Earthquake Report: Papua New Guinea: Update #1

The aftershocks are still coming in! We can use these aftershocks to define where the fault may have slipped during this M 7.5 earthquake. As I mentioned yesterday in the original report, it turns out the fault dimension matches pretty well with empirical relations between fault length and magnitude from Wells and Coppersmith (1994).
The mapped faults in the region, as well as interpreted seismic lines, show an imbricate fold and thrust belt that dominates the geomorphology here (as well as some volcanoes, which are probably related to the slab gap produced by crust delamination; see Cloos et al., 2005 for more on this). I found a fault data set and include this in the aftershock update interpretive poster (from the Coordinating Committee for Geoscience Programmes in East and Southeast Asia, CCOP).
I initially thought that this M 7.5 earthquake was on a fault in the Papuan Fold and Thrust Belt (PFTB). Mark Allen pointed out on twitter that the ~35km hypocentral depth is probably too deep to be on one of these “thin skinned” faults (see Social Media below). Abers and McCaffrey (1988) used focal mechanism data to hypothesize that there are deeper crustal faults that are also capable of generating the earthquakes in this region. So, I now align myself with this hypothesis (that the M 7.5 slipped on a crustal fault, beneath the thin skin deformation associated with the PFTB. (thanks Mark! I had downloaded the Abers paper but had not digested it fully.)

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 plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.5 earthquake, 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.
  • I include some inset figures.

  • In the upper right corner is a general overview of the plate boundaries and mapped faults in the region. I place a blue star in the general location of the M 7.5 epicenter. The fault lines on this figure also come from CCOP.
  • In the lower right corner is a plot showing vertical land motion for GPS sites along a north-south profile. Basically, this shows that the sites north of the FTB are currently uplifting at about 5 mm.yr and the sites north of the Bewani fault zone are uplifting an additional 10 mm/yr. This means that the crustal shortening associated with the collision of Australia with the Pacific/Caroline plates is partly being accumulated as elastic strain in the crust and is localized on these fault systems. While this profile is several tens of kilometers to the west of the M 7.5, this process is likely also happening where the M 7.5 occurred.
  • On the left are three figures from Abers and McCaffrey (1988).
    • The upper panel shows the extent of a portion of their analysis that is cogent for the M 7.5 sequence. The extent of this box is also outlined in a dashed yellow rectangle on the main map. The blue star represents the general location of the M 7.5 earthquake. There are no backthrusts mapped on this figure (the hypothesis for the M 7.5 source fault promoted in my original report and on social media).
    • This is a north-south cross section showing the focal mechanisms for 3 of the earthquakes in the map. This shows a south vergent fault as a possible source for the M ~5.x earthquakes studied by Abers and McCaffrey (1988). I am starting to favor an interpretation that the M 7.5 fault is south vergent.
    • The lowest panel shows the interpretation from Abers that these deeper crustal faults are responsible for the seismicity they studied (and I thank mark again that I may posit that these faults are responsible for the current seismicity).


  • Here is the original interpretive poster from my initial report here.

  • The same map without historic seismicity.

Some Relevant Discussion and Figures

  • Here is the tectonic map from Loulali et al. (2015).

  • Tectonic setting of the Papua New Guinea region. Topography and bathymetry are from SRTM(http://topex.ucsd.edu/www_html/srtm30_plus.html). Faults are mostly from the East and Southeast Asia (CCOP) 1:2000000 geological map (downloaded from http://www.orrbodies.com/resources/item/orr0052). AFTB, Aure Fold-and-Thrust Belt; OSZF, Owen Stainly fault zone; GF, Gogol fault; BTFZ, Bewani-Torricelli fault zone; RMFZ, Ramu-Markham fault zone; BSSL, Bismarck Sea Seismic Lineation.

  • Here is a map from Abers and McCaffrey (1988) that shows all the earthquakes included in their study (and the focal mechanisms). Inset “a” is the region shown on the aftershock poster above.

  • Map of focal mechanisms determined here, locations of cross sections in Figure 11, and shallow seismicity. Focal mechanisms are shown as lower hemisphere projections with the compressional quadrants shaded, and the P and T axes shown as solid and open circles, respectively. The sizes of the focal spheres are scaled to log (MO), according to the scale in the upper right, and are labeled by the event numbers in Table 1. Seismicity is from the ISC catalog, 1964-1984, and includes all events listed as being shallower than 70 km recorded by 25 or more stations, with M b • 5.0, and with standard deviations in latitude, longitude, or depth each not exceeding 20 km. Inset in lower left shows all large (M • 7.0), shallow (! 70 km) earthquakes in the period 1900-1985, from the catalog compiled by Everingham [1974] for events before 1971 and from Ganse and Nelson [1981, with supplement] for more recent events. Faults are labelled on Figure 1.

  • Here are all the 3 cross sections from Abers and McCaffrey (1988). The upper section is a and the lower section is c (from the above map).

  • Cross sections of seismicity and topography: a, b, and c refer to the profile locations on Figure 2. Vertical exaggeration is 10x for topography and lx for seismicity, as indicated by the vertical scale bars on right. Horizontal scale, indicated on profile a, is the same for all profiles. Focal spheres are plotted as back hemisphere projections, and compressional quadrants are filled.

  • This is the money shot, showing their interpretation (Abers and McCaffrey, 1988).

  • Cartoon showing how thin-skinned faulting mapped in PNG might be related to faulting in the basement, inferred from the earthquakes and other evidence discussed in the text. See Figure 11a for comparison to actual topography and earthquake mechanisms.

Earthquake Report: Papua New Guinea

This morning (local time in California) there was an earthquake in Papua New Guinea with, unfortunately, a high likelihood of having a good number of casualties. I was working on a project, so could not immediately begin work on this report.
This M 7.5 earthquake (USGS website) occurred along the Papua Fold and Thrust Belt (PFTB), a (mostly) south vergent sequence of imbricate thrust faults and associated fold (anticlines). The history of this PFTB appears to be related to the collision of the Australia plate with the Caroline and Pacific plates, the delamination of the downgoing oceanic crust, and then associated magmatic effects (from decompression melting where the overriding slab (crust) was exposed to the mantle following the delamination). More about this can be found in Cloos et al. (2005).
The USGS prepared a fault slip model that shows this earthquake may have ruptured a north vergent (south dipping) thrust fault.
There was a M 6.5 earthquake north of today’s M 7.5 earthquake in November 2017. These earthquakes are along different fault systems and likely are too distant to be related.
On 2018.02.26 I prepared an updated report here.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.5 earthquake, in addition to some relevant historic earthquakes. There was a M 6.6 earthquake to the southeast along the PFTB in 2000 and I include the moment tensor for this earthquake.

  • 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.
  • I include some inset figures.

  • In the lower right corner is a great figure showing the generalized plate tectonic boundaries in this region of the equatorial Pacific Ocean (Holm et al., 2016). I place a blue star in the general location of the M 6.5 earthquake (also plotted in other inset figures). This map shows the major plate boundary faults. Active subduction zones have shaded triangle fault symbols, while inactive subduction zones have un-shaded triangle fault line symbols.
  • In the lower left corner is a map showing the fault systems in the region (Cloos et al., 2005). The legend allows us to distinguish between active and inactive fault systems.
  • In the upper right corner is a figure from Baldwin et al. (2012). This figure shows a series of cross sections along this convergent plate boundary from the Solomon Islands in the east to Papua New Guinea in the west. Cross section ‘D’ is the most representative for the earthquakes today. I present the map and this figure again below, with their original captions.
  • In the upper left corner is cross section D-D’ that shows the PFTB. I placed the blue star along a north vergent fault that may be associated with today’s M 7.5. The faults are actually quite complex, so this schematic illustration may not be a perfect represetation of the fautls here.
  • In the center left is a plot showing the larger aftershocks (large enough to show up in USGS database, a global catalog). The rupture length of the fault that ruptured today may be ~160 km. Considering empirical relations developed by Wells and Coppersmith (1994), a 160 km fault length would generate a M 7.6-7.7 earthquake (close to M 7.5, given the empirical relations and the uncertainty with those relations).


  • The same map without historic seismicity.

  • Here is the interpretive poster from last November (this is the report).

  • Some Relevant Discussion and Figures

    • Here is the Holm et al. (2016) figure.

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

    • Koulali et al (2015) use GPS data to resolve the kinematics of the central-eastern Papua New Guinea region. The first figure below is a map that shows the GPS velocities in this region There are two cross section profiles labled on the map (the M 7.5 earthquake happened to the east of A-A’). Note the complicated and detaile dfault mapping (the balck lines). The convergence is generally perpendicular to the PFTB in the east and more oblique to the PFTB on the western portion of this map.

    • The GPS velocity field and 95 per cent confidence interval ellipses with respect to the Australian Plate. Red and blue vectors are the new calculated field and black vectors are from Wallace et al. (2004). The dashed rectangle shows the area of Fig. 3. The blue dashed lines correspond to the location of profiles shown in Fig. 4. Note that the velocity scales for the red and blue vectors are different (see the lower right corner for scales). The black velocities are plotted at the same scale as the red vectors.

    • Here are the two profiles. The red and blue lines plot vertical land motion (VLM) rates in mm/yr and show strain accumulates across the region. Today’s earthquake happened in the region labeled ‘Highland FTB.’ The plot shows that ~5 mm/yr of strain accumulates in this fault system.

    • Profiles A–A& and B–B& from Fig. 2 showing model fit to GPS observations. Red symbols and lines are the GPS observed and modelled velocities, respectively, for the profile-normal component. Blue symbols and lines correspond to the profile-parallel component. The green and pink lines corresponds to the model using the Ramu-Markham fault geometry from Wallace et al. (2004), south of Lae. Grey profiles show the projected topography. The seismicity is from the ISC catalogue for events > Mw 3.5 (1960–2011).

    • Here is a comparison of the proposed fault length shown on the poster with fault scaling relations from Wells and Coppersmith (1994). The upper panel is figure 9 and the lower panel is figure 17. I include figure captions for these figures below. Presuming a fault length of 160 km, the magnitude would be between 7.5 and 8.

    • Figure 9. (a) Regression of surface rupture length on magnitude (M). Regression line shown for all-slip-type relationship. Short dashed line indicates 95% confidence interval. (b) Regression lines for strike-slip, reverse, and normal-slip relationships. See Table 2 for regression coefficients. Length of regression lines shows the range of data for each relationship.

      Figure 17. Regression lines for stable continental region (SCR) earthquakes and non-SCR continental earthquakes. (a) Regression of surface rupture length on magnitude (M). (b) Regression of rupture area on magnitude (M).

    • Here is the USGS Pager Alert. More can be found about the PAGER alerts here.
    • PAGER provides shaking and loss estimates following significant earthquakes anywhere in the world. These estimates are generally available within 30 minutes and are updated as more information becomes available. Rapid estimates include the number of people and names of cities exposed to each shaking intensity level as well as the likely ranges of fatalities and economic losses. PAGER does not consider secondary effects such as landslides, liquefaction, and tsunami in loss estimates at this time.
    • This shows that there is a 42% chance that there will be between 100 and 1,000 casualties. We can only hope that there are fewer (which is possible).

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

    • This map shows plate velocities and euler poles for different blocks. I include the figure caption below as a blockquote. The PFTB is shown as a kelly-green band of color.

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

    • This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). These deep earthquakes are nearest the cross section D (though are much deeper than these shallow cross sections). I include the figure caption below as a blockquote.

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

    • UPDATE (23:00 pacific time): This is one of the ground motion visualizations from IRIS. The red and blue colors represent the upward or downward motion recorded on seismometers. Note the background motions along the coast of WA, OR, and CA have high amplitudes (darker red and darker blue). This is probably due to the storm that is hitting the region (the wind blows trees, buildings, etc. and the waves pound the earth, both of which are recorded on seismometers). This is the first time that I noticed this phenomena on one of these visualizations. There are probably many other examples.
    • Another cool thing is that about half way through the animation, the seismic waves that were traveling west from the earthquake, travel around the globe, and then are seen here, traveling from teh east coast to the west coast. This is common to most all of these visualizations.

    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:

    Social Media

      References:

    • Baldwin, S.L., Monteleone, B.D., Webb, L.E., Fitzgerald, P.G., Grove, M., and Hill, E.J., 2004. Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea in Nature, v. 431, p/ 263-267, doi:10.1038/nature02846.
    • Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region, Annu. Rev. Earth Planet. Sci., v. 40, pp. 495-520.
    • Cloos, M., Sapiie, B., Quarles van Ufford, A., Weiland, R.J., Warren, P.Q., and McMahon, T.P., 2005, Collisional delamination in New Guinea: The geotectonics of subducting slab breakoff: Geological Society of America Special Paper 400, 51 p., doi: 10.1130/2005.2400.
    • Hamilton, W.B., 1979. Tectonics of the Indonesian Region, USGS Professional Paper 1078.
    • Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
    • Holm, R. and Richards, S.W., 2013. A re-evaluation of arc-continent collision and along-arc variation in the Bismarck Sea region, Papua New Guinea in Australian Journal of Earth Sciences, v. 60, p. 605-619.
    • Holm, R.J., Richards, S.W., Rosenbaum, G., and Spandler, C., 2015. Disparate Tectonic Settings for Mineralisation in an Active Arc, Eastern Papua New Guinea and the Solomon Islands in proceedings from PACRIM 2015 Congress, Hong Kong ,18-21 March, 2015, pp. 7.
    • Holm, R.J., Rosenbaum, G., Richards, S.W., 2016. Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting in Eartth Science Reviews, v. 156, p. 66-81.
    • Johnson, R.W., 1976, Late Cainozoic volcanism and plate tectonics at the southern margin of the Bismarck Sea, Papua New Guinea, in Johnson, R.W., ed., 1976, Volcanism in Australia: Amsterdam, Elsevier, p. 101-116
    • Koulali, A., tregoning, P., McClusky, S., Stanaway, R., Wallace, L., and Lister, G., 2015. New Insights into the present-day kinematics of the central and western Papua New Guinea from GPS in GJI, v. 202, p. 993-1004, doi: 10.1093/gji/ggv200
    • Sapiie, B., and Cloos, M., 2004. Strike-slip faulting in the core of the Central Range of west New Guinea: Ertsberg Mining District, Indonesia in GSA Bulletin, v. 116; no. 3/4; p. 277–293
    • Tregoning, P., McQueen, H., Lambeck, K., Jackson, R. Little, T., Saunders, S., and Rosa, R., 2000. Present-day crustal motion in Papua New Guinea, Earth Planets and Space, v. 52, pp. 727-730.
    • Wells, D., l., and Coppersmith, K.J., 1994. New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement in BSSA, vol. 84, no. 4, pp. 974-1002

    Earthquake Report: 1971 Sylmar, CA

    This earthquake was the second earthquake in the state of CA to lead to major changes in how people in the state handled earthquake hazards and risk and today is the 47th anniversary of this earthquake. The first important earthquake was the 1933 Long Beach Earthquake, which led to major changes in the building code (first in Long Beach, then later adopted by the entire state). These changes in the building code have continued to evolve and improve, eventually adopted globally. The 1971 M 6.7 Sylmar Earthquake (a little larger than the M 6.4 damaging earthquake sequence recently that happened in Taiwan) caused major damage to buildings and other infrastructure in southern CA (e.g a hospital was destroyed, which caused many casualties). The 1906 San Francisco Earthquake was important too, so I don’t want the SAF to feel left out. Though the 1933 Long Beach and 1971 Sylmar earthquakes seem to have led to more significant changes in how people approach earthquake hazards and risk.
    A major positive result from the Sylmar Earthquake was the Alquist Priolo Act. The AP Act created a requirement to characterize all the active faults in the state of CA and to regulate how to consider how structures could be built in relation to these active faults. More about the AP Act can be found here. After several years of no support from the state, the CA Geological Survey has recently supported work in this regard, resulting in an update of their guidelines in how to apply the AP Act in Special Publication 42.
    I put together a commemorative #EarthquakeReport interpretive poster to discuss the tectonics of the region. The San Andreas fault (SAF) system is the locus of ~75% of the Pacific-North America plate boundary motion. The SAF is in some places a mature fault with a single strand and in other places, there are multiple strands (e.g. the Elsinore, San Jacinto, and SAF in southern CA or the Maacama, Bartlett Springs, and SAF in northern CA). In southern CA, the SAF makes a bend (called the “Big Bend”) that forms a region of compression. This compression is realized in the form of thrust faults and folds, creating uplift forming the mountain ranges like the Santa Monica Mountains. Some of these thrust faults breach the ground surface and some are blind (they don’t reach the surface).
    In 1971 there was a large earthquake (M 6.7) that caused tremendous amounts of damage in southern CA. A hospital was built along one of the faults and this earthquake caused the hospital to collapse killing many people. The positive result of this earthquake is that the Alquist Priolo Act was written and passed in the state legislature. I plot the moment tensor for the 1971 earthquake (Carena and Suppe, 2002).
    Then, over 2 decades later, there was the M 6.7 Northridge Earthquake. This earthquake was very damaging. Here is a page that links to some photos of the damage. Here is the USGS website for this 1971 M 6.7 Sylmar 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 ≥ 4.5.
    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.7 earthquake, in addition to some of the significant earthquakes in southern CA.

    • 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 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 a legend showing the relative age of most recent activity for faults shown on the map. These faults are from the USGS Active Fault and Fold Database. More can be found about this database here.
    • I include some inset figures.

    • In the upper left corner is a map of the faults in southern CA (Tucker and Dolan, 2001). Strike-slip faults (like the SAF) have arrows on either side of the fault desginating the relative motion across the fault. Thrust faults have triangle barbs showing the convergence direction (the triangles are on the side of the fault that is dipping into the Earth).
    • Below this fault map is a low-angle oblique block diagram showing the configuration of thrust faults in the region of the Big Bend. These thrust faults are forming the topography in southern CA. The 1971 and 1994 earthquakes occurred along thrust faults similar to the ones shown in this block diagram.
    • In the upper right corner is a cross section of seismicity associated with the 1971 and 1994 earthquakes (Tsutsumi and Yeats, 1994). 1971 main and aftershocks are in blue and 1994 main and aftershocks are in red. Note how both earthquakes occurred along blind thrust faults. Also note that these faults were dipping in opposite directions (1971 dips to the north (south vergent) and 1994 dips to the south (north vergent).
    • In the lower right corner is another figure showing the aftershocks from the 1971 and 1994 earthquakes (Fuis et al., 2003). This shows their seismic velocity model (with fault interpretations). The 1971 and 1994 earthquake focal mechanisms are shown.
    • In the lower left corner is an illustration that shows the Likelihood of an earthquake with M ≥ 6.7 for the next 30 years. This is based upon the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3). More about UCERF3 can be found here. I placed a blue star in the general location of the 1971 Sylmar Earthquake.


    • Here is the same map, but the MMI is plotted as contours.


    Some Relevant Discussion and Figures

    • Here is the fault map from Tucker and Dolan (2001).

    • Regional neotectonic map for metropolitan southern California showing major active faults. The Sierra Madre fault is a 75-km-long active reverse fault that extends along the northern edge of the metropolitan region. Fault locations are from Ziony and Jones (1989), Vedder et al. (1986), Dolan and Sieh (1992), Sorlien (1994), and Dolan et al. (1997, 2000b). Closed teeth denote reverse fault surface trace; open teeth on dashed lines show upper edge of blind thrust fault ramps. Strike-slip fault surface traces shown by double arrows. Star denotes location of Oak Hill paleoseismologic trench site of Bonilla (1973). CSI, Clamshell-Sawpit fault; ELATB, East Los Angeles blind thrust system; EPT, Elysian park blind thrust fault; Hol Fl, Hollywood fault; PHT, Puente Hills blind thrust fault; RMF, Red Mountain fault; SCII, Santa Cruz Island fault; SSF, Santa Susana fault; SJcF, San Jacinto fault; SJF, San Jose fault; VF, Verdugo fault; A, Altadena study site of Rubin et al. (1998); LA, Los Angeles; LB, Long Beach; LC, La Crescenta; M, Malibu; NB, Newport Beach; Ox, Oxnard; P, Pasadena; PH, Port Hueneme; S, Horsethief Canyon study site in San Dimas; V, Ventura. Dark shading denotes mountains.

    • This is a figure that is based upon Fuis et al. (2001) as redrawn by UNAVCO that shows the orientation of thrust faults in this region of southern CA. Below the block diagram is a map showing the location of their seismic experiment (LARSE = Line 1; Fuis et al., 2003).

    • Schematic block diagram showing interpreted tectonics in vicinity of LARSE line 1. Active faults are shown in orange, and moderate and large earthquakes are shown with orange stars and attached dates, magnitudes, and names. Gray half-arrows show relative motions on faults. Small white arrows show block motions in vicinities of bright reflective zones A and B (see Fig. 2A). Large white arrows show relative convergence direction of Pacific and North American plates. We interpret a master decollement ascending from bright reflective zone A at San Andreas fault, above which brittle upper crust is imbricating along thrust and reverse faults and below which lower crust is flowing toward San Andreas fault (brown arrows) and depressing Moho. Fluid injection, indicated by small lenticular blue areas, is envisioned in bright reflective zones A and B.


      Shaded relief map of Los Angeles region, southern California, showing Quaternary faults (thin black lines, dotted where buried), shotpoints (gray and orange filled circles), seismographs (gray and orange lines), air-gun bursts (dashed yellow lines), and epicenters of earthquakes .M 5.8 since 1933 (focal mechanisms with attached magnitudes: 6.7a—Northridge [Hauksson et al., 1995], 6.7b—San Fernando [Heaton, 1982], 5.9—Whittier Narrows [Hauksson et al., 1988], 5.8—Sierra Madre [Hauksson, 1994], 6.3—Long Beach [Hauksson, 1987]). Faults are labeled in red; abbreviations: HF—Hollywood fault, MCF—Malibu Coast fault, MHF—Mission Hills fault, NHF—Northridge Hills fault, RF—Raymond fault, SF—San Fernando surface breaks, SSF—Santa Susana fault, SMoF—Santa Monica fault, SMFZ—Sierra Madre fault zone, VF—Verdugo fault. NH is Newhall.

    • Here are the figures from Hauksson et al. (1995) showing the regions effected by earthquakes in southern CA.

    • (A) Significant earthquakes of M >= 4.8 that have occurred in the greater Los Angeles basin area since 1920. Aftershock zones are shaded with cross hatching, including the 1994 Northridge earthquake. Dotted areas indicate surface rupture, including the rupture of the 1857 earthquake along the San Andreas fault. (B) Lower hemisphere focal mechanisms (shaded quadrants are compressional) for significant earthquakes that have occurred since 1933 in the greater Los Angeles area.

    • Here is the seismicity cross section plot from Tsutsumi and Yeats (1999).

    • Cross section down to 20 km depth across the central San Fernando Valley, including the 1971 Sylmar and 1994 Northridge earthquake zones. See Figure 2 for location of the section and Figure 3 for stratigraphic abbreviations. Wells are identified in the Appendix. Aftershock data for the 1971 (blue) and 1994 (red) earthquakes within a 10-km-wide strip including the line of this section are provided by Jim Mori at Kyoto University. Abbreviation for faults: MHF, Mission Hills fault; NHF, Northridge Hills fault; SSF, Santa Susana fault.

    • Here is the figure from Fuis et al. (2003) showing their interpretation of seismic data from the region. These data are from a seismic experiment also plotted in the map above. The panel on the left is A and the panel on the right is B. This is their figure 3.

    • Cross section along part of line 2 with superposition of various data layers. A: Tomographic velocity model plus line drawing extracted from reflection data (see text); heavier black lines represent better-correlated or higher-amplitude phases. B: Velocity model plus relocated aftershocks of 1971 San Fernando and 1994 Northridge earthquakes (brown and blue dots, respectively); main shock focal mechanisms (far hemispheres) are red (San Fernando; Heaton, 1982) and blue (Northridge; Hauksson et al., 1995). Aftershocks are projected onto line 2 from up to 10 km east.

    • This is a smaller scale cross section from Fuis et al. (2003) showing a broader view of the faults in this region. This shows the velocity model color legend that also applies to the above figure. This is their figure 4.

    • Similar to Fig. 3, with expanded depth and distance frame. See caption for Fig. 3 for definition of red, magneta, and blue lines; orange line—interpreted San Andreas fault (SAF); yellow lines—south-dipping reflectors of Mojave Desert and northern Transverse Ranges; “K” —reflection of Cheadle et al. (1986), which is out of plane of this section. SAF is not imaged directly; interpretation is based on approximate northward termination of upper reflections (best constrained) in San Fernando reflective zone (magenta lines). (See similar interpretation for SAF on line 1—Fig. 5.) Wells shown in Mojave Desert are (s) H&K Exploration Co., (t) Meridian Oil Co. (Dibblee, 1967). For well color key, see caption for Fig. 3. Thin, dashed yellow-orange line—estimated base of Cenozoic sedimentary rocks in Mojave Desert based on velocity. Darker, multicolored region (above region of light violet) represents part of velocity model where resolution ≥ 0.4 (see color bar).

    • Here is a fascinating figure from Carena and Suppe (2002) showing the 3-dimensional configuration of the faults involved in the 1971 and 1994 earthquakes.

    • Perspective view, looking from the SE, of the modeled Northridge and San Fernando thrusts. The Northridge thrust stops at a depth of about 6 km, and its upper tip east of the lateral ramp (Fig. 4) terminates almost against the San Fernando thrust, as was suggested by Morti et al. (1993). The San Fernando thrust loser tip is at a depth o 13 km, whereas the Northridge thrust lower tip is at 32 km.

    • Here is a map view of the Carena and Suppe (2002) interpretation of these fault planes.

    • Schematic geological map showing the position of the main faults and folds, as well as the depth contours (contour interval = 1 km) of the Northridge (solid) and San Fernando (dashed) thrusts.

    • Here is a structural cross section across this region (Carena and Suppe, 2002).

    • Cross-section through the San Fernando Valley with projected aftershocks of the 1994 Northridge earthquake and of the 1971 Sylmar earthquake. The Northridge aftershocks are projected from a distance of 1 km or less on each side of the cross-section (main shock projected from 2 km W), whereas those of the Sylmar earthquake are projected from 1.5 km or less (main shock projected from 5 km ESE). The sources that we used for near-surface geology and structure are Dibblee (1991) and a seismic line (Fig. 11). The large N-S changes in Upper Tertiary stratigraphic thicknesses in this region (Dibblee, 1991, 1992a), prevent detailed stratigraphic correlation across fault blocks (this figure and Fig. 12). This face suggests that the shallow faults and possible the deeper San Fernando thrust itself, are reactivating old normal faults of the southern margin of the Ventura Basin (Yeats, et al., 1994; Huftle and Yeats, 1996; Tsutsumi and Yeats, 1999). Location of cross-section is in Fig. 13.

    • Here is a comparison of the ground shaking intensity for these two earthquakes (1971 Sylmar vs. 1994 Northridge). These earthquakes had similar magnitudes, but the 1994 earthquake had a higher MMI. The upper panels are the USGS Shakemaps, which are model based estimates of shaking intensity, based on Ground Motion Predicti0on Equations (GMPE; attenuation relations). The lower panels plot two different sets of data. The orange lines are regression lines that represent how shaking intensity diminishes (attenuates) with distance from the earthquake. These are regressions based upon these GMPE relations. More about GMPE relations can be found here. The dots are data from real observations made by people who have reported this on the USGS Did You Feel It? website for each of these earthquakes. More about the DYFI program can be found here.

    Some Background Materials

    • 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. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
    • Strike Slip:

      Compressional:

      Extensional:

    Documentaries


    Social Media

    References

    • Carena, S. and Supper, J., 2002. Three-dimensional imaging of active structures using earthquake aftershocks: the Northridge thrust, California in Journal of Structural Geology, v. 24, p. 887-904.
    • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
    • Fuis, G.S>, Ryberg, T., Godfrey, N.J>, Okaya, D.A., and Murphy, J.M., 2001. Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California in Geology, v. 29, no. 1. p. 15-18.
    • Fuis, G.S. et al., 2003. Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II in Geology, v. 31, no. 2, p. 171-174.
    • Hauksson, E., Jones, L.M., and Hutton, K., 1995. The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects in Journal of Geophysical Research, v., 100, no. B7, p. 12235-12355.
    • Tsutsumi, H. and Yeats, R.S., 1999. Tectonic Setting of the 1971 Sylmar and 1994 Northridge Earthquakes in the San Fernando Valley, California in BSSA, v. 89, p. 1232-1249.
    • Tucker, A.Z. and Dolan, J.F., 2001. Paleoseismologic Evidence for a ~8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault, Northern Los Angeles Metropolitan Region, California in BSSA, v. 91, no. 2, p. 232-249.

    Earthquake Report: Burma!

    There was an earthquake in Burma today! The epicenter plotted very close to the Sagaing fault (SF), a major dextral (right-lateral) strike-slip fault system, part of the plate boundary between the India and Eurasia plates. This fault system accommodates much of the dextral relative movement between these two plates.
    I initially thought this would be a strike-slip earthquake. However, the USGS fault plane solution (moment tensor, read more about them below) shows that this was a thrust (compressional) earthquake. There is a region of uplift to the west of the SF, where there is a fold and thrust belt (the Bago-Yoma Range). This region may be experiencing compression due to the relative plate motion here and the orientation of the SF (strain partitioning). There is a GPS rate map below that shows geodetic motion oblique to the SF, showing compression.
    There were two M 7.2 and M 7.4 earthquakes just to the southeast in 1930 and an earthquake in 1994. The 1994 earthquake was a dextral strike-slip earthquake, but the 1930 earthquakes are too old to have this type of analytical results on the USGS website (see Sloan et al., 2017 figure below for the M 7.3 1930 earthquake, which shows a strike-slip mechanism).

    Below is my interpretive poster

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 (and down to M ≥ 4.5 in a second poster).
    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.0 earthquake, 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.

      I include some inset figures.

      • In the upper left corner, is a map from Maurin and Rangin (2009) that shows the regional tectonics at a regional scale. The Sunda Trench is formed along the Sumatra-Andaman subduction zone, where the India plate subducts beneath the Eurasia, Burma, and Sunda plates. The Sagaing fault is the right-lateral strike-slip plate boundary fault between the Burma and Sunda plates. The black arrows show the relative plate motions between the India : Sunda and India : Burma plates. The Sagaing fault links with the Sumatra fault via the Andaman spreading ridge system. I place a blue star in the general location of today’s earthquake sequence.
      • To the right of the Maurin and Rangin (2009) map is a map from Wang et al (2014) that shows how the Sangaing fault can be broken up into segments. Warm colors are higher elevation than cooler colors. Other than national boundaries, red and black lines represent faults. I place a blue star in the general location of today’s earthquake sequence.
      • In the lower left corner is a figure from Sloan et al. (2017) that shows the fault systems here along with the GPS derived plate motions. On the left, we can see the triangle-barbed red lines, which are ~north-south striking thrust faults in the Indo-Burmese Wedge (“Ranges” on the map). I place a blue star in the general location of today’s earthquake sequence.
      • In the lower right corner is a large scale view of the earthquake faults and historic seismicity of this region (Wang et al., 2014). These authors also plotted some moment tensor data for historic earthquakes. I place a blue star in the general location of today’s earthquake sequence.
      • In the upper right corner is a map showing historic earthquakes on the Sagaing fault (Hurukawa and Maung, 2011). The right panel shows where the authors hypothesize that there is a seismic gap north of 20 degrees latitude, north of where this M 6.0 earthquake happened. I place a blue star in the general location of today’s earthquake sequence.


    • Here is the same map for USGS historic seismicity for earthquakes M ≥ 4.5. This map shows nicely how seismicity gets deeper to the east along the Sumatra-Andaman subduction zone (the Sunda Trench) along the southern part of the poster. This also shows how seismicity also deepens to the east along the Indo-Burmese we3dge (IBW), which is the convergent plate boundary system to the west of the SF.


    USGS Earthquake Pages

    Some Relevant Discussion and Figures

    • 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 Maurin and Rangin (2009) 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 (http://www.globalcmt.org/CMTsearch.html) 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.

    • Cartoon showing the tectonic evolution of the Indo-Burmese Wedge from late Miocene to present.

    • 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 the map showing the SF fault segments (Wang et al., 2014).

    • Fault segments and historical earthquakes along the central and southern parts of the Sagaing fault. Green dots show relocated epicenters from Hurukawa and Phyo Maung Maung [2011]. Dashed and solid gray boxes surround segments of the fault that ruptured in historical events. NTf = Nanting fault; Lf = Lashio fault; KMf = Kyaukme fault; PYf = Pingdaya fault; TGf = Taunggyi fault.

    • Here is the Curray (2005) plate tectonic map.

    • Tectonic map of part of the northeastern Indian Ocean. Modified from Curray (1991).

    • Here is the Sloan et al. (2017) map showing the faults and GPS derived plate motion.

    • Seismotectonic map of Myanmar (Burma) and surroundings. Faults are from Taylor & Yin (2009) with minor additions and adjustments. GPS vectors show velocities relative to a fixed India from Vernant et al. (2014), Gahalaut et al. (2013), Maurin et al. (2010) and Gan et al. (2007). Coloured circles indicateMw > 5 earthquakes from the EHB catalogue. Grey events are listed for depths <50 km, yellow for depths of 50–100 km and red for depths >100 km. The band of yellow and red earthquakes beneath the Indo-Burman Ranges represents the Burma Seismic Zone. The dashed black line shows the line of the cross-section in Figure 2.13. ASRR, Ailao Shan–Red River Shear Zone.

    • Here is a Sloan et al. (2017) map that shows fault plane solutions (including the 1930 M 7.3 SF earthquake) for earthquakes in the region.

    • Seismotectonic map of Myanmar (Burma). Faults are from Taylor & Yin (2009) with minor additions and adjustments. GPS vectors show velocities relative to a fixed Eurasia from Maurin et al. (2010). Slip rate estimates on the Sagaing Fault are given in blue and are from a, Bertrand et al. (1998); b, Vigny et al. (2003); c, Maurin et al. (2010); and d, Wang et al. (2011). Major earthquakes (Ms ≥7) are shown by yellow stars for the period 1900–76 from International Seismological Centre (2011) and by red stars for the period 1836–1900 from Le Dain et al. (1984). The location and magnitude of theMb 7.5 1946 earthquake is taken from Hurukawa&Maung Maung (2011). Earthquake focal mechanisms are taken from the GCMT catalogue (Ekström et al. 2005) and show Mw ≥5.5 earthquakes, listed as being shallower than 30 km in the period 1976–2014. IR, Irrawaddy River; CR, Chindwin River; HV, Hukawng Valley; UKS, Upper Kachin State; SF, Sagaing Fault; KF, Koma Fault. The inset panel is an enlargement of the area within the dashed grey box. It shows the dense GPS network in this area.

    • This map shows that the region where today’s M 6.0 earthquake is located is in the region of uplifted regions along the SF.

    • Regional setting, and fault geometries and uplift distribution associated with the Sagaing Fault.

    • Here is a comprehensive map showing the complicated tectonics of this region (Sloan et al., 2017).

    • Regional tectonic setting of the Andaman Sea Region modified from Morley (2017). See text for explanation of labels A–E. The locations of Figures 2.15– 2.17 are indicated.

    • This map shows how Rangin (2017) hypothesizes about the platelets formed along the plate boundary.

    • Extension of the Burma–Andaman–Sumatra microplate (shown in green). The Burma Platelet is the northern part in Myanmar. Active faults are shown in red and inactive faults in purple. The post-Santonian magnetic anomalies and associated transform faults of the Indian and Australian plates are suggested in blue. Left-lateral red arrows along the 90° E Ridge illustrate left-lateral motion between the Indian and Australian plates. India/Eurasia relative motion is shown with a yellow arrow, India/Sunda motion with purple arrows and Australia/Sunda motion with black arrows (modified from Rangin 2016).

    • This is a great summary figure from Ranging (2017) showing how these plates and platelets interact in this region.

    • Structural map of the active buckling of the Burma Platelet considered not to be rigid. The curved Sagaing Fault, Lelong, Kaladan and coastal faults outline this arched platelet. WSW extrusion of the platelet is outlined by the NE–SW diffuse dextral shear south of the South Assam Shear Zone into the north and by the left lateral Pyay-Prome shear zone in the south. The western margin (CSM: collapsing Sunda margin) of this platelet is affected by dextral wrench and active collapse of the continental margin, but no sign of active subduction was found. This platelet is bracketed tectonically between the drifted 90° E Ridge and the accreted volcanic ridges into the south and the Eurasian Buttress (Himalayas and Shillong) into the north. The East Himalaya Crustal Flow (EHCF; large curved red arrow) imaged in the East Himalaya Syntaxis (EHS) is induced by the Tibet Plateau collapse and could be an important component of the tectonic force causing the platelet buckling. The Burma Platelet is jammed between the Accreted Volcanic Ridges in the south, and the Shillong Plateau crustal block in the north, participate to the buckling of the Myanmar Platelet. BBacc, Bay of Bengal attenuated continental crust (Rangin & Sibuet 2017); CMB, Central Myanmar Basins; CMF, Churachandpur-Mao Fault (Gahalaut et al. 2013).

      References:

    • Curray, J.R., 2005. Tectonics and history of the Andaman Sea Region in Journal of Asian Earth Sciences, v. 25, p. 187-232.
    • Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
    • Hurukawa, N. and Maung, P.M., 2011. Two seismic gaps on the Sagaing Fault, Myanmar, derived from relocation of historical earthquakes since 1918 in GRL, v. 38, L01310, doi:10.1029/2010GL046099
    • Maurin, T. and Rangin, C., 2009. Structure and kinematics of the Indo-Burmese Wedge: Recent and fast growth of the outer wedge in Tectonics, v. 28, TC2010, doi:10.1029/2008TC002276
    • Rangin, C., 2017. Active and recent tectonics of the Burma Platelet in Myanmar in BARBER, A. J., KHIN ZAW & CROW, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, v. 48, p. 53–64, https://doi.org/10.1144/M48.3
    • Sloan, R.A., Elliot, J.R., Searle, M.P., and Morley, C.K., 2017. Active tectonics of Myanmar and the Andaman Sea in BARBER, A. J., KHIN ZAW & CROW, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, v. 48, p. 19–52, https://doi.org/10.1144/M48.2
    • Wang, Y., K. Sieh, S. T. Tun, K.-Y. Lai, and T. Myint, 2014. Active tectonics and earthquake potential of the Myanmar region in J. Geophys. Res. Solid Earth, 119, 3767–3822, doi:10.1002/2013JB010762.

    Earthquake Report: Iraq

    A month and a half ago, I was attending the PATA conference and an earthquake hit Iran and Iraq the night before our first field trip. Thus, I did not have the time to address this earthquake at the time. I am preparing this report in support of my annual summary.
    This was a damaging earthquake and is the most deadly for 2017. Over 500 people were killed and thousands were injured.
    I post lots of material below that was developed in the 6 weeks following the earthquake.
    There is a page here with some photos of the damage: Earthquake-Report.com.

    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 1917-2017 with magnitudes M > 6.5.
    I plot the USGS fault plane solutions (moment tensors in blue) for the M 7.3 earthquake.

    • 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. Based upon the tectonics associated with the San Andreas and Maacama faults, I interpret this M 4.3 earthquake to be a right-lateral strike-slip fault.
    • 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.

    Here are the USGS pages for the main earthquake in this sequence.

    I include some inset figures.

    • In the lower right corner I include a map that shows the major plate boundary and major crustal faults in the region, as well as relative plate motions plotted as arrows (Taymaz et al.,
      2007). I place a green star in the general location of the M 7.3 earthquake. Note that this M 7.3 earthquake happened along the Bitis-Zagros Fold Belt.
    • In the upper right corner is a map that shows the results of interfereometric RADAR analyses as prepared by GSI in Japan. This map shows a region of subsidence to the southwest of the M 7.3 epicenter (the largest orange circle) and a region of uplift to the northeast of this M 7.3 earthquake. More about this map below.
    • To the left of this interferogram, I include a basic tectonic map of this region (Woudloper, 2009). Maps with local (larger) scale have much more detailed views of the faulting. I place a green star in the general location of this M 7.3 earthquake.
    • In the upper left corner are two maps that show how Earth’s surface moved during the earthquake (and shortly afterwards). The left panel shows east-west motion and the right panel shows up-down motion (this looks similar to the figure in the upper right corner.
    • In the lower left corner I place a map that shows the large scale details of the crustal faults in the Bitis-Zagros Fold Belt (Allen et al., 2013). I place a green star in the general location of this M 7.3 earthquake.
    • To the right of this fault map is a cross section A-A.’ The location of this cross section is designated by a blue line on the map in the lower left corner, as well as on the main interpretive poster map.


    • Here is a comparison between the “Did You Feel It?” map and the Shakemap. Both maps represent shaking intensity with the same scale, the MMI scale (described above). The DYFI map on the left is based on peoples’ observations as they report using the USGS DYFI website. The map on the right is the result of numerical simulations of shaking intensity. Below each map are regressions of those data.

    • This map shows the plate boundary and intraplate faults of the region. Also shown are the relative plate motions as black arrows. Note how the Bitis-Zagros Fold Belt (BZFB) is a dextral oblique (right-lateral thrust) fault system. This fault system is part of the Alpide belt, which is oriented parallel to the Arabia-Anatolia relative plate motion (ergo the strike-slip motion).

    • (a) Seismicity of the Eastern Mediterranean region and surroundings reported by USGS–NEIC during 1973–2007 with magnitudes for M . 3 superimposed on a shaded relief map derived from the GTOPO-30 Global Topography Data taken after USGS. Bathymetry data are derived from GEBCO/97–BODC, provided by GEBCO (1997) and Smith & Sandwell (1997a, b). (b) Summary sketch map of the faulting and bathymetry in the Eastern Mediterranean region, compiled from our observations and those of Le Pichon & Angelier (1981), Taymaz (1990), Taymaz et al. (1990, 1991a, b); S¸arogˇlu et al. (1992), Papazachos et al. (1998), McClusky et al. (2000) and Tan & Taymaz (2006). Large black arrows show relative motions of plates with respect to Eurasia (McClusky et al. 2003). Bathymetry data are derived from GEBCO/97–BODC, provided by GEBCO (1997) and Smith & Sandwell (1997a, b). Shaded relief map derived from the GTOPO-30 Global Topography Data taken after USGS. NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; NEAF, North East Anatolian Fault; EPF, Ezinepazarı Fault; PTF, Paphos Transform Fault; CTF, Cephalonia Transform Fault; PSF, Pampak–Sevan Fault; AS, Apsheron Sill; GF, Garni Fault; OF, Ovacık Fault; MT, Mus¸ Thrust Zone; TuF, Tutak Fault; TF, Tebriz Fault; KBF, Kavakbas¸ı Fault; MRF, Main Recent Fault; KF, Kagˇızman Fault; IF, Igˇdır Fault; BF, Bozova Fault; EF, Elbistan Fault; SaF, Salmas Fault; SuF, Su¨rgu¨ Fault; G, Go¨kova; BMG, Bu¨yu¨k Menderes Graben; Ge, Gediz Graben; Si, Simav Graben; BuF, Burdur Fault; BGF, Beys¸ehir Go¨lu¨ Fault; TF, Tatarlı Fault; SuF, Sultandagˇ Fault; TGF, Tuz Go¨lu¨ Fault; EcF, Ecemis¸ Fau; ErF, Erciyes Fault; DF, Deliler Fault; MF, Malatya Fault; KFZ, Karatas¸–Osmaniye Fault Zone.

    • The Alpide Belt, shown in this map, is a convergent plate boundary that extends from Australia to Portugal. This map shows the westernmost extent of this system. The convergence here drives uplift of the Himalayas and the European Alps. Subduction along the Makran and Sunda subduction zones are also part of this system.

    • This is a great map showing some details of the tectonics associated with the Arabia plate (Stern and Johnson, 2010).

    • Simpli”ed map of the Arabian Plate, with plate boundaries, approximate plate convergence vectors, and principal geologic features. Note location of Central Arabian Magnetic Anomaly (CAMA).

    • This map (Allen et al., 2013) shows focal mechanisms (fault plane solutions) for earthquakes associated with the BZFB. GPS velocities are also plotted in blue (rates of motion at points on the earth, measured in mm per year), relative to Iran.

    • (a) Regional topography and seismicity of the Arabia-Eurasia collision. Large dots are epicenters of earthquakes of M >6 from 1900 to 2000 [Jackson, 2001], small dots are epicenters from the EHB catalogue 1964–1999, M >5. Red arrows show GPS-derived velocity with respect to Asia from Sella et al. [2002]. A= Alborz; TIP = Turkish-Iranian plateau; Z = Zagros. (b) Seismicity of the Zagros: focal mechanisms reported in Nissen et al. [2011] and references therein. Note the scarcity of thrusts above the smoothed 1250m regional elevation contour (derived using a Gaussian filter with a radius of 50 km). Earthquake epicenters are accurate to within 20 km [Nissen et al., 2011]. GPS vectors are from Walpersdorf et al. [2006]. MZRF =Main Zagros Reverse Fault (Zagros suture).

    • This map shows a detailed view of faults and folds in the BZFB (Allen et al., 2013).

    • (a) Location map and major structures of the Zagros Simply Folded Belt, Iran. Derived from NIOC [1975, 1977], Berberian [1995], Hessami et al. [2001], Blanc et al. [2003], Agard et al. [2005], and Babaie et al. [2006]. Key to fault abbreviations: B = Borazjan; Iz = Izeh; K= Kazerun; KB= Kareh Bas; Kh = Khanaqin; S = Sarvestan; SP = Sabz-Pushan; BL = Balarud Line; A= Kuh-e Asmari. b) Earthquake epicentres across the Zagros, from Nissen et al. [2011] and references therein, divided by fault type. MZRF =Main Zagros Reverse Fault.


    • This is cross section A-A’ from the map above (also on poster). Note the thrust faults and the strike-slip faults represented in this section (Allen et al., 2013). While this section is to the south of the M 7.3 earthquake, it still represents the generalized tectonics in the region (dextral oblique plate boundary).

    • (a) Cross-section through the Dezful Embayment and the Bakhtyari Culmination.

    • The Geospatial Information Authority of Japan (GSI) conducted some analyses using Synthetic Aperture Radar (SAR). “Two or more line-of-sight displacements with different observing directions can be decomposed to quasi east-west and up-down components.” They describe their interpretation below.



    • Large displacement (~90 cm upward and ~50 cm westward) has been detected around 20 km NNW of Sarpol-e Zahab. Around the epicenter, ~30 cm downward and ~35 cm westward displacement has been detected.

    • Here is a map that displays an estimate of seismic hazard for the region (Jenkins et al., 2010). This comes from Giardini et al. (1999).

    • The Global Seismic Hazard Map. Peak ground acceleration (pga) with a 10% chance of exceedance in 50 years is depicted in m/s2. The site classification is rock everywhere except Canada and the United States, which assume rock/firm soil site classifications. White and green correspond to low seismicity hazard (0%-8%g), yellow and orange correspond to moderate seismic hazard (8%-24%g), pink and dark pink correspond to high seismicity hazard (24%-40%g), and red and brown correspond to very high seismic hazard (greater than 40%g).

    Other Social Media Posts

    • Here is a plot showing historic seismicity from Dr. Jascha Polet (Cal Poly Pomona Seismologist).

      References

    • Allen, M.B., Saville, C., Blac, E.K-P., Talebian, M., and Nissen, E., 2013. Orogenic plateau growth: Expansion of the Turkish-Iranian Plateau across the Zagros fold-and-thrust belt in Tectonics, v. 32, p. 171-190, doi:10.1002/tect.20025
    • Giardini, D., Grunthal, G., Shedlock, K., Zhang. P., and Global Seismic Hazards Program, 1999. Global seismic hazards map: Accessed on Jan. 9, 2007 at http://www.seismo.ethz.ch/GSHAP.
    • Jenkins, Jennifer, Turner, Bethan, Turner, Rebecca, Hayes, G.P., Sinclair, Alison, Davies, Sian, Parker, A.L., Dart, R.L., Tarr, A.C., Villaseñor, Antonio, and Benz, H.M., compilers, 2013, Seismicity of the Earth 1900–2010 Middle East and vicinity (ver 1.1, Jan. 28, 2014): U.S. Geological Survey Open-File Report 2010–1083-K, scale 1:7,000,000, https://pubs.usgs.gov/of/2010/1083/k/.
    • Stern, R.J. and Johnson, P., 2010. Continental lithosphere of the Arabian Plate: A geologic, petrologic, and geophysical synthesis in Earth-Science Reviews, v. 101, p. 29-67.
    • Taymaz, T., Yilmaz, Y., and Dilek, Y., 2007. The geodynamics of the Aegean and Anatolia: introduction in Geological Society, London, Special Publications, v. 291; p. 1-16, doi:10.1144/SP291.1
    • Woudloper, 2009. Tectonic map of southern Europe and the Middle East, showing tectonic structures of the western Alpide mountain belt.

    Earthquake Report: China #2!

    We had another earthquake in China today! This one along the northern Tian Shan Mountains, on the other side of the orogenic wedge from the earthquakes from earlier today. This M 6.3 earthquake is along a thrust fault, while the earlier M 6.5 earthquake had a strike-slip (slightly oblique) sense of motion.
    Here is the USGS website for this M 6.3 earthquake.
    This is my Earthquake Report for the M 6.5 earthquake from earlier today.
    Here is a report from The Earth Observatory of Singapore.
    Here is a report from earthquake-report.com.

    Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS epicenters from 2007-2017 for magnitudes M ≥ 4.5.
    I also include the USGS moment tensor for today’s earthquake.

    • 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. Based upon the series of earthquakes and the mapped faults, I interpret this M 5.1 earthquake as a left-lateral strike-slip earthquake related to slip associated with the Gorda plate.
    • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

      I include some inset figures in the poster.

    • In the upper left corner are two figures showing the tectonic regime (upper panel) and the major fault locations overlain upon topography (lower panel). This is from a paper that discusses how convergence in the Late Carboniferous contributed to the regional tectonics today (Han et al., 2011). I include blue stars in the general location of the M 6.3 earthquake.
    • In the upper right corner is another large scale geologic map of the region (Han et al., 2011). This map shows the different geologic units, sorted by age. Major cities and basins are labeled.
    • In the lower left corner I include a figure from Yang et al. (2008) that shows the medium scale fault mapping in this region (more detailed than the other maps on the poster). GPS velocities and slip rates calculated for various faults in the region are also plotted. Today’s earthquake appears associated with the Northern Tianshan Marginal fault, a thrust fault that crosses the Tainshan Mountains (See Li et al., 2016 map below).


    • Here is the poster from the M 6.5 earthquake.

    Below are some figures that help explain today’s seismicity

    • Here is a figure from Han et al. (2011) that shows the major tectonic and geologic features. I include a subset of their very long figure caption below in blockquotes (you are welcome, for my not including their entire caption).

    • (a) The Central Asian Orogenic Belt is the tectonic assembly of continental and oceanic terranes between the European craton in the west, the Siberian craton in the east, and the North China and Tarim cratons in the south due to closure of the Paleo-Asian Ocean in the Phanerozoic (modified from Şengör et al., 1993; Jahn et al., 2000). (b) Topographic and sketch tectonic map of the western segment of the Tian Shan in China–Kyrgyzstan contiguous regions. KNTS — Kyrgyzstan North Tian Shan, KMTS — Kyrgyzstan Middle Tian Shan, KSTS — Kyrgyzstan South Tian Shan, and CSTS — Chinese South Tian Shan, AISNQF — Atbashy–Inylchek–South Nalati–Qawablak Fault, TFF — Talas–Fergana Fault, NL — Nikolaev Line, NTT — North Tarim Thrust, and NTSF — North Tian Shan Fault.

    • Here is a larger scale figure from Han et al. (2011) that shows the geology and faulting. I include their figure caption below in blockquotes.

    • Geological map of the western segment of the South Tian Shan Orogen and adjacent tectonic units (modified from IGCAGS, 2006). AISNQF — Atbashy–Inylchek–South Nalati–Qawablak Fault, TFF — Talas–Fergana Fault, NL — Nikolaev Line, and NTT — North Tarim Thrust.

    • Here is a series of figures from Yang et al. (2008) that show the earthquake fault slip rates and block rotation rates, along with Global Positioning System (GPS) analyses results. The uppermost figure is in the interpretive poster above. The middle panel shows the GPS locations and GPS transect regions. The lower two panels show GPS velocities along the transects plotted in the middle panel map. The profiles show how tectonic strain is accumulating across the faults in the region (where there are inflections in the velocities). I include their figure caption below in blockquotes.

    • Horizontal movement velocity field of the Tianshan Mountains relative to stable Eurasia plate. The arrows show movement rate and its orienta-tion with the error ellipse, at a 95% confidence level. TA, Talas-Fergana fault; KT, Kindyktash fault; BOK, Boluokenu fault; PMT, Main Pamir thrust; MAK, Markansu fault; PC, Puchang fault; MDKA, Maidan-Karatieke fault; KT, Kepingtage thrust fault; BL, Beiluntai fault; QL, Qiulitage fracture; XD, Xingdi fracture; BLK, Balikun fracture.


      Profile configuration. A, B, C, D, E, and F are longitudinal profiles across the Tianshan Mountains for Figure 4. G, H, I, J, and K are profiles across the WN-SE trending strike-slip faults for Figure 3.


      The velocity profiles across the WN-SE trending strike-slip faults in the Tianshan Mountains. G and H, the northwest and southeast sections of Talas-Fergana fault respectively; I, indyktash fault; J and K, the west and east sections of Boluokenu fault respectively. The horizontal axis represents distance from GPS sites to central point of profile, and the vertical axis shows GPS velocity.


      GPS velocity profiles across the Tianshan Mountains illustrated in Figure 2. The horizontal axis represents distance from GPS sites to central point of profile, and the vertical axis shows GPS velocity.

    • Here are some figures from Li et al. (2016) that shows detailed fault maps for this region, along with a low-angle oblique block diagram for the region outlined in yellow on the map. The M 6.3 earthquake is just to the west of the block diagram, but the structure is representative.I include their figure caption below in blockquotes.

    • Geomorphological and Tectonic features of the Tianshan Mountains. (A) Study area and earthquakes that were used for the formation of the receiver function image, which were selected from more than 500 earthquakes from a USGS database that was created during this study’s data collection. (B) Geomorphologic and tectonic features of the Tianshan Mountains, which show their segmentation with latitude and zoning with longitude, Cenozoic faults36 and Paleozoic subduction zones9,11–14,62, the asymmetry of structural deformation near the surface on both sides12, the crust’s velocity and direction from GPS data60,61, and the clockwise rotation of the Tarim Blocks22,40. The primary DEM data that were used for the geomorphological features in (B) are in the SRTM GTOPO 30 format and were provided by NASA and downloaded from http://glcf.umiacs.umd.edu in 2010.


      Cartoon map of Segment C in the Tianshan Mountains. (A) Geomorphologic features of Segment C in the Tianshan Mountains. (B) Deep structures and the driving mechanism for the uplift of the mountains during the Cenozoic. The abbreviations are the same as those in Fig. 2. The primary DEM data that were used for the geomorphological features in (A) are in the SRTM GTOPO 30 format and were provided by NASA and downloaded from http://glcf.umiacs.umd.edu in 2010.

      References:

    • Han, B-F., He, G-Q., Wang, X-C., and Guo, Z-J., 2011. Late Carboniferous collision between the Tarim and Kazakhstan–Yili terranes in the western segment of the South Tian Shan Orogen, Central Asia, and implications for the Northern Xinjiang, western China in Earth-Science Reviews, v. 109, p. 74-93
    • Kirby, E., Harkins, N., Wang, E., Shi, X., Fan, C., and Burbank, D., 2007. Slip rate gradients along the eastern Kunlun fault in Tectonics, v. 26, doi:10.1029/2006TC002033
    • Li, J. et a., 2016., Mantle Subduction and Uplift of Intracontinental Mountains: A Case Study from the Chinese Tianshan Mountains within Eurasia in Scientific Reports, DOI: 10.1038/srep28831
    • Yang, S., et al., 2008. The deformation pattern and fault rate in the Tianshan Mountains inferred from GPS observations in Science in China Series D: Earth Sciences, v. 51, no. 8, p. 1064-1080
    • Yong, L., Allen, P.A., Densmore, A.L., and Qiang, X., 2003. Evolution of the Longmen Shan Foreland Basin (Western Sichuan, China) during the Late Triassic Indosinian Orogeny in Basin Research, v. 15, p. 115-138
    • Yong, Z., HongSheng, M., Jian, L., SiDao, N., YingChun, L., and ShengJi, W./, 2009. Source mechanism of strong aftershocks (Ms≥5.6) of the 2008/05/12 Wenchuan earthquake and the implication for seismotectonics in Science in China Series D: Earth Sciences, v. 52, no. 6, p. 739-753, doi: 10.1007/s11430-009-0074-3
    • Zheng, Y-F., Xiao, W-J., and Zhao, G., 2013. Introduction to tectonics of China in Gondwana Research, v. 23, p. 1189-1206.

    Earthquake Report: Turkey

    We just had a good shaker in western Turkey. At the moment, there are over 400 reports of ground shaking to the USGS “Did you Feel It?” web page. The USGS PAGER report estimates that there may be some casualties (though a low number of them), but that the economic loss estimate is higher (35% chance of between 10 and 100 million USD).
    This earthquake appears to have been along a normal fault named either the Bodum fault (NOA; Helenic Seismic Network) or the Ula-Oren fault (GreDASS; Greek Database of Seismogenic Sources). The inset map shows the faults and fault planes from the GreDASS database. A third name for this fault is the Gökova fault (Kurt et al., 1999).
    Here is the USGS website for this earthquake.
    There is lots of information on the European-Mediterranean Seismological Centre (EMSC) page here.

    Below is my interpretive poster for this earthquake.

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS earthquake epicenters from 1917-2017 for magnitudes M ≥ 6.5. This is also the time and magnitude range of earthquakes in the inset map.

    • 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 plot moment tensors for the M 6.3 earthquake. Based upon the series of earthquakes and the mapped faults, I interpret this M 6.7 earthquake to be a normal fault (extensional) earthquake.
    • 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 from the (Database of Individual Seismogenic Sources (DISS), Version 3.2.0), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

      I include some inset figures in the poster.

    • In the lower left corner I include a map of the regional tectonics (Dilek and Sandvol, 2009). I place a green star in the general location of today’s M 6.7 earthquake.
    • In the lower right corner is a figure from Jolivet et al. (2013) that shows focal mechanisms for earthquakes across the Aegean-Anatolian region. Earthquakes plotted in the region of today’s M 6.7 (the green star) are all normal (extensional) earthquakes (with one extensional oblique).
    • In the upper right corner is a tectonic map of western Eurasia and northern Africa (Dilek, 2006). Today’s earthquake lies near the cross section G-G (in yellow). I also show the general location of this cross-section on the main map.
    • Below this map is a figure showing a north-south cross section through this region (Dilek, 2006), G-G on the above map. This shows the subduction zone in the south, the transform fault (North Anatolian fault) in the north, and the Aegean Extensional Province in the center. Today’s earthquake is along the southern boundary of the core complex, which is in the center of this extensional province.
    • In the upper left corner is a larger scale map showing the same earthquakes as the main map. I also include the faults and fault planes from the GreDASS database. I also label the larger earthquakes in this region. Note the 2017 M 6.3 Lesbos earthquake in the north. Here is my earthquake report for that earthquake. Note the flare up of seismicity in the 1950s, possibly beginning in 1948.


    • Here is the same poster, but with USGS earthquake epicenters from 2007-2017 with magnitude M ≥ 4.5.

    • There was a small tsunami recorded at the Bodum tide gage. Here is the source.

    • Here is the tectonic map from Dilek and Sandvol (2009).

    • Tectonic map of the Aegean and eastern Mediterranean region showing the main plate boundaries, major suture zones, fault systems and tectonic units. Thick, white arrows depict the direction and magnitude (mm a21) of plate convergence; grey arrows mark the direction of extension (Miocene–Recent). Orange and purple delineate Eurasian and African plate affinities, respectively. Key to lettering: BF, Burdur fault; CACC, Central Anatolian Crystalline Complex; DKF, Datc¸a–Kale fault (part of the SW Anatolian Shear Zone); EAFZ, East Anatolian fault zone; EF, Ecemis fault; EKP, Erzurum–Kars Plateau; IASZ, Izmir–Ankara suture zone; IPS, Intra–Pontide suture zone; ITS, Inner–Tauride suture; KF, Kefalonia fault; KOTJ, Karliova triple junction; MM, Menderes massif; MS, Marmara Sea; MTR, Maras triple junction; NAFZ, North Anatolian fault zone; OF, Ovacik fault; PSF, Pampak–Sevan fault; TF, Tutak fault; TGF, Tuzgo¨lu¨ fault; TIP, Turkish–Iranian plateau (modified from Dilek 2006).

    • Below is a series of figures from Jolivet et al. (2013). These show various data sets and analyses for Greece and Turkey.
    • Upper Panel (A): This is a tectonic map showing the major faults and geologic terranes in the region. The fault possibly associated with today’s earthquake is labeled OU on the map, for the Ula-Oren fault.
    • Lower Panel (B): This shows historic seismicity for the region. Note the general correlation with the faults in the upper panel.

    • A: Tectonic map of the Aegean and Anatolian region showing the main active structures
      (black lines), the main sutures zones (thick violet or blue lines), the main thrusts in the Hellenides where they have not been reworked by later extension (thin blue lines), the North Cycladic Detachment (NCDS, in red) and its extension in the Simav Detachment (SD), the main metamorphic units and their contacts; AlW: Almyropotamos window; BD: Bey Daglari; CB: Cycladic Basement; CBBT: Cycladic Basement basal thrust; CBS: Cycladic Blueschists; CHSZ: Central Hellenic Shear Zone; CR: Corinth Rift; CRMC: Central Rhodope Metamorphic Complex; GT: Gavrovo–Tripolitza Nappe; KD: Kazdag dome; KeD: Kerdylion Detachment; KKD: Kesebir–Kardamos dome; KT: Kephalonia Transform Fault; LN: Lycian Nappes; LNBT: Lycian Nappes Basal Thrust; MCC: Metamorphic Core Complex; MG: Menderes Grabens; NAT: North Aegean Trough; NCDS: North Cycladic Detachment System; NSZ: Nestos Shear Zone; OlW: Olympos Window; OsW: Ossa Window; OSZ: Ören Shear Zone; Pel.: Peloponnese; ÖU: Ören Unit; PQN: Phyllite–Quartzite Nappe; SiD: Simav Detachment; SRCC: South Rhodope Core Complex; StD: Strymon Detachment; WCDS: West Cycladic Detachment System; ZD: Zaroukla Detachment. B: Seismicity. Earthquakes are taken from the USGS-NEIC database. Colour of symbols gives the depth (blue for shallow depths) and size gives the magnitude (from 4.5 to 7.6).

    • Upper Panel (C): These red arrows are Global Positioning System (GPS) velocity vectors. The velocity scale vector is in the lower left corner. The main geodetic (study of plate motions and deformation of the earth) signal here is the westward motion of the North Anatolian fault system as it rotates southward as it traverses Greece. The motion trends almost south near the island of Crete, which is perpendicular to the subduction zone.
    • Lower Panel (D): This map shows the region of mid-Cenozoic (Oligo-Miocene) extension (shaded orange). It just happens that there is still extension going on in parts of this prehistoric extension.

    • C: GPS velocity field with a fixed Eurasia after Reilinger et al. (2010) D: the domain affected by distributed post-orogenic extension in the Oligocene and the Miocene and the stretching lineations in the exhumed metamorphic complexes.

    • Upper Panel (E): This map shows where the downgoing slab may be located (in blue), along with the volcanic centers associated with the subduction zone in the past.
    • Lower Panel (F): This map shows the orientation of how seismic waves orient themselves differently in different places (anisotropy). We think seismic waves travel in ways that reflects how tectonic strain is stored in the earth. The blue lines show the direction of extension in the asthenosphere, green lines in the lithospheric mantle, and red lines for the crust.

    • E: The thick blue lines illustrate the schematized position of the slab at ~150 km according to the tomographic model of Piromallo and Morelli (2003), and show the disruption of the slab at three positions and possible ages of these tears discussed in the text. Velocity anomalies are displayed in percentages with respect to the reference model sp6 (Morelli and Dziewonski, 1993). Coloured symbols represent the volcanic centres between 0 and 3 Ma after Pe-Piper and Piper (2006). F: Seismic anisotropy obtained from SKS waves (blue bars, Paul et al., 2010) and Rayleigh waves (green and orange bars, Endrun et al., 2011). See also Sandvol et al. (2003). Blue lines show the direction of stretching in the asthenosphere, green bars represent the stretching in the lithospheric mantle and orange bars in the lower crust.

    • Upper Panel (G): This is the map showing focal mechanisms in the poster above. Note the strike slip earthquakes associated with the North Anatolian fault and the thrust/reverse mechanisms associated with the thrust faults.

    • G: Focal mechanisms of earthquakes over the Aegean Anatolian region.

    • Here is a figure showing a north-south cross section through this region, from ~95 million years ago until about 2 million years ago (Dilek and Sandvol, 2009). This figure shows how the regional tectonics have developed over time, with the modern subduction zone in the south, the North Anatolian transform fault in the north, and an extensional metamorphic core complex in the center (“Core Complex” on cross section). Today’s earthquake is along the southern boundary of this core complex.

    • Late Mesozoic–Cenozoic geodynamic evolution of the western Anatolian orogenic belt as a result of collisional and extensional processes in the upper plate of north-dipping subduction zone(s) within the Tethyan realm.

    • This is a great figure showing another interpretation to explain the extension in this region (slab rollback and mantle flow) from Brun and Sokoutis (2012).

    • Mantle flow pattern at Aegean scale powered by slab rollback in rotation around vertical axis located at Scutary-Pec (Albania). A: Map view of flow lines above (red) and below (blue) slab. B: Three-dimensional sketch showing how slab tear may accommodate slab rotation. Mantle fl ow above and below slab in red and blue, respectively. Yellow arrows show crustal stretching.

    • Finally, here is a map showing tectonic domains (Taymaz et al., 2007).

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

    References

    • Basili R., G. Valensise, P. Vannoli, P. Burrato, U. Fracassi, S. Mariano, M.M. Tiberti, E. Boschi (2008), The Database of Individual Seismogenic Sources (DISS), version 3: summarizing 20 years of research on Italy’s earthquake geology, Tectonophysics, doi:10.1016/j.tecto.2007.04.014
    • Brun, J.-P., Sokoutis, D., 2012. 45 m.y. of Aegean crust and mantle flow driven by trench retreat. Geol. Soc. Am., v. 38, p. 815–818.
    • Caputo, R., Chatzipetros, A., Pavlides, S., and Sboras, S., 2012. The Greek Database of Seismogenic Sources (GreDaSS): state-of-the-art for northern Greece in Annals of Geophysics, v. 55, no. 5, doi: 10.4401/ag-5168
    • Dilek, Y., 2006. Collision tectonics of the Mediterranean region: Causes and consequences in Dilek, Y., and Pavlides, S., eds., Postcollisional tectonics and magmatism in the Mediterranean region and Asia: Geological Society of America Special Paper 409, p. 1–13
    • Dilek, Y. and Sandvol, E., 2006. Collision tectonics of the Mediterranean region: Causes and consequences in Dilek, Y., and Pavlides, S., eds., Postcollisional tectonics and magmatism in the Mediterranean region and Asia: Geological Society of America Special Paper 409, p. 1–13
    • DISS Working Group (2015). Database of Individual Seismogenic Sources (DISS), Version 3.2.0: A compilation of potential sources for earthquakes larger than M 5.5 in Italy and surrounding areas. http://diss.rm.ingv.it/diss/, Istituto Nazionale di Geofisica e Vulcanologia; DOI:10.6092/INGV.IT-DISS3.2.0.
    • Ersoy, E.Y., Cemen, I., Helvaci, C., and Billor, Z., 2014. Tectono-stratigraphy of the Neogene basins in Western Turkey: Implications for tectonic evolution of the Aegean Extended Region in Tectonophysics v. 635, p. 33-58.
    • Jolivet, L., et al., 2013. Aegean tectonics: Strain localisation, slab tearing and trench retreat in Tectonophysics, v. 597-598, p. 1-33
    • Kokkalas, S., et al., 2006. Postcollisional contractional and extensional deformation in the Aegean region in GSA Special Papers, v. 409, p. 97-123.
    • Kurt, H., Demirbag, E., and Kuscu, I., 1999. Investigation of the submarine active tectonism in the Gulf of Gokova, southwest Anatolia–southeast Aegean Sea, by multi-channel seismic reflection data in Tectonophysics, v. 305, p. 477-496
    • Papazachos, B.C., Papadimitrious, E.E., Kiratzi, A.A., Papazachos, C.B., and Louvari, E.k., 1998. Fault Plane Solutions in the Aegean Sea and the Surrounding Area and their Tectonic Implication, in Bollettino Di Geofisica Terorica Ed Applicata, v. 39, no. 3, p. 199-218.
    • Taymaz, T., Yilmaz, Y., and Dilek, Y., 2007. The geodynamics of the Aegean and Anatolia: introduction in Geological Society Special Publications, v. 291, p. 1-16.
    • Wouldloper, 2009. Tectonic map of southern Europe and the Middle East, showing tectonic structures of the western Alpide mountain belt. Only Alpine (tertiary) structures are shown.

    Earthquake Report: Arctic!

    Well, I put this and my next earthquake report together shortly after these earthquakes happened, but I was otherwise busy before I could present them online.
    There was an earthquake in the Arctic on 2017.01.08, along the channel of one of the major northwest passages. At first, I thought: “intraplate!” This earthquake is not along a plate boundary (though there are many examples of intraplate earthquakes). What led to this seismicity? Perhaps it is due to intraplate deformation along pre-existing fault systems. Perhaps it is related to internal deformation of the crust due to stressed from post-glacial rebound. I am still not sure. There is sparce historic seismicity here and I only spent a few hours looking through the literature. If anyone has an explanation, I would love to hear their ideas. One confounding factor is that this region is covered in ice at least most of the year, so there is probably a limitation to the subsurface geophysical exploration data (e.g. seismic reflection/refraction, seismic tomography, etc.).

    Below is my interpretive poster for this earthquake.

    I plot the seismicity from the 1900 until today, with color representing depth and diameter representing magnitude (see legend). Here is my html query for the USGS NEIC database. I present USGS moment tensors for some of the larger magnitude earthquakes. There was an earthquake with a magnitude of M = 7.7 in 1933. There has been some work on that earthquake, so I plot the focal mechanism for that earthquake from Bent (2002).
    There have been earthquakes in this region, notably a M 5.8 earthquake in 1987, which has a focal mechanism (plotted) almost identical to this 2017 M 5.8 earthquake (can we say “characteristic?” heheh). There was another earthquake after the 2016 M 5.8, a M = 85.2 on 2016.01.09. Below are some of the earthquakes plotted on this interpretive poster below. The earthquakes with moment tensors or focal mechanisms have their magnitude in bold.
    Anthony Lomax prepared a first motion mechanism for this 2017.01.08 M 5.8 earthquake that suggest a more strike-slip earthquake. Lomax states they interpret these data to be of poor quality (probably due to the azimuthal seismologic coverage). I present the Lomax focal mechanism below. The USGS moment tensor suggests that this is compressional and slightly oblique (largely consistent with the Lomax focal mechanism).
    This earthquake may be along faults related to the Devon fault or others that may be responsible for the formation of Barrow Strait. It is difficult to tell without more data.

    • I place a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. The moment tensor shows northeast-southwest compression, perpendicular to the compression at the “Big Bend.”
    • I also include the shaking intensity on the map (as a raster, not as contours, due to the small scale of this map). These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
    • I include some additional information like the possible location of the Devon fault, as published by Indigenous and Northern Affairs Canada (green line). I include the approximate location of the cross section from this publication (with some insets described below) as a dashed yellow line.

      I include some inset figures in the poster (there is not much literature about the tectonics of this region, in my very brief review: please let me know of any additional sources!).

    • In the lower left corner is a map that shows the estimated thickness of sediment overlying Cretaceous oceanic crust. The location of the cross section (on the right) is demarcated as a dashed yellow line on this map and on the main map. The general region mapped in this inset map is outlined as a gray rectangle on the main map.
    • To the right of the map is a cross section from the same publication. Note the sub-vertical faults, with the basin forming Devon fault on the northern boundary of the Lancaster Basin (labeled “Lancaster Sound” on the map.


    • Here is a map showing seismicity in the region of the 1933 earthquake (Bent, 2002). I include the original figure caption below in blockquote.

    • Seismicity in and near Baffin Bay. Circles (scaled to magnitude) indicate epicentres of earthquakes of magnitude less than 6.0. Larger earthquakes are represented by stars and date. Earthquakes of magnitude 5.0 and greater are plotted for the period 1900–1996, magnitudes 4.0–4.9 for 1960–1996, 3.0–3.9 for 1970–1996 and 2.0–2.9 for 1980–1996. See the text for completeness periods for various magnitudes. Epicentres are from the Canadian Earthquake Epicentre File (CEEF). The 2000 m bathymetry contour is indicated by the dashed line. Black triangles indicate communities in which the London Times reported that the 1933 earthquake had been felt; white triangles are communities in which the earthquake had been reported not felt; grey triangles are communities shown for geographic reference only.

    • Here is the map of the Cenezoic/Mesozoic sedimentary basins (INAC). The major basin bounding faults are labeled (e.g. Devon fault)

    • lsopach (thousands of metres) of Mesozoic-Cenozoic strata, Lancaster Sound and adjacent areas.

    • Here is the cross section designated by the dashed yellow lines in the interpretive poster. Note that the Devon fault is the big player on the northern boundary of the basin. With this single cross section, it is difficult to understand the structural relations between these different faults.

    • Schematic cross-section, Lancaster Sound Basin to Bylot Basin.

    References:

    Earthquake Report: 1994 Northridge!

    Today is the commemoration of the 1994 M 6.7 Northridge Earthquake. I was living in Arcata at the time (actually in my school bus in a driveway to save money). I remember calling my mom from a pay phone to talk about the earthquake (I did not have a phone at the time; turns out Pac Bell did not want to install a phone in my bus and I probably could not afford it anyways). She lived in Long Beach, but the damage affected the entirety of southern California.
    https://earthquake.usgs.gov/earthquakes/eventpage/ci3144585/executive
    I put together a commemorative #EarthquakeReport interpretive poster to discuss the tectonics of the region. The San Andreas fault (SAF) system is the locus of ~75% of the Pacific-North America plate boundary motion. The SAF is in some places a mature fault with a single strand and in other places, there are multiple strands (e.g. the Elsinore, San Jacinto, and SAF in southern CA or the Maacama, Bartlett Springs, and SAF in northern CA). In southern CA, the SAF makes a bend (called the “Big Bend”) that forms a region of compression. This compression is realized in the form of thrust faults and folds, creating uplift forming the mountain ranges like the Santa Monica Mountains. Some of these thrust faults breach the ground surface and some are blind (they don’t reach the surface).
    In 1971 there was a large earthquake (M 6.7) that caused tremendous amounts of damage in southern CA. A hospital was built along one of the faults and this earthquake caused the hospital to collapse killing many people. The positive result of this earthquake is that the Alquist Priolo Act was written and passed in the state legislature. I plot the moment tensor for the 1971 earthquake (Carena and Suppe, 2002).
    Then, over 2 decades later, there was the M 6.7 Northridge Earthquake. This earthquake was very damaging. Here is a page that links to some photos of the damage. I plot the moment tensor for this earthquake in my interpretive poster below.

    Below the 2017 report, see the UPDATE from 2019, the 25 Year Commemoraion

    Below is my interpretive poster for this earthquake.

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

    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. The moment tensor shows northeast-southwest compression, perpendicular to the compression at the “Big Bend.”
    • 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 plot the fault lines from the USGS Quaternary Fault and Fold Database. I include a legend showing how colors represent the USGS estimates for the most recent activity along each of these faults. More can be found about this database here.

      I include some inset figures in the poster.

    • In the upper right corner is a map of the faults in southern CA (Tucker and Dolan, 2001). Strike-slip faults (like the SAF) have arrows on either side of the fault desginating the relative motion across the fault. Thrust faults have triangle barbs showing the convergence direction (the triangles are on the side of the fault that is dipping into the Earth).
    • To the left of this fault map is a low-angle oblique block diagram showing the configuration of thrust faults in the region of the Big Bend. These thrust faults are forming the topography in southern CA. The 1971 and 1994 earthquakes occurred along thrust faults similar to the ones shown in this block diagram.
    • In the lower right corner is a figure that shows some historic earthquakes in this region (Hauksson et al., 1995). The upper panel shows the affected areas from these earthquakes in hatchured polygons. The lower panel shows the focal mechanisms for these earthquakes.
    • In the upper left corner I include the USGS “Did You Feel It?” shakemap. This map uses the MMI scale mentioned above. These are results from the USGS DYFI reporting website. So, these are real observations, compared to the MMI contours in the main map, which is based upon ground motion modeling of the earthquake.
    • Below the DYFI map is a cross section of seismicity associated with the 1971 and 1994 earthquakes (Tsutsumi and Yeats, 1994). 1971 main and aftershocks are in blue and 1994 main and aftershocks are in red. Note how both earthquakes occurred along blind thrust faults. Also note that these faults were dipping in opposite directions (1971 dips to the north (south vergent) and 1994 dips to the south (north vergent).
    • In the lower left corner is another figure showing the aftershocks from the 1971 and 1994 earthquakes (Fuis et al., 2003). On the left panel is their seismic velocity model (with fault interpretations) and on the right panel shows the seismicity plotted on the velocity model. I present this figure below.


    Some Relevant Discussion and Figures

    • Here is the fault map from Tucker and Dolan (2001).

    • Regional neotectonic map for metropolitan southern California showing major active faults. The Sierra Madre fault is a 75-km-long active reverse fault that extends along the northern edge of the metropolitan region. Fault locations are from Ziony and Jones (1989), Vedder et al. (1986), Dolan and Sieh (1992), Sorlien (1994), and Dolan et al. (1997, 2000b). Closed teeth denote reverse fault surface trace; open teeth on dashed lines show upper edge of blind thrust fault ramps. Strike-slip fault surface traces shown by double arrows. Star denotes location of Oak Hill paleoseismologic trench site of Bonilla (1973). CSI, Clamshell-Sawpit fault; ELATB, East Los Angeles blind thrust system; EPT, Elysian park blind thrust fault; Hol Fl, Hollywood fault; PHT, Puente Hills blind thrust fault; RMF, Red Mountain fault; SCII, Santa Cruz Island fault; SSF, Santa Susana fault; SJcF, San Jacinto fault; SJF, San Jose fault; VF, Verdugo fault; A, Altadena study site of Rubin et al. (1998); LA, Los Angeles; LB, Long Beach; LC, La Crescenta; M, Malibu; NB, Newport Beach; Ox, Oxnard; P, Pasadena; PH, Port Hueneme; S, Horsethief Canyon study site in San Dimas; V, Ventura. Dark shading denotes mountains.

    • This is a figure that is based upon Fuis et al. (2001) as redrawn by UNAVCO that shows the orientation of thrust faults in this region of southern CA. Below the block diagram is a map showing the location of their seismic experiment (LARSE = Line 1; Fuis et al., 2003).

    • Schematic block diagram showing interpreted tectonics in vicinity of LARSE line 1. Active faults are shown in orange, and moderate and large earthquakes are shown with orange stars and attached dates, magnitudes, and names. Gray half-arrows show relative motions on faults. Small white arrows show block motions in vicinities of bright reflective zones A and B (see Fig. 2A). Large white arrows show relative convergence direction of Pacific and North American plates. We interpret a master decollement ascending from bright reflective zone A at San Andreas fault, above which brittle upper crust is imbricating along thrust and reverse faults and below which lower crust is flowing toward San Andreas fault (brown arrows) and depressing Moho. Fluid injection, indicated by small lenticular blue areas, is envisioned in bright reflective zones A and B.


      Shaded relief map of Los Angeles region, southern California, showing Quaternary faults (thin black lines, dotted where buried), shotpoints (gray and orange filled circles), seismographs (gray and orange lines), air-gun bursts (dashed yellow lines), and epicenters of earthquakes .M 5.8 since 1933 (focal mechanisms with attached magnitudes: 6.7a—Northridge [Hauksson et al., 1995], 6.7b—San Fernando [Heaton, 1982], 5.9—Whittier Narrows [Hauksson et al., 1988], 5.8—Sierra Madre [Hauksson, 1994], 6.3—Long Beach [Hauksson, 1987]). Faults are labeled in red; abbreviations: HF—Hollywood fault, MCF—Malibu Coast fault, MHF—Mission Hills fault, NHF—Northridge Hills fault, RF—Raymond fault, SF—San Fernando surface breaks, SSF—Santa Susana fault, SMoF—Santa Monica fault, SMFZ—Sierra Madre fault zone, VF—Verdugo fault. NH is Newhall.

    • Here are the figures from Hauksson et al. (1995) showing the regions effected by earthquakes in southern CA.

    • (A) Significant earthquakes of M >= 4.8 that have occurred in the greater Los Angeles basin area since 1920. Aftershock zones are shaded with cross hatching, including the 1994 Northridge earthquake. Dotted areas indicate surface rupture, including the rupture of the 1857 earthquake along the San Andreas fault. (B) Lower hemisphere focal mechanisms (shaded quadrants are compressional) for significant earthquakes that have occurred since 1933 in the greater Los Angeles area.

    • Here is the seismicity cross section plot from Tsutsumi and Yeats (1999).

    • Cross section down to 20 km depth across the central San Fernando Valley, including the 1971 Sylmar and 1994 Northridge earthquake zones. See Figure 2 for location of the section and Figure 3 for stratigraphic abbreviations. Wells are identified in the Appendix. Aftershock data for the 1971 (blue) and 1994 (red) earthquakes within a 10-km-wide strip including the line of this section are provided by Jim Mori at Kyoto University. Abbreviation for faults: MHF, Mission Hills fault; NHF, Northridge Hills fault; SSF, Santa Susana fault.

    • Here is the figure from Fuis et al. (2003) showing their interpretation of seismic data from the region. These data are from a seismic experiment also plotted in the map above. The panel on the left is A and the panel on the right is B. This is their figure 3.

    • Cross section along part of line 2 with superposition of various data layers. A: Tomographic velocity model plus line drawing extracted from reflection data (see text); heavier black lines represent better-correlated or higher-amplitude phases. B: Velocity model plus relocated aftershocks of 1971 San Fernando and 1994 Northridge earthquakes (brown and blue dots, respectively); main shock focal mechanisms (far hemispheres) are red (San Fernando; Heaton, 1982) and blue (Northridge; Hauksson et al., 1995). Aftershocks are projected onto line 2 from up to 10 km east.

    • This is a smaller scale cross section from Fuis et al. (2003) showing a broader view of the faults in this region. This shows the velocity model color legend that also applies to the above figure. This is their figure 4.

    • Similar to Fig. 3, with expanded depth and distance frame. See caption for Fig. 3 for definition of red, magneta, and blue lines; orange line—interpreted San Andreas fault (SAF); yellow lines—south-dipping reflectors of Mojave Desert and northern Transverse Ranges; “K” —reflection of Cheadle et al. (1986), which is out of plane of this section. SAF is not imaged directly; interpretation is based on approximate northward termination of upper reflections (best constrained) in San Fernando reflective zone (magenta lines). (See similar interpretation for SAF on line 1—Fig. 5.) Wells shown in Mojave Desert are (s) H&K Exploration Co., (t) Meridian Oil Co. (Dibblee, 1967). For well color key, see caption for Fig. 3. Thin, dashed yellow-orange line—estimated base of Cenozoic sedimentary rocks in Mojave Desert based on velocity. Darker, multicolored region (above region of light violet) represents part of velocity model where resolution ≥ 0.4 (see color bar).

    • Here is a fascinating figure from Carena and Suppe (2002) showing the 3-dimensional configuration of the faults involved in the 1971 and 1994 earthquakes.

    • Perspective view, looking from the SE, of the modeled Northridge and San Fernando thrusts. The Northridge thrust stops at a depth of about 6 km, and its upper tip east of the lateral ramp (Fig. 4) terminates almost against the San Fernando thrust, as was suggested by Morti et al. (1993). The San Fernando thrust loser tip is at a depth o 13 km, whereas the Northridge thrust lower tip is at 32 km.

    • Here is a map view of the Carena and Suppe (2002) interpretation of these fault planes.

    • Schematic geological map showing the position of the main faults and folds, as well as the depth contours (contour interval = 1 km) of the Northridge (solid) and San Fernando (dashed) thrusts.

    • Here is a structural cross section across this region (Carena and Suppe, 2002).

    • Cross-section through the San Fernando Valley with projected aftershocks of the 1994 Northridge earthquake and of the 1971 Sylmar earthquake. The Northridge aftershocks are projected from a distance of 1 km or less on each side of the cross-section (main shock projected from 2 km W), whereas those of the Sylmar earthquake are projected from 1.5 km or less (main shock projected from 5 km ESE). The sources that we used for near-surface geology and structure are Dibblee (1991) and a seismic line (Fig. 11). The large N-S changes in Upper Tertiary stratigraphic thicknesses in this region (Dibblee, 1991, 1992a), prevent detailed stratigraphic correlation across fault blocks (this figure and Fig. 12). This face suggests that the shallow faults and possible the deeper San Fernando thrust itself, are reactivating old normal faults of the southern margin of the Ventura Basin (Yeats, et al., 1994; Huftle and Yeats, 1996; Tsutsumi and Yeats, 1999). Location of cross-section is in Fig. 13.

    UPDATE 2019.01.17 25 Year Commemoration

    I present some updates to this earthquake report for the 17 January 1994 M 6.7 Northridge Earthquake. First I present a new poster with some updated figures, then I review some of the new knowledge that we have gained over the years since 1994.
    I presented a Temblor post about what would happen if there were a repeat of the Northridge earthquake today, during the partial federal government shutdown. Here is that article.
    Below are two interpretive posters that allow one to compare the shaking intensities from the 1994 Northridge earthquake and an hypothetical earthquake on several segments of the southern San Andreas fault.
    I won’t review the background for this poster as it includes the same background information as the poster I made 2 years ago (read above).
    I include some of the major earthquakes in the region, including a mechanism for the 1993 M = 6.4 Long Beach earthquake (Hauksson and Gross, 1991).

      I include some inset figures in the poster.

    • In the upper left corner is a figure that shows (A) GPS velocities and remote sensing (InSAR) based surface velocities, (B) a cross section showing how these plate motions result in compression, and (C) a low angle oblique block model view of the tectonic blocks configured to accommodate the plate motions (Daout et al., 2016).
    • In the lower left corner is a low angle oblique view of the San Andreas fault and other faults that are underlying much of the LA basin and its millions of inhabitants (Daout et al., 2016).
    • In the upper right corner is a figure from Rollins et al. (2018) that shwos the fault geometry, GPS plate motion rates, and historic earthquake mechanisms for the LA basin.
    • In the lower right corner shows how much plate motion is proportioned onto the different major blind thrust faults in the southland.

    I present the poster in 2 formats. First we see the USGS shakemap from the 1994 quake. Then we see a shakemap from a “scenario” earthquake, an earthquake of size that we speculate to occur on several segments of the San Andreas fault.
    The USGS prepares scenario earthquakes so that we have an estimate of the potential for damage to people and their belongings from an earthquake of a given size in a given location. Here is an interface that allows one to browse all of the USGS scenario earthquakes.

    • This is the map showing shaking intensities from the 1994 earthquake.



    • Here is the Daout et al. (2016) introduction figure. Note the large remote sensing velocities (LOS) in the LA basin.

    • Seismotectonic setting of the Southern California fault system. (a) GPS data in the ITRF08 reference frame highlighting a uniform velocity field despite the complex three-dimensional geometry of the faults systems. InSAR velocitiy map is derived from Envisat descending track 170 [from Liu et al., 2014]. Black rectangle defines the profile perpendicular to the SAF. Major strikes-slip faults including the San Andreas Fault (SAF), Whittier Fault (WF), San Jacinto Fault (SJF), and the Elsinore Fault (EF) are in black. Major thrust faults including the Sierra Madre Thrust fault (SMT), the Elysian Park Thrust (EPT), the Puente Hills Thrusts (PHT), San Gorgonio Pass (SGP), and the North Frontal Thrusts (NFT) are in red. (b) Simplified kinematic sketch illustrating how the obliquity of the SAF creates a local shortening (red vector) between the Mojave Block (MB) and Los Angeles (LA). (c) Simplified three-dimensional model across the profile PP′ illustrating how the geometry of the ramp-décollement system partitions the uniform velocity field and controls the amount of shortening and uplift along the various blocks.

    • This figure shows how they modeled the subsurface faults and how their model results fit the remote sensing (InSAR) and GPS data. They used a Bayesian statistical technique, which is why there are so many possible fault geometries in panel B.

    • Comparison between the prior and posterior models. (a) Two-dimensional prior model based on the tectonic review along the profile PP′ defined in Figure 1. Black lines (red dashed lines, respectively) represent slipping (locked, respectively) sections of the faults; arrows indicate relative direction of the movement on faults. The SAF is associated with two thick black dashed lines and a question mark as we have no constraints of its deep geometry. We use this configuration and the conservation of motion along each junction to explore the various parameters defined in this figure. Insert is a simplified two-dimensional block model illustrating the relation between the block geometries and longitudinal velocities along the structures. (b) Posterior geometries in agreement with the data (blue lines) and average geometry (black lines). (c) InSAR LOS velocities (grey points) and GPS projected in the LOS direction (black squares) and average model obtained. (d) Profile-perpendicular (blue markers), profile-parallel (green markers), and vertical (red markers) GPS velocities with their associated uncertainties. Average model obtained (blue, green, and red lines) along profiles.

    • Here is the block model from Daout et al. (2016) that shows their modeled fault slip rates for each of these faults.

    • Three-dimensional schematic block model across the SGM [after Fuis et al., 2001b] superimposed to the digital elevation model, the seismicity (yellow dots), the Moho model (red line), and interpreted active faults summarizing the average interseismic strike-slip (back arrows) and dip-slip (red arrows) rates extracted from the Bayesian exploration. Shallow faults (dashed lines) that formed a complex three-dimensional system at the surface [Plesch et al., 2007] are locked during the interseismic period, while the ramp-décollement system (solid lines) decouples the upper crust from the lower crust and partitioned the observed uniform velocity field (blue vector) at the downdip end of the
      structures.

    • Here are the GPS observations used by Rollins et al. (2018) to conduct their study evaluating the seismogenic locking on the blind thrust faults (like the Puente Hills Thrust) beneath Los Angeles. These faults may pose a greater hazard to Angelinos than the San Andreas fault. Take another look at the two interpretive posters above. Chekc out the shaking in the LA basin from both the 1994 Northridge quake and the Scenario San Sandreas fault earthquake. You may notice a slight increase in shaking intensity from the 1994 earthquake. Note: the Puente Hills Thrust is even closer to the LA Basin than the Northridge quake. The Compton fault, similar to the Puente Hills, is hypothesized to generate a M = 7.45 earthquake, which would release a substantially larger earthquake than in 1994.

    • (a) Tectonics and shortening in the Los Angeles region. Dark blue arrows are shortening-related GPS velocities relative to the San Gabriel Mountains (Argus et al., 2005). Contours are uniaxial strain rate (rate of change of εxx) in the N ~5° E direction estimated from the GPS using the method of Tape et al. (2009). Background shading is the shear modulus at 100-m depth in the CVM*, a heterogeneous elastic model based on the Community Velocity Model (Süss & Shaw, 2003; Shaw et al., 2015) that we create and use in this study (section 4). Black lines are upper edges of faults, dashed for blind faults. Epicenters of the 1971, 1987, and 1994 earthquakes are from Southern California Earthquake Data Center; focal mechanisms are from Heaton (1982) for 1971 and Global CMT Catalog for 1987 and 1994. Profile A-A0 follows LARSE line 1 (Fuis et al., 2001) onshore and line M-M0 of Sorlien et al. (2013) offshore. SGF = San Gabriel Fault; SSF = Santa Susana Fault. VF = Verdugo Fault. SAF = San Andreas Fault. CuF = Cucamonga Fault. A-DF = Anacapa-Dume Fault. SMoF = Santa Monica Fault. HF = Hollywood Fault. RF = Raymond Fault. UEPF = Upper Elysian Park Fault. ChF = Chino Fault. WF = Whittier Fault. N-IF = Newport-Inglewood Fault. PVF = Palos Verdes Fault. (b) GPS velocities on islands. (c) Tectonic setting. Black lines and pairs of half-arrows, respectively, are major faults and their slip senses. Black arrow is Pacific Plate velocity relative to North American plate from Kreemer et al. (2014). GF = Garlock Fault. SJF = San Jacinto Fault. EF = Elsinore Fault. SB = Santa Barbara. LA = Los Angeles. SD = San Diego.

    • Here is a cross section showing the fault geometry used by Rollins et al. (2018).

    • (a) Cross sections of faults, structure, north-south contraction, and seismicity along profile A-A0 . Red lines are fault surfaces as meshed here (Figure 3), dashed where uncertain (Shaw & Suppe, 1996; Shaw & Shearer, 1999; Fuis et al., 2012). Geometries of basin, basement, and mantle are from Shaw et al. (2015); geometry of base of Fernando Formation (boundary between beige and tan units of the basin) is interpolated from Sorlien et al. (2013; offshore), Wright (1991; coastline to Whittier Fault), and Yeats (2004; Whittier Fault to Sierra Madre Fault); topography is from Fuis et al. (2012). (b) Projections of Argus et al. (2005) GPS velocities (relative to San Gabriel Mountains) onto the direction N 5° E and 1σ uncertainties. Note that stations on Palos Verdes are plotted left of the coastline as the offshore section of profile A-A0 passes alongside Palos Verdes (Figure 1a). (c) Seismotectonic features. Distribution of shear modulus is from the CVM*, the heterogeneous elastic model used in this study (section 4). Translucent white circles are relocated 1981–2016 M ≥ 2 earthquakes whose epicenters lie within the mesh area of the three thrust faults and decollement (Hauksson et al., 2012 and updated). PVF = Palos Verdes Fault; N-IF = Newport-Inglewood Fault; WF = Whittier
      Fault.

    • This is a great map showing the depth of the faults in the region from Rollins et al. (2018).

    • Meshed geometries of the three main thrust faults beneath the Los Angeles basin (section 4), colored by depth, and 1981–2016 M ≥ 2.5 earthquakes within the mesh area from Hauksson et al. (2012 and updated), scaled by magnitude (white-filled circles). Gray-filled circles are 1981–2016 M ≥ 4.5 earthquakes outside the mesh area. Inferred paleoearthquakes are from Rubin et al. (1998) and Leon et al. (2007, 2009). SAF = San Andreas Fault.

    • There are a great many more fantastic details about the Rollins et al. (2018) analysis in their paper, so please read it (search for the preprint that is lurking around online). This map is the main summary figure, showing the amount of seismic energy (moment deficit) that each fault accumulates each year. Warmer colors mean that the fault is accumulating more strain each year. The more strain that is accumulated, the more energy could potentially be released during an earthquake. Some suggest that larger strain accumulation rates may also lead to more frequent earthquakes, but this is a complicated issue and we don’t know yet what the real answer is… so exciting.

    • Estimates of moment deficit accumulation rate from combining the four interseismic strain accumulation models. (a) Spatial distribution of moment deficit accumulation rate per area. (Values are on the order of ~108 N m -1 yr -1 as the moment deficit accumulation rate per patch is on the order of 1015 N m -1 yr -11 [Figure S11] and the patches are a few kilometers (a few thousand meters) on a side.) (b) Unified PDF of moment deficit accumulation rate (dark blue object) formed by combining the PDFs from the four strain accumulation models. The PDF would follow the red curve if strain accumulation updip of the tips of the Puente Hills and Compton faults (PHF and CF) were counted.

    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

    • Carena, S. and Supper, J., 2002. Three-dimensional imaging of active structures using earthquake aftershocks: the Northridge thrust, California in Journal of Structural Geology, v. 24, p. 887-904.
    • Daout, S., S. Barbot, G. Peltzer, M.-P. Doin, Z. Liu, and R. Jolivet, 2016. Constraining the kinematics of metropolitan Los Angeles faults with a slip-partitioning model in Geophys. Res. Lett., v. 43, p. 11,192–11,201 doi:10.1002/2016GL071061.
    • Fuis, G.S>, Ryberg, T., Godfrey, N.J>, Okaya, D.A., and Murphy, J.M., 2001. Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California in Geology, v. 29, no. 1. p. 15-18.
    • Fuis, G.S. et al., 2003. Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II in Geology, v. 31, no. 2, p. 171-174.
    • Hauksson, E. and Gross, S., 1991, Source Parameters of the 1933 Long Beach Earthquake in BSSA, v. 81, no. 1., p. 81-98
    • Hauksson, E., Jones, L.M., and Hutton, K., 1995. The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects in Journal of Geophysical Research, v., 100, no. B7, p. 12235-12355.
    • Rollins, C., Avouac, J.-P., Landry, W., Argus, D. F., & Barbot, S. (2018). Interseismic strain accumulation on faults beneath Los Angeles, California. Journal of Geophysical Research: Solid Earth, 123. https://doi.org/10.1029/2017JB015387
    • Tsutsumi, H. and Yeats, R.S., 1999. Tectonic Setting of the 1971 Sylmar and 1994 Northridge Earthquakes in the San Fernando Valley, California in BSSA, v. 89, p. 1232-1249.
    • Tucker, A.Z. and Dolan, J.F., 2001. Paleoseismologic Evidence for a 8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault, Northern Los Angeles Metropolitan Region, California in BSSA, v. 91, no. 2, p. 232-249.

    Earthquake Report: Java Sea!

    Last night as I was finishing work for the day, I noticed an earthquake in the Java Sea, just north of western Java. Here is the USGS website for this M 6.6 earthquake. This earthquake is extensional and plots very deep along the subduction zone beneath Java.
    In the map below I plot the epicenters of earthquakes from the past 30 days of magnitude greater than M = 2.5. The epicenters have colors representing depth in km. The USGS plate boundaries are plotted vs color. The USGS modeled estimate for ground shaking is plotted with contours of equal ground shaking using 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. 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 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.
    The subduction of the India-Australia plate, northwards beneath the Sunda plate, forms a subduction zone trench (labeled Sunda Trench in the map below). I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. The hypocentral depth plots this close to the location of the fault as mapped by Hayes et al. (2012). So, the earthquake is either in the downgoing slab, or in the upper plate and a result of the seismogenic locked plate transferring the shear strain from a fracture zone in the downgoing plate to the upper plate.
    Today’s earthquake has an hypocentral depth of 415 km, while the slab depth estimate from Hayes et al. (2013) is greater than 620 km. This is a pretty good match. The moment tensor shows northeast-southwest extension, so this earthquake is possibly in the down going slab where there is either down-slab tension (the subducting plate is pulling the plate down, causing extension) or due to “bending moment normal faults” (if the plate is bending downwards, this causes extension in the top of the plate and compression in the lower part of the plate). Based upon these observations, I suspect this earthquake is in the downgoing Indo-Australia plate.

      I include some inset figures.

    • In the upper right corner are some figure insets from Jones et al. (2010). This is a report on the regional seismicity. The panel on the right is a map showing seismicity vs. depth (color of circle) and magnitude (diameter of circle). There are two cross sections (A-A’ and B-B’) that sample seismicity limited to the rectangular boxes shown on the map. The seismicity cross sections show the general location of the India-Australia slab as it subducts beneath the Sunda plate. On the left are legends for the map and the cross sections. I place a yellow circle for the general location of the epicenter of this M 6.6 earthquake.
    • Below Jones et al. (2010), I present two more cross sections of seismicity (Hengesh and Whitney, 2016). The lower right cross section is position in eastern Java.
    • In the lower left corner is a figure I prepared using SRTM (Space Shuttle Radar Topography Mission) bathymetric and topographic data (Smith and Sandwell, 1997). I plot USGS earthquake epicenters for earthquakes with magnitudes greater than, or equal to, M = 6.5, for the period from 1916 to present. Circle diameter represents earthquake magnitude. Plate motion rates are from Bock et al. (2003). Outline of the Bengal and Nicobar fans is from Stow (1990). Relative plate motion along the subduction zone is increasingly oblique, south to north. I place a red circle for the general location of the epicenter of this M 6.6 earthquake.
    • Above the seismicity map is a geodetic-tectonic fault map from Hengesh and Whitney (2016). Seismicity is plotted vs. magnitude (diameter of circle) and depth (color of circle). Relative plate motion and GPS geodetic plate motion rates are plotted as scaled and labeled vectors. I place a red circle for the general location of the epicenter of this M 6.6 earthquake.


    • Here is a figure showing the regional geodetic motions (Bock et al., 2003). I include their figure caption below as a blockquote.

    • Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, New Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals.

    • In addition to the orientation of relative plate motion (that controls seismogenic zone and strain partitioning), the Indo Australia plate varies in crustal age (Lasitha et al., 2006). I include their figure caption below as a blockquote.

    • Tectonic sketch map of the Sumatra–Java trench-arc region in eastern Indian Ocean Benioff Zone configuration. Hatched line with numbers indicates depth to the top of the Benioff Zone (after Newcomb and McCann13). Magnetic anomaly identifications have been considered from Liu et al.14 and Krishna et al.15. Magnitude and direction of the plate motion is obtained from Sieh and Natawidjaja11. O indicates the location of the recent major earthquakes of 26 December 2004, i.e. the devastating tsunamigenic earthquake (Mw = 9.3) and the 28 March 2005 earthquake (Mw = 8.6).

    • Here is a figure showing the regional gravity anomalies, supporting the interpretations of Hengesh and Whitney, 2016. I include their figure caption below as a blockquote.

    • Merged free-air and isostatic gravity anomalies and inferred Quaternary active faults along the western margin of Australia [Geoscience Australia, 2009]. Note the association of faults with areas of high gravity anomaly associated with former rift margin basins.

    • Here is a figure showing the tectonic interpretations of Hengesh and Whitney, 2016. I include their figure caption below as a blockquote.

    • 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. [2006]. 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.

      Recent Seismicity

      There have been several large magnitude earthquakes in this part of the Alpide belt in historic times, including some great earthquakes (

      • 2015.11.08 M 6.1 and M 6.4 Earthquakes

      • The interesting things about these two earthquakes is that they are not on the subduction zone fault interface. The M = 6.4 earthquake is shallow (USGS depth = 7.7 km). Note how the subduction zone is mapped to ~120-140 km depth near the M 6.4 earthquake. The Andaman Sea is a region of backarc spreading and forearc sliver faulting. Due to oblique convergence along the Sunda trench, the strain is partitioned between the subduction zone fault and the forearc sliver Sumatra fault. In the Andaman Sea, there is a series of en echelon strike-slip/spreading ridges. The M 6.4 earthquake appears to have slipped along one of these strike-slip faults. I interpret this earthquake to be a right lateral strike-slip earthquake, based upon the faults mapped in this region. The smaller earthquakes align in a west-southwest orientation. These may be earthquakes along the spreading center, or all of these earthquakes may be left lateral strike slip faults aligned with a spreading ridge. More analyses would need to be conducted to really know.
      • Here is a map showing moment tensors for the largest earthquakes since the 26 December 2004 Mw = 9.15 Megathrust Great Sumatra-Andaman subduction zone (SASZ) earthquake. Below is a map showing the earthquake slip contours. The beginning of this series started with the Mw 9.15 and Mw = 8.7 Nias earthquakes. There were some other earthquakes along the Mentawaii patch to the south (Mw = 8.5, 7.9, and 7.0). These were also subduction zone earthquakes, but failed to release the strain that had accumulated since the last large magnitude earthquakes to have slipped in this region in 1797 and 1833. In 2012 we had two strike slip earthquakes in the outer rise, where the India-Australia plate flexes in response to the subduction. At first I interpreted these to be earthquakes on northeast striking faults since those the orientation of the predominant faulting in the region. The I-A plate has many of these N-S striking fracture zones, most notably the Investigator fracture zone (the most easterly faults shown in this map as a pair of strike slip faults that head directly for the epicenter of yesterday’s earthquake). However, considering the aftershocks and a large number of different analyses, these two earthquakes (the two largest strike slip earthquakes EVER recorded!) were deemed to have ruptured northwest striking faults. We called these off fault earthquakes, since the main structural grain is those N-S striking fracture zones. Also of note is the focal depth of these two large earthquakes (Mw 8.2 & 8.6). These earthquakes ruptured well into the mantle. Before the 2004 SASZ earthquake and the 2011 Tohoku-Oki earthquake (which also probably ruptured into the mantle), we would not have expected earthquakes in the mantle.

        While we were at sea offshore Sumatra, there was a CBC (Canada) film maker aboard recording material for a film on Cascadia subduction zone earthquakes. This is a dity that he made for us.

      • link to the embedded video below. (45 mb mp4)
      • YT link to the embedded video below.
      • Here is a map showing the historic earthquake regions. Earthquake slip contours are shown for the 2004 and 2005 earthquakes. Some references for these earthquake sources include: Newcomb and McMann, 1987; Rivera et al., 2002; Abercrombie et al., 2003; Natawidjaja et al., 2006; Konca et al., 2008; Bothara, 2010; Kanamori et al., 2010; Philibosian et al., 2012.


        This map shows the magnitude of these historic earthquakes overlain upon a map showing the magnetic anomalies.

          References:

        • Abercrombie, R.E., Antolik, M., Ekstrom, G., 2003. The June 2000 Mw 7.9 earthquakes south of Sumatra: Deformation in the India–Australia Plate. Journal of Geophysical Research 108, 16.
        • Bothara, J., Beetham, R.D., Brunston, D., Stannard, M., Brown, R., Hyland, C., Lewis, W., Miller, S., Sanders, R., Sulistio, Y., 2010. General observations of effects of the 30th September 2009 Padang earthquake, Indonesia. Bulletin of the New Zealand Society for Earthquake Engineering 43, 143-173.
        • Chlieh, M., Avouac, J.-P., Hjorleifsdottir, V., Song, T.-R.A., Ji, C., Sieh, K., Sladen, A., Hebert, H., Prawirodirdjo, L., Bock, Y., Galetzka, J., 2007. Coseismic Slip and Afterslip of the Great (Mw 9.15) Sumatra-Andaman Earthquake of 2004. Bulletin of the Seismological Society of America 97, S152-S173.
        • Harris, R. A. (2006), Rise and fall of the Eastern Great Indonesian arc recorded by the assembly, dispersion and accretion of the Banda Terrane,
          Timor, Gondwana Res., 10, 207–231.
        • Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
        • Hengesh, J.V. and Whitney, B.B., 2016. Transcurrent reactivation of Australia’s western passive margin: An example of intraplate deformation from the central Indo-Australian plate in Tectonics, v. 35, doi:10.1002/2015TC004103.
        • 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 SurveyOpen-File Report 2010–1083-N, 1 sheet, scale 1:5,000,000,http://dx.doi.org/10.3133/ofr20101083N.
        • Kanamori, H., Rivera, L., Lee, W.H.K., 2010. Historical seismograms for unravelling a mysterious earthquake: The 1907 Sumatra Earthquake. Geophysical Journal International 183, 358-374.
        • Konca, A.O., Avouac, J., Sladen, A., Meltzner, A.J., Sieh, K., Fang, P., Li, Z., Galetzka, J., Genrich, J., Chlieh, M., Natawidjaja, D.H., Bock, Y., Fielding, E.J., Ji, C., Helmberger, D., 2008. Partial Rupture of a Locked Patch of the Sumatra Megathrust During the 2007 Earthquake Sequence. Nature 456, 631-635.
        • Lasitha, S., Radhakrishna, M., Sanu, T.D., 2006. Seismically active deformation in the Sumatra–Java trench-arc region: geodynamic implications in Current Science, v. 90, p. 690-696.
        • Natawidjaja, D.H., Sieh, K., Chlieh, M., Galetzka, J., Suwargadi, B., Cheng, H., Edwards, R.L., Avouac, J., Ward, S.N., 2006. Source parameters of the great Sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls. Journal of Geophysical Research 111, 37.
        • Newcomb, K.R., McCann, W.R., 1987. Seismic History and Seismotectonics of the Sunda Arc. Journal of Geophysical Research 92, 421-439.
        • Philibosian, B., Sieh, K., Natawidjaja, D.H., Chiang, H., Shen, C., Suwargadi, B., Hill, E.M., Edwards, R.L., 2012. An ancient shallow slip event on the Mentawai segment of the Sunda megathrust, Sumatra. Journal of Geophysical Research 117, 12.
        • Rigg, J. W., and R. Hall (2011), Structural and stratigraphic evolution of the Savu Basin, Indonesia, Geol. Soc. London Spec. Publ., 355(1), 225–240.
        • Rivera, L., Sieh, K., Helmberger, D., Natawidjaja, D.H., 2002. A Comparative Study of the Sumatran Subduction-Zone Earthquakes of 1935 and 1984. BSSA 92, 1721-1736.
        • Sieh, K., Natawidjaja, D.H., Meltzner, A.J., Shen, C., Cheng, H., Li, K., Suwargadi, B.W., Galetzka, J., Philobosian, B., Edwards, R.L., 2008. Earthquake Supercycles Inferred from Sea-Level Changes Recorded in the Corals of West Sumatra. Science 322, 1674-1678.
        • Smith, W.H.F., Sandwell, D.T., 1997. Global seafloor topography from satellite altimetry and ship depth soundings: Science, v. 277, p. 1,957-1,962.
        • Storchak, D. A., D. Di Giacomo, I. Bondár, E. R. Engdahl, J. Harris, W. H. K. Lee, A. Villaseñor, and P. Bormann (2013), Public release of the ISC-GEM global instrumental earthquake catalogue (1900–2009), Seismol. Res. Lett., 84(5), 810–815, doi:10.1785/0220130034.
        • Stow, D.A.V., et al., 1990. Sediment facies and processes on the distal Bengal Fan, Leg 116, ODP Texas & M University College Station; UK distributors IPOD Committee NERC Swindon, p. 377-396.