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.

Earthquake Report: India!

Today we had a good sized earthquake in eastern India, within the India-Burmese wedge (IBW). The IBW is a part of the convergent plate boundary between the India plate to the west and the Burma (part of Eurasia) plate to the east. This plate boundary has been evolving since India came into the scene about 60 Ma (Curray, 2005). Prior to that, this boundary is thought to have been primarily convergent. Once India came into the region, prior to colliding with Asia (about?), this margin began to accommodate right lateral (dextral) shear. There is a major strike-slip fault, the Saging fault (SF), to the east of the IBW (Wang et al., 2014). The SF accommodates most of this shear, but some continues to be accommodated in the IBW (Maurin and Rangin, 2009).
Below is a map where I plot the epicenter for today’s M 6.7 earthquake, along with the moment tensor. I also include some inset maps. The lower left inset map is from Curray (2005) and shows the regional tectonics. The map on the right (Maurin and Rangin, 2009) shows the details of faulting in this region. In the upper right corner there is a cross section that is the east-west bold black line (at 22 degrees North) in the Maurin and Rangin (2009) map. This cross section is a little south of today’s earthquake, but is still relevant.
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.
Today’s 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 figure caption for the Maurin and Rangin (2009) map and cross section in the above map.

(a) General structural map of the Indo-Burmese ranges. The arrow shows the motion of the India Plate with respect to the Burma Plate [Socquet et al., 2006]. Figures 3, 5, 7, 8, and 13 are located with black boxes. The black dashed line is the trace of the buried incipient Chittagong Coastal Fault (C. C. Fault) discussed in this paper. The gray dashed line is the approximate position of the deformation front above the de´collement, in the western boundary of the outer wedge (see text for details). The gray area shows the position of the strong negative Bouguer gravity anomaly produced by the Sylhet Trough. (b) E-W synthetic cross section based on field observations and industrial multichannel seismic data discussed in this paper. The cross section is located as a thick black line in the map. Ages and thicknesses are based on unpublished well records and previously published sedimentological studies (see text for details). OIBW, outer Indo-Burmese Wedge; IIBW, inner Indo-Burmese Wedge.

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.

Here is a different cross section that shows how they interpret this plate boundary to have an oblique sense of motion (it is a subduction zone with some strike slip motion). Typically, these different senses of motion would be partitioned into different fault systems (read about forearc sliver faults, like the Sumatra fault. I mention this in my report about the earthquakes in the Andaman Sea from 2015.07.02). This cross section is further to the south than the one on the interpretation map above. I include their figure caption as a blockquote below.

Present cross section based on industrial multichannel seismics and field observations. The seismicity from USGS catalog and Engdahl [2002] is represented as black dots. Focal mechanisms from Global CMT ( catalog are also represented.

This figure shows the interpretation from Maurin and Rangin (2009) about how the margin has evolved over the past 10 Ma.

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.

Early reports show that four people have been killed. AP report.
Here is a cross section that shows seismicity for this region. The earthquakes are plotted as focal mechanisms. This comes from Jacha Polet, Professor of Geophysics at Cal Poly Pomona.

Here is a map showing the seismicity and focal mechanisms, also from Jacha Polet.

Earthquake Report: 2015 Summary M GT 7

Here I summarize the global seismicity for 2015. I limit this summary to earthquakes with magnitude greater than or equal to M 7.0. I reported on all but one of these earthquakes.

    I include summaries of my earthquake reports in sorted into three categories. One may also search for earthquakes that may not have made it into these summary pages (use the search tool).

  • Magnitude
  • Region
  • Year

Annual Summary Poster

Here is the map where I show the epicenters as white circles. I also plot the USGS moment tensors for each earthquake, with arrows showing the sense of motion for each earthquake.
I placed a moment tensor / focal mechanism legend in the lower left corner of the map. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
In some cases, I am able to interpret the sense of motion for strike-slip earthquakes. In other cases, I do not know enough to be able to make this interpretation (so I plot both solutions).

  • 2015.07.18 Santa Cruz Islands

  • 2015.07.27 New Guinea

  • 2015.07.27 New Guinea update #1 updated interpretation
  • 2015.07.27 New Guinea update #2 animations and seismic records

  • Compilation Map from here

  • 2015.09.16 Illapel, Chile

  • 2015.09.16 Illapel, Chile update #1 new maps and tsunami data
  • 2015.09.16 Illapel, Chile update #2 tsunami observations
  • 2015.09.16 Illapel, Chile update #3 more tsunami observations
  • 2015.09.16 Illapel, Chile update #4 historic tsunami comparisons
  • 2015.09.16 Illapel, Chile update #5
    • First Map

    • Second Map

    • Third Map (made in November 2015 following a M 6.8 earthquake). Here is the first report and the second report for that M 6.8 earthquake.

    • Regional Historic Earthquake Comparison Map #1

    • Large Scale Historic Earthquake Comparison Map

    • Regional Historic Earthquake Comparison Map #2 (from here)

    • Historic Tsunami Comparisons
      • Here are the NOAA Center for Tsunami Research websites for the three tsunamis plotted in the map below, plus the one from 2015.09.16 not shown on the map below.

      • 1960.05.22 M 9.5 (There is no page for the 1960 earthquake, so this map is located on the 2010 page.)/li>
      • 2010.02.27 M 8.8
      • 2014.04.01 M 8.2
      • 2015.09.16 M 8.3

      Here is the map. These three maps use the same color scale. There is not yet a map with this scale for the 2015 tsunami, so we cannot yet make the comparison.

      Here is an animation of these three tsunami from the US NWS Pacific Tsunami Warning Center (PTWC). This is the YouTube link.

  • 2015.10.20 Vanuatu

  • 2015.10.26 Afghanistan

  • 2015.10.26 Afghanistan update #1
  • Global Map

  • Local Map

  • 2015.11.18 Solomon Islands

  • 2015.11.24 Peru

  • 2015.11.24 Peru update #1

    • Updated Map

  • 2015.11.24 Argentina/Brazil
  • 2015.12.04 Southeast Indian Ridge

  • 2015.12.07 Tajikistan

    • References:

    • See Earthquake Reports for the references in the maps from those individual earthquakes.

    Earthquake Report: Southeast Indian Ridge!

    While at the HSU Geology Club Rock Auction last night, there was a large earthquake south of the SE Indian Ridge, between the SE India Ridge and the Kerguelen Plateau. Here is the USGS web page for this M 7.1 earthquake. The SE India Ridge is an oceanic spreading center where oceanic crust is formed between Australia and Antarctica.
    I placed a moment tensor / focal mechanism legend in the lower left corner of the map. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    Below is my interpretation map. I placed the USGS moment tensor near the epicenter plotted as an orange circle. I also label the Kerguelen Plateau (KP), Broken Ridge (BR), and the Ninetyeast Ridge. The Kerguelen Plateau and Broken Ridge are parts of a Large Igneous Province (LIP). LIPs are thought to form when a Hot Spot (HS) initially breaks through the crust. This initial HS volcanism leads to a very large volume of magma production and produce extensive flood basalts. Some of these are oceanic (e.g. Ontong Java Plateu) and some are continental (e.g. Columbia River Flood Basalts). I place a pair of dashed yellow arrows that show how these two parts (KP & BR) were earlier formed along the SE India Ridge. I also label the Ninetyeast Ridge, which is an overthickened region of the India-Australia plate. We think that the crust is overthickened here because as the crust formed, it spread across a hot spot, creating thick oceanic crust. There is more about this LIP and the Ninetyeast Ridge in Frey et al. (2002).
    Based on the presence of northeast striking fracture zones (strike slip faults associated with differential spreading along the spreading ridge), I interpret this earthquake as a left-lateral strike-slip earthquake. I could be wrong. I do not have access to any high resolution bathymetry of this region, if it might exist, to help me interpret the structure of this region. We can remember that these data may not help us by taking a look at the largest strike-slip earthquakes ever recorded. These happened in April 2012, offshore of Sumatra in a region of the India plate oceanic crust between the Ninetyeast Ridge and the 2004/2005 Sumatra-Andaman subduction zone earthquakes.

    Here is a map of the Indian Ocean that shows the age of the oceanic crust in color (Bernard et al., 2007).

    Here is a map from some GSA publication (but they do not list where it is from, so I cannot really provide a reference. here is the link). This map shows the global distribution of LIPs, colored vs. their classification (continental, silicic, and oceanic).

    Frey et al. (2002) is an IODP summary report for analyses of basalts collected along this LIP and the Ninetyeast Ridge. Below I place their figure caption in blockquote.

    Basalt provinces in the eastern Indian Ocean. Oceanic features attributed to the Kerguelen plume include the Kerguelen Plateau, Broken Ridge, Ninetyeast Ridge, Kerguelen archipelago, and Heard and McDonald Islands. Several continental basalt provinces have also been associated with the Kerguelen plume; these include Cretaceous lamprophyres in Antarctica and northeast India (solid diamonds), the Rajmahal Traps in northeast India, and Bunbury Basalt (BB) in Southwest Australia. Solid circles = locations of igneous basement sites drilled by DSDP and ODP, open circle = Eltanin piston core. Also indicated are dredge locations (solid squares = igneous rock recovery, open square = sediment dredge by Petuna Explorer). NKP, CKP and SKP = northern, central, and southern Kerguelen Plateau, respectively.

    Frey et al. (2007) plot the summary of the ODP drill holes to basalt formed in this region. Below I place their figure caption in blockquote.

    Summary of ODP drill holes on the Kerguelen Plateau and Broken Ridge that recovered volcanic rocks from the uppermost igneous basement (see Fig. F1, p. 36, for locations). Data for Leg 183 (Sites 1136, 1137, 1138, 1139, 1140, 1141, and 1142) are from Coffin, Frey, Wallace, et al. (2000), Coffin et al. (2002), and Duncan (2002). Data for other sites are from Barron, Larsen, et al. (1989), and Schlich, Wise, et al. (1989). Radiometric ages (40Ar/39Ar) for mafic volcanics are in bold.

    Frey et al. (2007) plot the volume of basalt formed in this region. Below I place their figure caption in blockquote.

    Estimated Kerguelen hotspot magma output since 130 Ma (Coffin et al., 2002). Analytical uncertainties for 40Ar/39Ar ages are variable, but 2-values are generally <5 m.y. Therefore, ages were assigned to various portions of the province (Fig. F1, p. 36) in 5-m.y. bins (diamonds) for the purpose of calculating the hotspot magma flux. The dashed line between 95 and 85 Ma indicates assumed Ninetyeast Ridge crust of that age buried beneath the Bengal Fan, produced at the same rate as Ninetyeast Ridge and Skiff Bank crust from 85 to 35 Ma.

    Their LIP volume vs. time plot is similar to that from the Columbia River Flood Basalt Group volume vs time. Below is a plot from the Martin et al. (2005). Below I place their figure caption in blockquote.

    Volume of Columbia River Basalt Group Eruptions Over Time. R1, N1, R2, and N2 are magnetostratigraphic units of the Grande Ronde Basalt.

    Frey et al. (2007) show their interpreted plate motion reconstruction. These maps show the configuration of the LIP through time. Below I place their figure caption in blockquote.

    Plate reconstructions of the southern Indian Ocean region of Coffin et al. (2002), using the hotspot reference frame of Müller et al. (1993). Red stars = possible reconstructed positions of the Kerguelen hotspot (after Müller et al., 1993); those labeled “K” assume that the Kerguelen archipelago is the current location of the Kerguelen hotspot, and those labeled “H” assume that Heard Island is the hotspot’s current location. Black shading = magmatism associated with the Kerguelen hotspot, diamonds = lamprophyres (labeled with “L” in 110 Ma panel) as they have appeared through geologic time (see Fig. F1, p. 36, for current locations of individual igneous complexes). Dashed line = a possible northern boundary for Greater India. IND = India, ANT = Antarctica, AUS = Australia. A, B. Seafloor spreading initiated at ~133 Ma between Western Australia and Greater India and at ~125 Ma between Australia and Antarctica. This model assumes breakup between India and Antarctica at ~133 Ma, although the timing of this event is not well known. The Bunbury Basalt (BB) of Southwest Australia erupted close to these breakup events in both time and space. Continental portions of Elan Bank (EB) and the Southern Kerguelen Plateau (unknown dimensions) remained attached to Greater India at these times. The Naturaliste Plateau (NP) also contains continental crust. C, D. Seafloor spreading continued between India, Antarctica, and Australia. The initial massive pulse of Kerguelen magmatism created the Southern Kerguelen Plateau (SKP), the Rajmahal Traps (RAJ), and Indian/Antarctic lamprophyres (L) from ~120 to ~110 Ma (Fig. F3, p. 38) and may be linked to breakup and separation between Elan Bank and Greater India. The Central Kerguelen Plateau (CKP) formed between ~105 and ~100 Ma and Broken Ridge (BR) between ~100 and ~95 Ma (Fig. F3, p. 38). Igneous basement of the Wallaby Plateau (WP) is not well characterized geochemically and has not been dated, but its age is inferred to lie between ~120 and ~100 Ma (Colwell et al., 1994).
    E, F. The hotspot generated the Ninetyeast Ridge (NER) and Skiff Bank (SB) as India continued its northward drift relative to Antarctica. G, H. At ~40 Ma, seafloor spreading commenced between the Central Kerguelen Plateau and Broken Ridge. The hotspot generated the Northern Kerguelen Plateau (NKP), and, since 40 Ma, as Broken Ridge and the Kerguelen Plateau have continued to separate, has produced the Kerguelen archipelago, Heard and McDonald Islands (Fig. F1, p. 36), and the chain of volcanoes between Kerguelen and Heard (Weis et al., 2002).


    • Bernard, F., et al., 2009. The Kerguelen plateau: Records from a long-living/composite microcontinent, , Marine and Petroleum Geology, doi:10.1016/j.marpetgeo.2009.08.011
    • Coffin, M.F., Pringle, M.S., Duncan, R.A., Gladczenko, T.P., Storey, R.D., Müller, R.D., and Gahagan, L.A., 2002. Kerguelen hotspot magma output since 130 Ma. J. Petrol., 43:1121–1140.
    • Duncan et al., 2002. A time frame for construction of the Kerguelen Plateau and Broken Ridge. J. Petrol., 43:1109–1120.
    • Frey et al., 2002. Kerguelen Plateau.
    • Larson, R.L., 1991. The latest pulse of Earth: evidence for a mid-Cretaceous superplume. Geology, 19:547–550.
    • Martin, B.S., et al., 2005. Goldschmidt Conference 2005: Field Trip Guide to the Columbia River Basalt Group.
    • Müller, R.D., Royer, J.Y., and Lawver, L.A., 1993. Revised plate motions relative to the hotspots from combined Atlantic and Indian-Ocean hotspot tracks. Geology, 21:275–278.
    • Schlich, R., Wise, S.W., Jr., et al., 1989. Proc. ODP, Init. Repts., 120: College Station, TX (Ocean Drilling Program).
    • Wallace, P., Cervantes, P., Weis, D., Ingle, S., Frey, F.A., Kieffer, B., and Moore, C.L., 2000. Explosive felsic volcanism on the Kerguelen Plateau (ODP Leg 183): geochemical characteristics and pre-eruptive volatile contents. EOS, Trans. Am. Geophys. Union, 81:432.
    • Weis, D., Frey, F.A., Schlich, R., Schaming, M., Montigny, R., Damasceno, D., Mattielli, N., Nicolaysen, K.E., and Scoates, J.S., 2002. Trace of the Kerguelen mantle plume: evidence from seamounts between the Kerguelen archipelago and Heard Island, Indian Ocean. Geochem. Geophys. Geosystems, 3 (Article), 10.1029/2001GC000251.

    Earthquake report: Sumatra Outer Rise Earthquakes

    The 2012 M 8.6 and M 8.2 Wharton Basin earthquakes happened shortly after I started preparing Earthquake Reports as earthjay. So, the material I had presented for this sequence was not very explanatory nor sophisticated (not that any of these reports are sophisticated). Today I take the opportunity to provide more on this.
    The M 8.6 earthquake was a strike-slip earthquake that happened in the middle of an oceanic plate. This was considered the largest magnitude intraplate earthquake to have occurred in historic time. I remember correspondence between myself and Dr. Lori Dengler, she asked about other examples and brought up a sequence near Macquarie in the 20th century. But these quakes were larger. There was a triggered event (M 8.2) about 2 hours after the M 8.6
    These “Great” (quakes with magnitude M ≥ 8) earthquakes happened in the middle of the India plate, just to the west of the Sumatra-Andaman trench. This deep sea trench is formed by a convergent plate boundary called a subduction zone. Here, at the Sumatra-Andaman subduction zone, the India plate (and further south, the Australia plate) dive beneath (subduct) the Sunda plate (part of the Eurasia plate). There was another intraplate earthquake in the Australia plate in 2016.
    Read more about different types of earthquakes here.
    About 8 years before there was a M 9.15 subduction zone earthquake on 26 December 2004, which was followed by a triggered M 8.6 earthquake further to the south on 28 March 2005. Our initial thoughts were that the 2012 events happened in a region of the oceanic crust that experienced an increase in stress from the 2004/2005 temblors. Dr. Dengler put together a one-pager about this as a possibility.

      The 2004 Sumatra-Andaman subduction zone earthquake led the beginning of over a decade of advancements in subduction zone science.

    • This earthquake caused a trans-oceanic tsunami that travelled across the Indian Ocean and was observed globally by tide gages and satellite platforms (like JASON, no relation).
    • The tsunami killed almost a quarter million people, including over 30,000 in Sri Lanka. There was no tsunami warning system in the Indian Ocean at the time, so nobody in India nor Sri Lanka knew the tsunami was coming.
    • This earthquake nucleated in the mantle, which was thought previously to be impossible (at least by most plate tectonicists).
    • Just like the 2001 Tohoku-oki M 9.0 earthquake, the 2004 event happened over a region that only had smaller (M~8) events in historic time. Thus, this large size of an earthquake was unexpected.
    • Read more about the 2004 Sumatra-Andaman subduction zone earthquake here.
      Some interesting observations for these Wharton Basin earthquakes.

      • Something else remarkable is that these earthquakes happened in ways that was moderately unexpected (at least to me, at the time; though, now with events like Kaikoura and Ridgecrest under my belt, this would not be unexpected if these events were to happen today).
      • This part of the India plate is sliced up by fracture zones, transform faults (strike-slip) that displace the sea-floor along north-south oriented faults. Slivers of crust move north relative to other slivers that move (relatively) to the south. We won’t spend much time discussing the absolute motions of plates.
      • The earthquake mechanisms (e.g. focal mechanisms or moment tensors) from these events were strike-slip. So, my original interpretation was that these earthquakes were along north-south oriented strike-slip faults. Even the aftershock pattern suggested this was true.
      • However, as more aftershocks rolled in and seismologists started studying these data, the initial interpretation was not completely correct. There were some major east-west faults involved in this sequence!!! This was very interesting.
      • Yet, there is more to remark about these earthquakes. This is their “extreme” depth.
      • Oceanic crust is, on average, about 7 km thick. Continental crust is about 25-30 km thick, on average. The M 8.6 and 8.2 were at 20 and 25 km depth respectively.
      • Turns out, the crust is very thick in this region (Dr. Satish Sing has worked on this). Also, given our education about earthquakes in the mantle, we have that going for us too (to help explain the depth for these earthquakes).
      • The ninetyeast ridge is a part of the oceanic crust that has been overthickened as the crust passed over a hotspot. Note in the map below how the ridge sticks up like a long mountain range, running north-south.

    Below is my interpretive poster for this earthquake

    • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 3.0 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.
    • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
    • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

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

    • In the upper right corner is a map showing the major tectonic plates, their boundaries, and some of the larger fault systems (from the Global Earthquake Model, GEM). I located the M 8.6 and 8.2 as yellow circles).
    • In the upper left corner I include an inset map showing the aftershocks for a 6 month period. I have not delineated the hypothetical faults so one may find their own conclusions.
      • However, check out the magnetic anomaly data (the red/blue regions oriented east-west).
      • These magnetic anomalies are formed when the crust is created at oceanic spreading centers.
      • Also, the magnetic poles flip every now and then (when the north pole becomes the south pole). When the crust if formed, it has the magnetic field at the time of formation preserved in the crust.
      • If crust was formed at a time in the past when the magnetic polarity is opposite of that from today, this creates a negative anomaly (in red). Regions of the crust that have the same polativy as today’s magnetic field, those regions are blue.
      • One may notice that these red/blue regions are east-west. This is because the oceanic spreading ridges where they were formed are oriented east-west.
      • Now take a look at how these magnetic anomalies are offset along north-south oriented lines. These are the strike-slip faults (the fracture zones) I mentioned earlier.
      • These magnetic anomalies are also visible in the main map.
    • In the lower center is a plot showing data from a buoy water pressure sensor (Heidarzadeh et al., 2017). These data show the seismic waves and resulting tsunami waves from each earthquake. I placed a white dot in the location of this buoy to the northwest of the temblors.
    • In the right center are two maps showing he seismic hazard and risk in Indonesia. I discuss this further below.
    • To the left of these hazard and risk maps is a figure from Jacob et al. (2014). This shows the age of the oceanic crust in the India-Australia plate. The age is in color (see legend) and note how these magnetic anomalies are offset across strike-slip faults.
    • To the left of the plate tectonic map is a figure that shows two maps. The outline of the 2004 and 2005 earthquake slip patches are in purple. The hypothetical faults that slipped in 2012 are shown as arrows with numbers relative to the time these faults slipped. The color fringes are their estimate of how much the stress in the India plate changed after teh 2004/2005 earthquakes, shown for different depths (10 km on the left and 40 km on the right). The warmer colors represent areas where there is an increase in stress. Note how the 2012 sequence is in a region of higher stress following the 2004/2005 quakes.
    • Here is the map with a month’s seismicity plotted.

    Seismic Hazard and Seismic Risk

    • These are the two maps shown in the map above, the GEM Seismic Hazard and the GEM Seismic Risk maps from Pagani et al. (2018) and Silva et al. (2018).
      • The GEM Seismic Hazard Map:

      • The Global Earthquake Model (GEM) Global Seismic Hazard Map (version 2018.1) depicts the geographic distribution of the Peak Ground Acceleration (PGA) with a 10% probability of being exceeded in 50 years, computed for reference rock conditions (shear wave velocity, VS30, of 760-800 m/s). The map was created by collating maps computed using national and regional probabilistic seismic hazard models developed by various institutions and projects, and by GEM Foundation scientists. The OpenQuake engine, an open-source seismic hazard and risk calculation software developed principally by the GEM Foundation, was used to calculate the hazard values. A smoothing methodology was applied to homogenise hazard values along the model borders. The map is based on a database of hazard models described using the OpenQuake engine data format (NRML). Due to possible model limitations, regions portrayed with low hazard may still experience potentially damaging earthquakes.
      • Here is a view of the GEM seismic hazard map for Indonesia.

      • The GEM Seismic Risk Map:

      • The Global Seismic Risk Map (v2018.1) presents the geographic distribution of average annual loss (USD) normalised by the average construction costs of the respective country (USD/m2) due to ground shaking in the residential, commercial and industrial building stock, considering contents, structural and non-structural components. The normalised metric allows a direct comparison of the risk between countries with widely different construction costs. It does not consider the effects of tsunamis, liquefaction, landslides, and fires following earthquakes. The loss estimates are from direct physical damage to buildings due to shaking, and thus damage to infrastructure or indirect losses due to business interruption are not included. The average annual losses are presented on a hexagonal grid, with a spacing of 0.30 x 0.34 decimal degrees (approximately 1,000 km2 at the equator). The average annual losses were computed using the event-based calculator of the OpenQuake engine, an open-source software for seismic hazard and risk analysis developed by the GEM Foundation. The seismic hazard, exposure and vulnerability models employed in these calculations were provided by national institutions, or developed within the scope of regional programs or bilateral collaborations.
    • Here is a view of the GEM seismic risk map for Indonesia.

    Tsunami Hazard

    • Here are two maps that show the results of probabilistic tsunami modeling for the nation of Indonesia (Horspool et al., 2014). These results are similar to results from seismic hazards analysis and maps. The color represents the chance that a given area will experience a certain size tsunami (or larger).
    • The first map shows the annual chance of a tsunami with a height of at least 0.5 m (1.5 feet). The second map shows the chance that there will be a tsunami at least 3 meters (10 feet) high at the coast.

    • Annual probability of experiencing a tsunami with a height at the coast of (a) 0.5m (a tsunami warning) and (b) 3m (a major tsunami warning).

    Other Report Pages

    Some Relevant Discussion and Figures

    • This is my interpretive poster for the 2004 and 2005 earthquakes. Read more here.

    • Here is a map from Jacob et a. (2014) that shows the structure of the eastern Indian Ocean. Figure text below.

    • Free-air gravity anomaly map derived from satellite altimetry [Sandwell and Smith, 2009] over the Wharton Basin area.

    • Here is the map from Jacobs et a. (2014). Figure text below.

    • Structure and age of the Wharton Basin deduced from free-air gravity anomaly [Sandwell and Smith, 2009; background colors] for the fracture zones (thin black longitudinal lines), and marine magnetic anomaly profiles (not shown) for the isochrons (thin black latitudinal lines). The plain colors represent the oceanic lithosphere created during normal geomagnetic polarity intervals (see legend for the ages of Chrons 20 to 34 according to the time scale of Gradstein et al. [2004]). Compartments separated by major fracture zones are labeled A to H. Grey areas: oceanic plateaus, thick black line: Sunda Trench subduction zone.

    • This is a fascinating figure from Jacob et al. (2014). This shows a reconstruction of the magntic anomalies for the oceanic crust as they are subducted beneath Eurasia.

    • Reconstitution of the subducted magnetic isochrons and fracture zones of the northern Wharton Basin using the finite rotation parameters deduced from our two- and three-plate reconstructions. (a) First the geometry is restored on the Earth surface, then (b) it is draped on the top of the subducting plate as derived from seismic tomography [Pesicek et al., 2010] shown by the thin dotted lines at intervals of 100 km (b). Colored dots: identified magnetic anomalies; colored triangles: rotated magnetic anomalies, solid lines; observed fracture zones and isochrons, dashed lines: uncertain or reconstructed fracture zones, dotted lines: reconstructed isochrons from rotated magnetic anomalies (two-plate and three-plate reconstructions), colored area: oceanic lithosphere created during normal geomagnetic polarity intervals (see legend for the ages; the colored areas without solid or dotted lines have been interpolated), grey areas: oceanic plateaus, thick line: Sunda Trench subduction zone.

    • Finally, these authors present what their reconstruction implicates about this plate boundary system.

    • The deviation of the Sunda Trench from a regular arc shape (dotted lines) off Sumatra is explained by the presence of the younger, hotter and therefore lighter lithosphere in compartments C–F, which resists subduction and form an indentor (solid line). The very young compartment G was probably part of this indentor before oceanic crust formed at slow spreading rate near the Wharton fossil spreading center approached subduction: The weaker rheology of outcropping or shallow serpentinite may have favored the restoration of the accretionary prism in this area. Further south, the deviation off Java is explained by the resistance of the thicker Roo Rise, an oceanic plateau entering the subduction.

    • Andrade and Rajendran (2014) present their interpretation of the seismotectonic context for the 2012 sequence. This is their map showing fracture zones and lineations (which may be earthquake faults).

    • P-axis orientations of earthquakes on the subducting Indo-Australian Plate overlaid on a trace of the diffuse India–Capricorn–Australia plate boundary region (pale yellow) and the diffuse triple junction (DTJ, warm yellow). Gray arrows indicate the convergence directions of the component plates (from Royer and Gordon, 1997). Data for earthquakes pre-1977 are fromStein and Okal (1978), Bergman and Solomon (1985), andPetroy andWiens (1989) and for the later period, fromNEIC and Global CMT. Broken Ridge (BR),

    • Here is the same map but with historic earthquake mechanisms plotted for events in the India plate.

    • Centroid moment tensors (colored by depth) of earthquakes on the Indian Ocean intraplate region and P-axis orientations from 1977 to 25 December 2004 (data source: Global CMT, with earthquake relocations from Engdahl et al., 2007). Numbered beachballs and cluster ‘R’ are discussed in the text. The Sumatra–Andaman plate boundary seismicity for the same period is represented by gray dots. Subevents of the 2000 Enggano and Cocos Islands events (focal mechanisms with black compressional quadrants) are from Abercrombie et al., 2003.

    • This is a large scale view of their interpretations of lineations that cut across the ninetyeast ridge.

    • (a) Centroid moment tensor solutions of the January (red) and April 2012 earthquakes (green)with respect to the E–W oriented fossil Wharton Ridge segments and its associated N–S trending fracture zones, and the E–Wlineations that dissect the Ninetyeast Ridge (NER). Other notations and data sources are the same as in Figs. 1 and 2. The source area of the April 2012 earthquakes (gray rectangle) is expanded in (b)which shows centroidmoment tensors (colored by depth) and P-axes in the year 2012; moment tensor solutions are from Global CMT.

    • Here are some maps that show the results from Wiseman and Bürgmann (2012). These show how they calculate the static coulomb stress (tectonic stress) to have changed after the 2004 and 2005 events.
    • The upper right map shows the changes exerted during the 2004 event, the lower right panel shows change in stress after the event, and the map on the left shows the total change in stress calculated by these authors.

    • Recent stress changes in the Indian Ocean. (a) Total stresses induced by the 2004 [Chlieh et al., 2007], 2005 [Konca et al., 2007], and January M7.2 ( php) earthquakes, resolved at the 20 km hypocentral depth of the mainshock on the orientation of the initial WNW-ESE (red) fault plane [Meng et al., 2012]. Gray circles mark the first 12 days of the aftershock sequence (NEIC catalog). (b) Coseismic stresses induced by the 2004 and 2005 earthquakes. The yellow focal mechanisms highlight the strike-slip earthquakes during the first year following the 2004 earthquake and the blue focal mechanisms depict the remaining strike-slip events before the 2012 mainshock (Global CMT catalog). (c) Cumulative postseismic stresses induced by the 2004 and 2005 earthquakes at the time of the 2012 earthquake.

    • This is the figure from the poster that shows how these stress changes varied with depth.

    • The effects of receiver depth. Coseismic stress changes resulting from the 2004 and 2005 earthquakes [Chlieh et al., 2007; Konca et al., 2007] resolved on the WNW-ESE (red) fault plane. Calculated at (a) 10 km depth, and (b) the deeper centroid depth of 40 km.

    • Here is a fascinating figure from Yadav et al. (2013) showing hypothetical changes in the elastic movement of the crust as a result of these earthquakes. They compare their model (the small arrows) with model and observations at GPS sites.

    • The 11 April 2012 earthquakes and coseismic offsets derived from GPS measurements at various IGS sites and at permanent GPS sites in the Andaman-Nicobar region (shown with black arrows with error bars). The bold gray arrows represent the compressional regime of the diffused plate boundary region (shaded with light gray color) between the Indian and Australian plates [Gordon et al., 1998]. The yellow dashed lines denote the rupture planes of the 11 April 2012 earthquakes [Yue et al., 2012]. Arrows with different colors show the simulated coseismic offsets due to the slip models by Yue et al. [2012] using the layered spherical earth [Pollitz, 1997]. The purple stars are other earthquakes discussed in the text. The north-south gray lines indicate the fracture planes in the Wharton and Central Indian basin.

    • This map from Yadav et a. (2013) shows the existence of north-south fracture zones using multibeam bathymetric maps (the purple map on the right) and seismic reflection profiles (the black and white plots on the right).

    • Satellite magnetic anomalies and swath bathymetry data. (a) North-south-oriented planes can be seen on the magnetic anomalies. These planes extend right up to the trench and appear to influence the tectonics of the subduction zone. Near the trench, they are characterized by strike-slip faulting (marked by yellow ellipses). If extended farther north, they coincide with the low-slip regions of the 2004 Sumatra-Andaman earthquake rupture [Chlieh et al., 2007].
      the pink dashed line with arrow marks the northern limit of the fast slip during the 2004 Sumatra-Andaman earthquake rupture [Lay et al., 2005], and it coincides with the north-south plane which accommodated part of the slip during the 11 April 2012 earthquakes. The slab pull force along the arc is also shown [Lallemand et al., 2005]. (b) Swath bathymetry data [Graindorge et al., 2008] showing a north-south-oriented fault. A, B, C, and D in the figure are the seismic lines across the fault. These near-vertical normal faults are now reactivated as left-lateral strike-slip faults.

    • Here is the map and plots of tsunami observations in the Indian Ocean from these Wharton Basin earthquakes (Heidarzadeh et al., 2017).

    • (a) Epicentres and mechanisms of the large strike-slip intraplate earthquakes in the Wharton Basin along with the locations of the DART and tide gauge stations used in this study. Focal mechanisms are from GCMT catalogue. The focal mechanisms for the 1928 and 1949 are from Petroy &Wiens (1989). TTT stands for tsunami travel time. (b) Inset showing the tectonic and bathymetric features along with north–south trending fracture zones. (c) Teleseismic stations used in this study to analyse the 2016 earthquake including both P (cyan circles) and SH (green circles) waves. (d,e) Tsunami waveforms for the 2012 and 2016 tsunamis, respectively.

    • These authors analyzed the spectral characteristics of the tsunami from these two events.

    • Comparison of the waveforms and spectra of the 2012 and 2016 off Sumatra tsunamis. (a) The observed waveforms of the 2012 and 2016 tsunamis. The time on the x-axis is from the origin time of the 2016 tsunami while the time of the 2012 event is shifted. (b) The spectra for the 2012 and 2016 tsunamis. The average spectra shown at the bottom row are normalized average. The term ‘Back.’ represents spectrum of background sea level waveforms. For the 2016 tsunami, the background spectrum is based on only the record of the Cocos Island station.

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      Basic & General References

    • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
    • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release,
    • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , /li>
    • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859.
    • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
    • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, ,
    • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889,
    • 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.
    • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006,
    • Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
    • Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1).
    • Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385,
    • Specific References

    • Andrade, V. and Rajendran, K., 2014. The April 2012 Indian Ocean earthquakes: Seismotectonic context and implications for their mechanisms in Tectonophysics, v. 617, p. 126-139,
    • Heidarzadeh, M., Harada, T., Satake, K., Ishibe, T., Takagawa, T., 2017. Tsunamis from strike-slip earthquakes in the Wharton Basin, northeast Indian Ocean: March 2016 Mw7.8 event and its relationship with the April 2012 Mw 8.6 event in GJI, v. 2110, p. 1601-1612, doi: 10.1093/gji/ggx395
    • Jacob, J., J. Dyment, and V. Yatheesh, 2014. Revisiting the structure, age, and evolution of the Wharton Basin to better understand subduction under Indonesia, J. Geophys. Res. Solid Earth, 119, 169–190, doi:10.1002/2013JB010285.
    • Yadav, R.K., Kundu, B., Gahalaut, K., Catherine, J., Gahalaut, V.K., Ambikapathy, A., and Naidu, MZ.S., 2013. Coseismic offsets due to the 11 April 2012 Indian Ocean earthquakes (Mw 8.6 and 8.2) derived from GPS measurements in Geophysical Research Letters, v. 40, p. 3389-3393, doi:10.1002/grl.50601
    • Wiseman, K. and Bürgmann, R., 2012. Stress triggering of the great Indian Ocean strike-slip earthquakes in a diffuse plate boundary zone in Geophysical research Letters, v. 39, L22304, doi:10.1029/2012GL053954

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    My original post, one of my first.

    looks like we just got some late aftershocks to the 2004 Sumatra-Andaman subduction zone earthquake. on 2012 April 11, there was a swarm of activity, with an Mw 8.6 and an Mw 8.1 event with strike slip focal mechanisms and moment tensors.
    the 8.6 is directly updip of the region of maximum slip from the 2004 Great earthquake. the 8.2 is updip from the 2005 Great earthquake. Todays two earthquakes are likely on one or two of the many fracture zones that strike generally north-south in the India-Australia plate. one of the better known fzs is the Investigator fracture zone, several hundred kms to the east of these epicenters.
    the fzs are thought to partly contribute to the segmentation of the subduction zone offshore sumatra. i have composed a map that shows Sumatera, the islands (forearc), the 2004 earthquake slip (in cm) modeled by Chlieh et al., 2007, some bathymetry with some structures labeled, and the epicenters of the two largest earthquakes from today mentioned above. my map