note Pavlof is along the eastern part of this map, approximately 900 km east of the Amlia fracture zone (which is just east of the largest cluster of earthquakes.


    Images from the 2014/11/12-16 eruption

  • In 2014/11/12 Pavlof Volcano began erupting. The report from the Global Volcanism Project for that eruption is here.
    • 2014/11/16 07:02 AM UTC – NASA EO-1 Advanced Land Imager image high temperature flowage deposit on the northwest flank of Pavlof Volcano. This shortwave infrared image is sensitive to very high temperatures. This flowage deposit likely contains both new lava and hot rock debris, but the distribution has not yet been determined. The deposit extends for about 3.3 miles (5.4 km) from the vent.

    • 2016/11/15 21:46 PM UTC – Satellite image from the USGS/NASA Landsat-8 satellite showing the eruption cloud at Pavlof Volcano on November 15 at 12:46 pm AKST (21:46 UTC). This is just a portion of the eruption cloud, which extended for more than 250 miles to the northwest at the time this image was collected. In this image, the distance from the erupting vent to the upper left corner of the image is 45 miles (70 km). The shadow of the eruption cloud on the underlying meteorological clouds can be seen in this image. Pilots reported the height of the cloud at 35,000 ft (10.7 km) above sea level.

    References:

  • Global Volcanism Program, 2015. Report on Pavlof (United States). In: wunderman, R (ed.), Bulletin of the Global Volcanism Network, 40:4. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.BGVN201504-312030.
  • Schaefer, J.R., Cameron, C.E., and Nye, C.J., 2014, Historically active volcanoes of Alaska, in Schaefer, J.R., Cameron, C.E., and Nye, C.J., Historically active volcanoes of Alaska: Alaska Division of Geological & Geophysical Surveys Miscellaneous Publication 133 v. 1.2, 1 sheet, scale 1:3,000,000. doi:10.14509/20181

Good Friday Earthquake: 1964/03/27 in Alaska

Today we commemorate the Good Friday Earthquake, which occurred on March 27, 1964. This is the second largest earthquake ever recorded on modern seismographic instruments. I summarize some of the information that we have about this earthquake, but this is far from a comprehensive display. The Good Friday Earthquake is one of the most studied earthquakes, along with the 20122/03/11 Tohoku-Oki earthquake. Much of what we learned about the 1964 earthquake was originally presented in a series of USGS Professional Papers here.

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., 2010). 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.

Earthquake Report: Bakersfield!

Today we had a M 4.9 earthquake northwest of Bakersfield in the middle of the southern San Joaquin Valley. At first, it looked like there were no faults here because I was looking at the USGS fault and fold database. However, upon looking at the California Geological Survey Jennings (1977) map, updated by Saucedo et al. (2010), I found that there are some pre-Quaternary faults mapped in this region. Here is the online fault map. Here is the USGS website for this earthquake, that was broadly felt across the entire region. Based upon the PAGER alert, there is actually a 24% chance of between 1 and 10 casualties. The felt area may include as many as 2.2 million people.

Here is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use the Modified Mercalli Intensity (MMI) scale. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. I plot the moment tensor and I interpret this earthquake to be a northwest striking right lateral strike-slip earthquake. This is based upon the mapped faults in the region and their sense of motion, which is synthetic to the San Andreas fault system to the west. I used this kml to make this map.

I include inset maps of the Saucedo et al. (2010) fault map, with larger scale maps for the southern San Joaquin Valley (upper center) and Bakersfield (lower left) areas. I plot the epicenter for today’s earthquake as a blue dot. In the Bakersfield scale map (lower left corner), one can see that there are several pre-Quaternary faults mapped in this region. Also, the other, more recently active faults are named and labeled with their most recent event age (e.g. the San Andreas to the west ruptured in 1857 and the White Wolf fault to the southeast ruptured in 1952).

There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.


Also as an inset, I include a map that shows earthquake epicenters for earthquakes of magnitude M ≥ 4.0 in this region. Here is the query that I used to create this map. There is a paucity of seismicity in this region.


Here are the two Saucedo et al. (2010) large scale maps. I made these using this fault interface.



Here is a map from Jascha Polet (Cal Poly Pomona Seismologist). Dr. Polet plots moment tensors and seismicity in this region, showing the lack of seismicity in this region. earthquakes are colored vs. depth (most earthquakes in this region are deep).


This earthquake was felt broadly, including my friends on the east side of the Sierra Nevada mountains.


Earthquake Report: Antarctic plate!

We just had an interesting mid-plate earthquake (not along a plate boundary). Hat Tip to Jascha Polet, who pointed out this is in the region of the 1998 M 8.1 earthquake, one of the largest strike-slip and mid-plate earthquakes ever recorded. I then learned that the seismicity in this region may be related to isostatic adjustments in the Antarctic plate! Here is the USGS website for this M 5.9 strike-slip earthquake.

Here is my interpretive map. I plot the USGS location as a yellow star. I also include some other figures as insets. I will discuss these below. I include a figure from Kreemer and Holt (2000) that shows focal mechanisms for earthquakes in the region plotted on a bathymetric map (seafloor topography). I also include a few maps from Das and Henry (2003). About a week ago, there was an earthquake along the Australia-Pacific plate boundary to the northeast of this earthquake (here is the Earthquake Report for that earthquake).

There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.


Here is the earthquake report interpretive poster for the recent earthquake to the northeast.


Here is the Kreemer and Holt (2000) figure 1, showing the focal mechanisms for earthquakes along the regional plate boundary faults, as well as the focal mechanisms from the earthquakes in the region of the 1998 M 8.1 earthquake. I include their figure caption below in blockquote.


Focal mechanisms are from the Harvard CMT catalog (1/77-6/99). The black focal mechanisms indicate the 1998 Antarctic plate event with (some of) its aftershocks. Bathymetry is from Smith and Sandwell [1994]. Transform locations are derived from satellite altimetry by Spitzak and DeMets [1996]. MRC is the Macquarie Ridge Complex and TJ is the Australia-Pacific- Antarctica triple junction.

Here is the first figure from Das and Henry (2003). They plot the epicenters and focal mechanisms for earthquakes from the 1998 swarm overlain upon the gravity anomaly map. I include their figure caption below in blockquote.


The 25 March 1998 Antarctic plate earthquake (with a seismic moment of 1.3  1021 N m). (a) Relocated aftershocks [Henry et al., 2000] for the period 25 March 1998 to 25 March 1999 are shown as diamonds, with the main shock epicenter shown by a star. Only those earthquakes which are located with the semimajor axis of the 90% confidence ellipse 20 km are shown. International Seismological Centre epicenters for the period 1 January 1964 to 31 July 1997 are shown as circles. Marine gravity anomalies from an updated version of Sandwell and Smith [1997], illuminated from the east, with contours every 20 mGal, are shown in the background in the epicentral region. Selected linear gravity features are identified by white lines and are labeled F1–F6. F1, F2, and their southward continuation to join F1a compose the George V fracture zone. F4–F6 compose the Tasman fracture zone. (b) An expanded view of the region of the aftershocks. The relocated aftershocks in the first 24 hours are shown as diamonds; the rest are shown as circles. The 90% confidence ellipses are plotted for the locations; earthquakes without confidence ellipses were not successfully relocated and are plotted at the National Earthquake Information Center (NEIC) locations. The yellow star shows the NEIC epicenter for the main shock, with the CMT mechanism of solution 5 from Henry et al. [2000]. Available Harvard CMT solutions for the aftershocks are plotted, linked with lines to their centroid locations and then to their relocated epicenters, and are identified by their dates (mmddyy). The location of the linear features identified on Figure 6a are shown by black arrows. (c) Final distribution of moment release for preferred solution 8 of Henry et al. [2000]. There are the same gravity anomalies, same linear features, and same epicenters as Figure 6b except that now only earthquakes which are located with the semimajor axis of the 90% confidence ellipse 20 km are shown. Two isochrons from Mu¨ller et al. [1997] are plotted as white lines. Superimposed graph shows the final moment density, with a peak density of 1.25  1019 N m km 1. Regions of the fault with 15% of this maximum value are excluded in this plot. The baseline of the graph is the physical location of the fault. The spatial and temporal grid sizes used in the inversion for the slip were 5 km 5 km and 3 s, respectively.

This is the continuation of the above figure. This shows their interpretation of the faults that slipped during this 1998 earthquake series. In their paper, Das and Henry (2003) discuss the relations between main shocks and aftershocks. At the time, the 1998 earthquake “was the largest crustal submarine intraplate earthquake ever recorded, the largest strike-slip earthquake
since 1977, and at the time the fifth largest of any type worldwide since 1977” (Das and Henry, 2003). This M 8.1 earthquake was interesting because it crossed the fracture zones that trend N-S in the area. This is especially interesting because this is also what happened during the 2012 Sumatra Outer Rise earthquakes. Toda and Stein (2000) model the coulomb stress changes associated with different slip models from the M 8.1 earthquake to estimate if the aftershocks were triggered by the main earthquake. I include their figure caption below in blockquote.


(d) Principal features of the main shock rupture process [from Henry et al., 2000]. Arrows show location and directivity for the first and second subevents. Arrows are labeled with start and end times of rupture segments. Focal mechanisms are shown for the initiation, the first subevent plotted at the centroid obtained by Henry et al. [2000], and the second subevent. (The second subevent is not well located, and the centroid location is not indicated.) The cross shows the centroid location of moment tensor of the total earthquake obtained by Henry et al. [2000], and the triangle shows the Harvard CMT centroid. The same aftershock epicenters as Figure 6c are shown. Linear gravity features are shown as shaded lines, and probable locations of tectonic features T1a and T3a associated with the gravity features F1a and F3a are shown as shaded dashed lines. (See Henry et al. [2000] for further details.)

Here is the Kreemer and Holt (2000) figure that shows their interpretation of the stress field. The first figure below shows their determination of the strain rates as modeled from tectonic stresses at the plate boundaries. Note the low strain rate in the area near the M 8.1 earthquake (plotted as a focal mechanism). The second figure below shows the averaged minimum horiztonal deviatoric stress field caused by by flexure in the crust following the last ice age. Based upon their analyses, they attribute the earthquake to possibly be the result of stresses in the Antarctic plate following the last deglaciation. I include their figure caption below in blockquote.


a) Grid in which a strain rate field is determined associated with the accommodation of relative plate motions [DeMets et al., 1994]. These motions are applied as boundary velocity conditions,
illustrated by the grey arrows. b) Principal axes of the strain rate field for the region where the Antarctic event occurred (indicated by CMT focal mechanism). Model strain rates in this
region are one order of magnitude lower than along the surrounding ridges and transforms.


Principal axes of the vertically averaged minimum horizontal deviatoric stress field caused by gravitational potential energy differences within the lithosphere. CMT focal mechanism of Antarctic plate earthquake is shown. a) ‘ice-age’ simulation. b) change in stress tensor field from ‘ice-age’ to present day determined by taking the tensorial difference between the two solutions.