Earthquake Report: Bengkulu (Sumatra)!

Last night (my time) while I was tending to other business, there was an earthquake along the Sunda Megathrust. Here is the USGS website for this M 6.4 earthquake.

This M 6.4 earthquake happened down-dip (“deeper than”) along the megathrust from the 2007.09.12 M 8.4 megathrust earthquake. Here is the USGS website for the M 8.4 earthquake. This M 6.4 earthquake occurred in a region of low seismogenic coupling (as inferred by Chlieh at al., 2008), albeit with sparse GPS data in this region. Chlieh et al. (2008) used coral geodetic and paleogeodetic data, along with Global Positioning System (GPS) observations, to constrain their model. Because there are no forearc islands in this part of the subduction zone, there are no GPS nor coral data with which to constrain their model (so it may underestimate the coupling %, i.e. coupling ratio).

Based upon the USGS fault plane slip model, this M 6.4 earthquake actually happened in a region of higher slip from the M 8.4 earthquake. We may consider this M 6.4 earthquake to be an aftershock of the M 8.4 earthquake.

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 1917-2017 for magnitudes M ≥ 7.

I also include the USGS moment tensor for today’s earthquake, as well as for the 2007 M 8.4 earthquake. I label the other epicenters with large magnitudes (2004, 2005, and 2012). Find more details about these earthquakes in my reports listed at the bottom of this page, above the references.

  • 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 poster.

  • In the upper right corner, I include a map from Hayes et al. (2013) that shows the epicenters of earthquakes from the past century or so. There is also a cross section that is in the region of the 2007 and 2017 M 8.4 and M 6.4 earthquakes. I also placed a C-C’ green line on the main map to show where this cross section is compared to the other features on my map. I placed a blue star in the general location of the M 6.4 earthquake.
  • In the lower left corner, I include a map I made that delineates the spatial extent for historic earthquakes along the Sunda Megathrust. This came from a paper that I had submitted to Marine Geology. I include this figure below with attributions to the publications that I used as references for this map. I outlined the slip patch for the M 8.4 earthquake in transparent orange.
  • In the upper left corner, I present a figure from Chlieh et al. (2008 ). These authors use GPS and coral geodetic and paleogeodetic data to constrain the proportion of the plate motion rates that are accumulated as tectonic strain along the megathrust fault. Basically, this means how much % that the fault is storing energy to be released in subduction zone earthquakes. This is just a model and is limited by the temporal and spatial extent of their observations which form the basis for their model. However, this is a well respected approach to estimate the potential for future earthquake (given the assumptions that I here mention).


  • I prepared this figure to show the difference in MMI Intensity for these two closely spaced earthquakes. The data here come from the USGS websites listed above.

  • Here is my map. I include the references below in blockquote.

  • Sumatra core location and plate setting map with sedimentary and erosive systems figure. A. India-Australia plate subducts northeastwardly beneath the Sunda plate (part of Eurasia) at modern rates (GPS velocities are based on regional modeling of Bock et al, 2003 as plotted in Subarya et al., 2006). Historic earthquake ruptures (Bilham, 2005; Malik et al., 2011) are plotted in orange. 2004 earthquake and 2005 earthquake 5 meter slip contours are plotted in orange and green respectively (Chlieh et al., 2007, 2008). Bengal and Nicobar fans cover structures of the India-Australia plate in the northern part of the map. RR0705 cores are plotted as light blue. SRTM bathymetry and topography is in shaded relief and colored vs. depth/elevation (Smith and Sandwell, 1997). B. Schematic illustration of geomorphic elements of subduction zone trench and slope sedimentary settings. Submarine channels, submarine canyons, dune fields and sediment waves, abyssal plain, trench axis, plunge pool, apron fans, and apron fan channels are labeled here. Modified from Patton et al. (2013 a).

  • This is the main figure from Hayes et al. (2013) from the Seismicity of the Earth series. There is a map with the slab contours and seismicity both colored vs. depth. There are also some cross sections of seismicity plotted, with locations shown on the map.

  • Here is a great figure from Philobosian et al. (2014) that shows the slip patches from the subduction zone earthquakes in this region.

  • Map of Southeast Asia showing recent and selected historical ruptures of the Sunda megathrust. Black lines with sense of motion are major plate-bounding faults, and gray lines are seafloor fracture zones. Motions of Australian and Indian plates relative to Sunda plate are from the MORVEL-1 global model [DeMets et al., 2010]. The fore-arc sliver between the Sunda megathrust and the strike-slip Sumatran Fault becomes the Burma microplate farther north, but this long, thin strip of crust does not necessarily all behave as a rigid block. Sim = Simeulue, Ni = Nias, Bt = Batu Islands, and Eng = Enggano. Brown rectangle centered at 2°S, 99°E delineates the area of Figure 3, highlighting the Mentawai Islands. Figure adapted from Meltzner et al. [2012] with rupture areas and magnitudes from Briggs et al. [2006], Konca et al. [2008], Meltzner et al. [2010], Hill et al. [2012], and references therein.

  • This is a figure from Philobosian et al. (2012) that shows a larger scale view for the slip patches in this region.

  • Recent and ancient ruptures along the Mentawai section of the Sunda megathrust. Colored patches are surface projections of 1-m slip contours of the deep megathrust ruptures on 12–13 September 2007 (pink to red) and the shallow rupture on 25 October 2010 (green). Dashed rectangles indicate roughly the sections that ruptured in 1797 and 1833. Ancient ruptures are adapted from Natawidjaja et al. [2006] and recent ones come from Konca et al. [2008] and Hill et al. (submitted manuscript, 2012). Labeled points indicate coral study sites Sikici (SKC), Pasapuat (PSP), Simanganya (SMY), Pulau Pasir (PSR), and Bulasat (BLS).

  • Here are a series of figures from Chlieh et al. (2008 ) that show their data sources and their modeling results. I include their figure captions below in blockquote.
  • This figure shows the coupling model (on the left) and the source data for their inversions (on the right). Their source data are vertical deformation rates as measured along coral microattols. These are from data prior to the 2004 SASZ earthquake.

  • Distribution of coupling on the Sumatra megathrust derived from the formal inversion of the coral and of the GPS data (Tables 2, 3, and 4) prior to the 2004 Sumatra-Andaman earthquake (model I-a in Table 7). (a) Distribution of coupling on the megathrust. Fully coupled areas are red, and fully creeping areas are white. Three strongly coupled patches are revealed beneath Nias island, Siberut island, and Pagai island. The annual moment deficit rate corresponding to that model is 4.0 X 10^20 N m/a. (b) Observed (black vectors) and predicted (red vectors) horizontal velocities appear. Observed and predicted vertical displacements are shown by color-coded large and small circles, respectively. The Xr^2 of this model is 3.9 (Table 7).

  • This is a similar figure, but based upon observations between June 2005 and October 2006.

  • Distribution of coupling on the Sumatra megathrust derived from the formal inversion of the horizontal velocities and uplift rates derived from the CGPS measurements at the SuGAr stations (processed at SOPAC). To reduce the influence of postseismic deformation caused by the March 2005 Nias-Simeulue rupture, velocities were determined for the period between June 2005 and October 2006. (a) Distribution of coupling on the megathrust. Fully coupled areas are red and fully creeping areas are white. This model reveals strong coupling beneath the Mentawai Islands (Siberut, Sipora, and Pagai islands), offshore Padang city, and suggests that the megathrust south of Bengkulu city is creeping at the plate velocity. (b) Comparison of observed (green) and predicted (red) velocities. The Xr^2 associated to that model is 24.5 (Table 8).

  • This is a similar figure, but based on all the data.

  • Distribution of coupling on the Sumatra megathrust derived from the formal inversion of all the data (model J-a, Table 8). (a) Distribution of coupling on the megathrust. Fully coupled areas are red, and fully creeping areas are white. This model shows strong coupling beneath Nias island and beneath the Mentawai (Siberut, Sipora and Pagai) islands. The rate of accumulation of moment deficit is 4.5 X 10^20 N m/a. (b) Comparison of observed (black arrows for pre-2004 Sumatra-Andaman earthquake and green arrows for post-2005 Nias earthquake) and predicted velocities (in red). Observed and predicted vertical displacements are shown by color-coded large and small circles (for the corals) and large and small diamonds (for the CGPS), respectively. The Xr^2 of this model is 12.8.

  • Here is the figure I included in the poster above.

  • Comparison of interseismic coupling along the megathrust with the rupture areas of the great 1797, 1833, and 2005 earthquakes. The southernmost rupture area of the 2004 Sumatra-Andaman earthquake lies north of our study area and is shown only for reference. Epicenters of the 2007 Mw 8.4 and Mw 7.9 earthquakes are also shown for reference. (a) Geometry of the locked fault zone corresponding to forward model F-f (Figure 6c). Below the Batu Islands, where coupling occurs in a narrow band, the largest earthquake for the past 260 years has been a Mw 7.7 in 1935 [Natawidjaja et al., 2004; Rivera et al., 2002]. The wide zones of coupling, beneath Nias, Siberut, and Pagai islands, coincide well with the source of great earthquakes (Mw > 8.5) in 2005 from Konca et al. [2007] and in 1797 and 1833 from Natawidjaja et al. [2006]. The narrow locked patch beneath the Batu islands lies above the subducting fossil Investigator Fracture Zone. (b) Distribution of interseismic coupling corresponding to inverse model J-a (Figure 10). The coincidence of the high coupling area (orange-red dots) with the region of high coseismic slip during the 2005 Nias-Simeulue earthquake suggests that strongly coupled patches during interseismic correspond to seismic asperities during megathrust ruptures. The source regions of the 1797 and 1833 ruptures also correlate well with patches that are highly coupled beneath Siberut, Sipora, and Pagai islands.

  • This figure shows the authors’ estimate for the moment deficit in this region of the subduction zone. This is an estimate of how much the plate convergence rate, that is estimated to accumulate as tectonic strain, will need to be released during subduction zone earthquakes.

  • Latitudinal distributions of seismic moment released by great historical earthquakes and of accumulated deficit of moment due to interseismic locking of the plate interface. Values represent integrals over half a degree of latitude. Accumulated interseismic deficits since 1797, 1833, and 1861 are based on (a) model F-f and (b) model J-a. Seismic moments for the 1797 and 1833 Mentawai earthquakes are estimated based on the work by Natawidjaja et al. [2006], the 2005 Nias-Simeulue earthquake is taken from Konca et al. [2007], and the 2004 Sumatra-Andaman earthquake is taken from Chlieh et al. [2007]. Postseismic moments released in the month that follows the 2004 earthquake and in the 11 months that follows the Nias-Simeulue 2005 earthquake are shown in red and green, respectively, based on the work by Chlieh et al. [2007] and Hsu et al. [2006].

  • For a review of the 2004 and 2005 Sumatra Andaman subduction zone (SASZ) earthquakes, please check out my Earthquake Report here. Below is the poster from that report. On that report page, I also include some information about the 2012 M 8.6 and M 8.2 Wharton Basin earthquakes.
    • I include some inset figures in the poster.
    • In the upper left corner, I include a map that shows the extent of historic earthquakes along the SASZ offshore of Sumatra. This map is a culmination of a variety of papers (summarized and presented in Patton et al., 2015).
    • In the upper right corner I include a figure that is presented by Chlieh et al. (2007). These figures show model results from several models. Each model is represented by a map showing the amount that the fault slipped in particular regions. I present this figure below.
    • In the lower right corner I present a figure from Prawirodirdjo et al. (2010). This figure shows the coseismic vertical and horizontal motions from the 2004 and 2005 earthquakes as measured at GPS sites.
    • In the lower left corner are the MMI intensity maps for the two SASZ earthquakes. Note these are at different map scales. I also include the MMI attenuation curves for these earthquakes below the maps. These plots show the reported MMI intensity data as they relate to two plots of modeled estimates (the orange and green lines). These green dots are from the USGS “Did You Feel It?” reports compared to the estimates of ground shaking from Ground Motion Prediction Equation (GMPE) estimates. GMPE are empirical relations between earthquakes and recorded seismologic observations from those earthquakes, largely controlled by distance to the fault, ray path (direction and material properties), and site effects (the local geology). When seismic waves propagate through sediment, the magnitude of the ground motions increases in comparison to when seismic waves propagate through bedrock. The orange line is a regression of data for the central and eastern US and the green line is a regression through data from the western US.


  • The 2004/2005 SASZ earthquakes also tended to load strain in the crust in different locations. On 2012.04.11 there was a series of strike-slip earthquakes in the India plate crust to the west of the 2004/2005 earthquakes. The two largest magnitudes for these earthquakes were M 8.6 and M 8.2. The M 8.6 is the largest strike-slip earthquake ever recorded.
  • On 2016.03.22 there was another large strike-slip earthquake in the India-Australia plate. This is probably related to this entire suite of subduction zone and intraplate earthquakes. I presented an interpretive poster about this M 7.8 earthquake here. Below is my interpretive poster for the M 7.8 earthquake. Here is the USGS website for this earthquake.
  • I include a map in the upper right corner that shows the historic earthquake rupture areas.

  • Here is a poster that shows some earthquakes in the Andaman Sea. This is from my earthquake report from 2015.11.08.

  • This map shows the fracture zones in the India-Australia plate.

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.
  • Bassin, C., Laske, G. and Masters, G., The Current Limits of Resolution for Surface Wave Tomography in North America, EOS Trans AGU, 81, F897, 2000.
  • Bock, Y., Prawirodirdjo, L., Genrich, J.F., Stevens, C.W., McCaffrey, R., Subarya, C., Puntodewo, S.S.O., Calais, E., 2003. Crustal motion in Indonesia from Global Positioning System measurements: Journal of Geophysical Research, v. 108, no. B8, 2367, doi: 10.1029/2001JB000324.
  • 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.
  • Chlieh, M., Avouac, J.P., Sieh, K., Natawidjaja, D.H., Galetzka, J., 2008. Heterogeneous coupling of the Sumatran megathrust constrained by geodetic and paleogeodetic measurements: Journal of Geophysical Research, v. 113, B05305, doi: 10.1029/2007JB004981.
  • DEPLUS, C. et al., 1998 – Direct evidence of active derormation in the eastern Indian oceanic plate, Geology.
  • DYMENT, J., CANDE, S.C. & SINGH, S., 2007 – Oceanic lithosphere subducting beneath the Sunda Trench: the Wharton Basin revisited. European Geosciences Union General Assembly, Vienna, 15-20/05.
  • Hayes, G. P., Wald, D. J., and Johnson, R. L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
  • Hayes, G.P., Bernardino, Melissa, Dannemann, Fransiska, Smoczyk, Gregory, Briggs, Richard, Benz, H.M., Furlong, K.P., and Villaseñor, Antonio, 2013. Seismicity of the Earth 1900–2012 Sumatra and vicinity: U.S. Geological Survey Open-File Report 2010–1083-L, scale 1:6,000,000, https://pubs.usgs.gov/of/2010/1083/l/.
  • JACOB, J., DYMENT, J., YATHEESH, V. & BHATTACHARYA, G.C., 2009 – Marine magnetic anomalies in the NE Indian Ocean: the Wharton and Central Indian basins revisited. European Geosciences Union General Assembly, Vienna, 19-24/04.
  • Ji, C., D.J. Wald, and D.V. Helmberger, Source description of the 1999 Hector Mine, California earthquake; Part I: Wavelet domain inversion theory and resolution analysis, Bull. Seism. Soc. Am., Vol 92, No. 4. pp. 1192-1207, 2002.
  • Ishii, M., Shearer, P.M., Houston, H., Vidale, J.E., 2005. Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array. Nature 435, 933.
  • 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.
  • Maus, S., et al., 2009. EMAG2: A 2–arc min resolution Earth Magnetic Anomaly Grid compiled from satellite, airborne, and marine magnetic measurements, Geochem. Geophys. Geosyst., 10, Q08005, doi:10.1029/2009GC002471.
  • Malik, J.N., Shishikura, M., Echigo, T., Ikeda, Y., Satake, K., Kayanne, H., Sawai, Y., Murty, C.V.R., Dikshit, D., 2011. Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India: Geology, v. 39, p. 559-562.
  • Meltzner, A.J., Sieh, K., Chiang, H., Shen, C., Suwargadi, B.W., Natawidjaja, D.H., Philobosian, B., Briggs, R.W., Galetzka, J., 2010. Coral evidence for earthquake recurrence and an A.D. 1390–1455 cluster at the south end of the 2004 Aceh–Andaman rupture. Journal of Geophysical Research 115, 1-46.
  • Meng, L., Ampuero, J.-P., Stock, J., Duputel, Z., Luo, Y., and Tsai, V.C., 2012. Earthquake in a Maze: Compressional Rupture Branching During the 2012 Mw 8.6 Sumatra Earthquake in Science, v. 337, p. 724-726.
  • 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.
  • Prawirodirdjo, P., McCaffrey,R., Chadwell, D., Bock, Y, and Subarya, C., 2010. Geodetic observations of an earthquake cycle at the Sumatra subduction zone: Role of interseismic strain segmentation, JOURNAL OF GEOPHYSICAL RESEARCH, v. 115, B03414, doi:10.1029/2008JB006139
  • 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.
  • Shearer, P., and Burgmann, R., 2010. Lessons Learned from the 2004 Sumatra-Andaman Megathrust Rupture, Annu. Rev. Earth Planet. Sci. v. 38, pp. 103–31
  • SATISH C. S, CARTON H, CHAUHAN A.S., et al., 2011 – Extremely thin crust in the Indian Ocean possibly resulting from Plume-Ridge Interaction, Geophysical Journal International.
  • 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.
  • Singh, S.C., Carton, H.L., Tapponnier, P, Hananto, N.D., Chauhan, A.P.S., Hartoyo, D., Bayly, M., Moeljopranoto, S., Bunting, T., Christie, P., Lubis, H., and Martin, J., 2008. Seismic evidence for broken oceanic crust in the 2004 Sumatra earthquake epicentral region, Nature Geoscience, v. 1, pp. 5.
  • 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.
  • Sorensen, M.B., Atakan, K., Pulido, N., 2007. Simulated Strong Ground Motions for the Great M 9.3 Sumatra–Andaman Earthquake of 26 December 2004. BSSA 97, S139-S151.
  • Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., McCaffrey, R., 2006. Plate-boundary deformation associated with the great Sumatra–Andaman earthquake: Nature, v. 440, p. 46-51.
  • Tolstoy, M., Bohnenstiehl, D.R., 2006. Hydroacoustic contributions to understanding the December 26th 2004 great Sumatra–Andaman Earthquake. Survey of Geophysics 27, 633-646.
  • Zhu, Lupei, and Donald V. Helmberger. “Advancement in source estimation techniques using broadband regional seismograms.” Bulletin of the Seismological Society of America 86.5 (1996): 1634-1641.

Posted in asia, earthquake, education, geology, Indian Ocean, Indonesia, plate tectonics, subduction, sumatra

Earthquake Report: Philippines!

There was a deep focus earthquake in the Philippines today. This shaker was located near the city of Manila (I live in Manila. Manila, California). The hypocenter was quite deep (168 km) so (a) had lesser shaking due to the distance to the earthquake from Earth’s surface and (b) was not related to the subduction zone fault. Seismicity associated with the megathrust fault is typically less than 40 km or so. As the oceanic lithosphere dives into the upper mantle, there are lots of processes that can lead to earthquakes (e.g. internal deformation due to bending of the slab). Many are familiar with extensional earthquakes in this region (e.g. the 2001 Nisqually Earthquake in Washington, USA). But, today’s earthquake is compressional.

Here is the USGS website for today’s M 6.2 earthquake.

There was a series of earthquakes earlier this year and here is my earthquake report for those earthquakes.

Here is a report from earthquake report dot 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 1917-2017 for magnitudes M ≥ 7.

I also include the USGS moment tensor for today’s earthquake, as well as for the other historic earthquake in the region that had a similarly deep focus (the 1985.04.23 M 7.0 earthquake). The 1985 earthquake was an extensional earthquake and slightly deeper (188 km).

  • 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 poster.

  • On the left side of the poster I include a small scale (upper panel) and a large scale (bottom panel) view of the regional tectonics (Zahirovic et al., 2014). Plate boundary fault symbology (and other features, like fracture zones) is shown in the legend. I place a blue star on the map in the general location of this earthquake epicenter.
  • In the upper right corner I include a map showing the seismicity and tectonic plate boundary faults for this region (Smoczyk et al., 2013). Earthquakes are plotted with color representing depth and diameter representing magnitude (see legend). To the left of the map I include the cross section C-C’ that shows the earthquake hypocenters.
  • In the lower right corner is another tectonic map (Fan et al., 2015) that shows a seismic tomographic cross section. Seismic Tomography is a way of using seismic waves (like X-Rays in a CT scan) to peer into Earth’s interior. Based upon how seismic waves travel at different velocities depending upon the type of material (e.g. oceanic crust or the asthenospheric mantle), we can deduce the location of subducted slabs inside the Earth. To the left of the map is a tomographic cross section with the subducting Sunda plate colored blue as it subducts to the east, forming the Manila Trench. This cross section F-F’ is 2 degrees north of today’s M 6.2 earthquake. Below the cross section is an illustration showing how the authors interpret the subducting slab to be configured beneath the Philippines.


  • Here is the poster from the earthquakes that happened earlier this year.

  • Below is a series of figures from Wu et al., 2016). These figures reveal information about the subduction zone forming the Manila Trench west of the northern Philippines. I will include their original figure caption in blockquote beneath the figures.
  • Here is a map showing the regional topography/bathymetry, along with the relative plate motions (in the inset map, note the vector for scale).

  • (a) Present-day Philippine Sea and East Asian tectonic setting. Plate motion azimuths in Figure 1a are shown relative to stable Eurasia from the MORVEL model [deMets et al., 2010]. Paleomagnetic sample locations in Figure 3 are shown by the colored symbols. (b) Present-day differential velocities across the Philippine Sea plate boundaries calculated from MORVEL. The southeast Philippine Sea plate is strongly edge coupled to the Pacific plate through the Caroline Sea, which has Pacific-like velocities. HB, Huatung Basin; AP, Amami Plateau; DR, Daito Ridge; ODR, Oki-Daito Ridge; BR, Benham Rise; KPR, Kyushu-Palau ridge; MS, Molucca Sea minor plate; BH, Bird’s Head minor plate; Hal, Halmahera.

  • This map shows the magnetic anomalies for the crust in the western Pacific. These magnetic anomalies help us interpret the age of the crust and the history of plate motion. As new oceanic crust is formed at mid ocean ridges, when Earth’s magnetic polarity flips, anomalies from parallel to the oceanic ridges. There is an inset map showing the plate age in colors. (Wu et al., 2016)

  • (a) EMAG2 gridded magnetic anomalies for the Philippine Sea and East Asia [Maus et al., 2009]. Plate motion azimuths as in Figure 1. (b) Philippine Sea gridded seafloor spreading model used in this study (modified from Müller et al. [2008] and Seton et al. [2012]). WPB, West Philippine Basin; SB, Shikoku Basin; PVB, Parece Vela Basin; MT, Mariana Trough; DRP, Daito ridges province; PB, Palau Basin; L, Luzon; KPR, Kyushu-Palau ridge; HB, Huatung Basin.

  • Here is a tomographic cross section in the region just north of the 2017.08.11 M 6.2 earthquake. Note the downgoing “Eurasia slab” highlighted showing the subducting Sunda plate. The Sunda plate is part of Eurasia, but is oceanic crust. There is also a larger scale cross section for the Mariana Trench. (Wu et al., 2016)

  • (a) MITP08 tomographic cross section oriented along the mean 0 to 50Ma Pacific convergence direction showing the subvertical Pacific “slab wall” under the central Marianas. (b) Maximum E-W width of the Pacific slab wall anomaly under the northern and central Marianas calculated from a 0% dVp cutoff along three transects. (c) Pacific slab wall from Figure 15a shown undistorted within spherical Earth model. Three possible Pacific slab areas A to C (dashed colored lines) were picked from the tomographic section that were guided by dVp cutoffs of 0.2% to 0%, respectively. We measured unfolded slab lengths between 3041 km and 4447 km for areas A to C using cross-sectional area unfolding (for method, see Figure 8). Our unfolded slab lengths were corrected for PREM density-depth changes [Dziewonski and Anderson, 1981] and assumed an incoming 100 km thick Pacific slab. (d) Total Pacific slab subduction times for slab areas A to C was 48 ± 10 Ma, based on a comparison of unfolded Pacific slab lengths to the Pacific convergence rate at the central Marianas from Seton et al. [2012].

  • Here are some more tomographic cross sections in the region. Cross section E-E’ is south of the 2017.08.11 M 6.2 earthquake and section D-D’ is just to the north of this earthquake. Section D-D’ is probably most representative of the subduction zone near this M 6.2 earthquake. (Wu et al., 2016)

  • (a to f) Interpreted slabs and mantle structure under East Asia from MITP08 tomography vertical cross sections. The study area is dominated by subhorizontal, relatively lower amplitude detached slabs at 500 to 1100 km depths that we call the East Asian Sea slabs. Inset map shows section locations. Seismicity shown by red dots. AUS, Australian craton; Ayu, deep Ayu Trough slab; BMS, Bismarck Sea; MS, Molucca Sea slab; NH, New Hebrides slab; OR, Ordos block; Pac, Pacific slabs; PP, proto-Pacific slabs; PSCS, proto-South China Sea slabs; PSP, Philippine Sea plate; PT, Philippine Trench slab; Ryu, Philippine Sea Ryukyu slab; SCS, South China Sea and Eurasian slabs; Shk, Philippine Sea Shikoku slab; SMar, southern Marianas detached slab; SolE, Solomon east slab; SolW, Solomon west slab; SolS, Solomon south slab; SS, Solomon Sea plate; Sulu, Sulu Sea slab; Su, Sunda slabs.

  • This figure shows details about the megathrust that forms the Philippine Trench to the southeast of the 2017.08.11 M 6.2 earthquake. I include this figure to show that the Philippine Trench subduction zone is unlikely to be responsible for today’s M 6.2 earthquake. (Wu et al., 2016)

  • Slab constraints for the SW Philippine Sea and surrounding areas. (a) Philippine Trench slab, Molucca Sea slabs, and detached “deep Ayu Trough” midslab maps. (b) Three-dimensional oblique view from west showing projected seismicity within 50 km of the Philippine Trench and Molucca Sea west midslab surfaces. (c) Unfolded Philippine Trench and Molucca Sea slabs colored by their dVp midslab seismic velocities. (d and e) MITP08 vertical tomographic cross sections and Benioff zone seismicity (red spheres) showing the interpreted fast-slab anomalies. Section locations are shown in Figure 21a. Note that the unfolded Molucca Sea slab in Figure 21c was the minimumlength model. A longer unfolded Molucca Sea slab is possible based on possible deeper (>900 km) anomalies in Figure 21b and the slab buckling in Figure 21e. PSP, Philippine Sea plate; Phil, Philippines; MS, Molucca Sea.

  • Here is the money shot, showing the subducting Sunda plate (labeled “Eurasia slab”) beneath the northern Philippines. h/t to JD Dianala for pointing out this figure in a Twitversation online. This is a fantastic paper that discusses the tectonics of the Philippine Sea plate and surrounding regions. (Wu et al., 2016)

  • Eurasian slab constraints shown by (a) map of Benioff zone seismicity within 50 km of the midslab surface, (b) Eurasia midslab depth map. (c) Unfolded Eurasian slab colored by its intraslab seismic velocity dVp. The unfolded Eurasian slab has a 400 to 500 km E-W width and has an eastern edge that terminates near Ishigaki at the Ryukyu Islands. Similarly, the unfolded northern proto-South China Sea detached slabs shown by the purple dashed line also have a similar eastern limit. This suggests a linked origin, namely, that the South China Sea opened as a back-arc basin through subduction of the proto-South China Sea. (d) Three dimensional visualization of the subvertical Eurasian midslab surface between Taiwan and Palawan. Coastlines in white.

  • This is a compilation of their (Wu et al., 2016) plate tectonic reconstruction. This is not important to interpret today’s earthquake, but it is interesting to know an alternate view of how the modern plate boundary configuration formed. (Wu et al., 2016)

  • Preferred Philippine Sea plate reconstruction Model 1 showing its origin near the Manus mantle plume (yellow dot). Stippled areas show the unfolded slab constraints from this study. Purple polygons show oceanic plateaus from Ishizuka et al. [2013] and the UTIG University of Texas LIPs database [Coffin, 2011]. HB, Huatung Basin.

  • Here is another view of the megathrust at the Manila Trench. This figure shows how the slab is more shallowly dipping in the north and more steeply dipping in the south (where the 2017.08.11 M 6.2 earthquake happened).

  • Here is a figure from Wang and Bilek (2014) that shows what proportion of the fault is creeping vs. being seismogenically coupled. Typically, people present figures like this and display the coupling ratio (what % of the plate motion is contributing to elastic strain), but this paper is about creeping faults, so they plot the opposite. One may look at it like this: a 20% creeping ratio = a 80% coupling ratio.

  • Creeping state and seismicity of the Manila Trench. Megathrust creeping ratio was determined by Hsu et al. (2012) by inverting GPS data from shown sites (triangles); Galgana et al. (2007) reported almost 100% creeping based on similar data.

  • This is an interesting figure to compare to the last one. Particularly in panel a, which shows the convergence rate as constrained by Global Positioning Systems (GPS) observations (Hsu et al., 2012). Note how the southern margin has a lower convergence rate (where the megathrust is creeping more).

  • Plate boundary deformation and trench parallel gravity anomaly along the Manila subduction zone. (a) Black vectors are GPS station velocities in the Sunda fixed reference frame. Error ellipses indicate 95% confidence intervals of GPS velocities. Blue vectors are velocities corrected for fault locking effect on the Philippine Fault (PHF in (b)). The yellow–red color scale indicates plate convergence rate. Bathymetry is shown in grey scale. The inset shows the regional geography with a red box indicating the study area (b) The seismicity is in the time period between 1973 and 2010 from NEIC (the US Geological Survey National Earthquake Information Center). The moment magnitude is in the range between 4.6 and 7.7. Color scale indicates focal depth. (c) The shaded relief topography and estimated free-air TPGA on the Manila subduction zone (Sandwell and Smith, 2009). The color bar indicates the amplitude of TPGA values. The black barbed and dashed lines denote the Manila Trench and 50 km slab iso-depth, respectively. (d) The Bouguer TPGA at the Manila subduction zone.

References:

  • Bock et al., 2003. Crustal motion in Indonesia from Global Positioning System measurements in JGR, v./ 108, no. B8, 2367, doi:10.1029/2001JB000324
  • Hall, R., 2011. Australia–SE Asia collision: plate tectonics and crustal flow in Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 75–109.
  • Fan, J-k., Wu, S-g., Spence, G., 2015. Tomographic evidence for a slab tear induced by fossil ridge subduction at Manila Trench, South China Sea in International Geology Review, v. 57, p. 998-1013, DOI: 10.1080/00206814.2014.929054
  • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
  • Hsu, Y-J., Yu, S-B., Song, T-R.A., and Bacolcol, T., 2012. Plate coupling along the Manila subduction zone between Taiwan and northern Luzo in Journal of Asian Earth Sciences, v. 51, p. 98-108
  • McCaffrey, R., Silver, E.A., and Raitt, R.W., 1980. Crustal Structure of the Molucca Sea Collision Zone, Indonesia in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands-Geophysical Monograph 23, p. 161-177.
  • Noda, A., 2013. Strike-Slip Basin – Its Configuration and Sedimentary Facies in Mechanism of Sedimentary Basin Formation – Multidisciplinary Approach on Active Plate Margins http://www.intechopen.com/books/mechanism-of-sedimentarybasin-formation-multidisciplinary-approach-on-active-plate-margins http://dx.doi.org/10.5772/56593
  • Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013. Seismicity of the Earth 1900–2012 Philippine Sea plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, 1 sheet, scale 1:10,000,000.
  • Waltham et al., 2008. Basin formation by volcanic arc loading in GSA Special Papers 2008, v. 436, p. 11-26.
  • Wang, K., and Bilek, S.L., 2014. Invited review paper: Fault creep caused by subduction of rough seafloor relief in Tectonophysics, v. 610, p./ 1-24.
  • Wu, J., J. Suppe, R. Lu, and R. Kanda, 2016. Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods, J. Geophys. Res. Solid Earth, 121, doi:10.1002/2016JB012923.
  • Zahirovic et al., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014.

Posted in asia, earthquake, education, geology, pacific, plate tectonics, subduction

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.

Posted in asia, collision, earthquake, education, geology, plate tectonics, strike-slip

Earthquake Report: China!

This morning (my time) there was a shallow earthquake in western China, near a region that was shocked by an M 7.9 earthquake in 2008 (the “Wenchuan Earthquake”).

Here is the USGS website for this M 6.5 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 also include USGS epicenters from 1917-2017 for magnitudes M ≥ 4.5.

I also include the USGS moment tensor for today’s earthquake. While it is possible that either nodal plane is correct, read below to see how I interpret today’s earthquake given the publications I include in this report (see discussion about inset figures).

This region lies along a boundary between extrusion tectonics related Kunlun and Haiyuan faults and the Longmen Shan thrust fault system. As the India subcontinent collides with Asia, central Asia is extruded to the east, forming these major strike-slip fault systems. The Haiyuan fault system experienced an M 8.3 earthquake on 1920.12.16 (shown on main map) and is one of the faults that is thought to experience superquakes and supercycles. There have been several M 7.8 earthquakes on the Kunlun fault (1932 and 2001)..

  • 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 lower right corner I include a map showing the tectonic domains in China (Zheng et al., 2013). I place a blue star in the general location of today’s M 6.5 earthquake (as in other figures).
  • In the center left, I include a tectonic map, showing the major faults in the region (Kirby et al., 2007). The inset map is below this figure in the lower left corner. This larger scale maps shows more detail of the faults (and their study sites). Today’s earthquake happened in the mountains of Min Shan, where there are numerous north-south striking thrust faults (e.g. the Minjiang fault and the Fuya fault; the Minjian fault is labeled MJ in the upper map).
  • To the left of the Zheng et al. (2013) map is a large scale map from Yong et al., 2009. This map shows more faults at this scale. The MJ (Mijiang fault) and HY (Fuya fault) are thrust faults and probably not related to today’s earthquake. The fault labeled DKL (I cannot yet determine the name of this fault, it is not listed in their publication) is the likely culprit for today’s earthquake. Given the moment tensor, shallow depth, and orientation of the DKL fault, I interpret today’s earthquake to be a west-northwest striking left-lateral strike-slip fault.
  • In the upper right corner I include a larger scale map with the same earthquakes as plotted in the main map. I highlight the earthquakes associated with the 2008 M 7.9 Wenchuan Earthquake sequence. This earthquake triggered tens of thousands of landslides and killed many people. There was also a series of earthquakes (with an M 6.6 mainshock) to the southwest of the 2008 earthquakes, which were probably related to the Wenchuan Earthquake. To the south of today’s M 6.5 earthquake, there was a series of southward propagating thrust fault earthquakes [possibly/probably] along the Fuya fault system.


  • Here is the fault block map from Zheng et al. (2013). I include their figure caption below in blockquotes.

  • Simplified tectonic map of China showing major cratonic blocks and orogenic belts (modified after Zhao et al., 2001). Circled asterisks denote the UHP metamorphic terranes in the central orogenic belt of China (Zheng et al., 2012), which occur from west to east: Southwest Tianshan, Altyn, North Qaidam, North Qinling, and Dabie and Sulu.

  • Here are the two maps from Kirby et al. (2007). I include their figure caption below in blockquotes.

  • Tectonic map showing major and minor active faults in eastern Tibet. Abbreviations are as follows; BJ, Bailong Jiang fault; E, Elashan fault; MJ, Min Jiang fault; R, Riyueshan fault; T, Tazang fault. Epicentral locations and focal mechanism solutions of recent seismicity along the Kunlun fault are compiled from USGS (http://neic.usgs.gov/neis/epic/epic_circ.html), Harvard CMT catalog (http://www.globalcmt.org/CMTsearch.html), and Molnar and Lyon-Caen [1989]. Topographic base is generated from the Shuttle Radar Topography Mission (SRTM) data.


    Map of the eastern segment of the Kunlun fault showing location of active faults and major physiographic features. Active faults are represented by heavy lines; dashed where recent activity is inferred. Yellow River (Huang He) is shown as blue line. White boxes represent slip rate estimates (mm/yr) [Van der Woerd et al., 2002b]. Locations of new slip rate determinations are labeled in white circles (1, 2, 3). Focal mechanisms are compiled from USGS (http://neic.usgs.gov/neis/epic/epic_circ.html).

  • Here is the large scale map from Yong et al. (2009). I include their figure caption below in blockquotes.

  • Focal mechanisms of Wenchuan earthquake aftershocks with Ms≥5.6. The numbers 1-12 are the indexes of the aftershocks (1―10 are the same as Table 2). The focal mechanism of the Wenchuan main event comes from the result of Harvard CMT solution. The black lines are the Quaternary faults. The abbreviated names of the faults are the same as Figure 2.

  • Here is a figure from Yong et al. (2003) that shows the development of these large strike-slip faults (e.g. the Kunlun fault) prior to the collision of India with Asia. These faults, at this time, were thrust faults. The Longmen Shan fault system is labeled LSFB in the lower panel. I include their figure caption below in blockquotes.

  • Cartoons showing geological evolution from South China Block passive margin flaking a deep remnant ocean basin accumulating sediments of the Songpan-Ganzi Complex in (a), to oceanic closure, telescoping of the Songpan-Ganzi Complex and South China margin and formation of the Longen Shan Foreland Basin as a flexural foredeep in (G). Modified from Harrowfield (2001).

  • Here is a map from Jacha Polet, a seismologist at Cal Poly Pomona. They plot moment tensors from their compiled historic database.

  • Here is a figure from Gorum et al. (2008) that shows the epicenters from the Wenchuan Earthquake sequence, along with faults and historic earthquakes. I include their figure caption below in blockquotes.

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

  • Here is a figure from Gorum et al. (2008) that shows the relations between earthquake slip (proxy for ground shaking) and landslide concentration (the number of landslides per square kilometer). I include their figure caption below in blockquotes.

  • Landslide concentration in relation to the total co-seismic slip distribution of the fault rupture (co-seismic slip distribution model is from Shen et al., 2009). a: The landslide concentration; b: contour lines of landslide concentration clipped for the projected fault plane boundary; c: the co-seismic slip distribution; d: contour lines of co-seismic slip distribution.

    References:

  • Goldfinger, C., Ikeda, Y., Yeats, R.S., and Ren, J., 2013. Superquakes and Supercycles in Seismological Research Letters, v. 84, no. 1, doi: 10.1785/0220110135
  • 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
  • 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.

Posted in asia, earthquake, education, geology, plate tectonics, strike-slip, Transform

Earthquake Report: Gorda plate

We just had an earthquake in the Gorda plate. The USGS magnitude is 5.1. This earthquake happened a few kilometers southwest of the 2014 M 6.8 earthquake. Based upon the orientation of the faults in the region, today’s earthquake may have occurred on the same fault as the 2014 earthquake (but it is really difficult to tell and just as likely did not).

The last earthquake report I prepared for a Gorda plate earthquake happened on 2016.09.25. Here is my report for that earthquake.

Here is the USGS website for this 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 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 the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    I include some inset figures in the poster.

  • In the upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004)
  • Below the CSZ map is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. Today’s earthquake did not occur along the CSZ, so did not produce crustal deformation like this. However, it is useful to know this when studying the CSZ.
  • In the lower left corner is a figure from Rollins and Stein (2010). In their paper they discuss how static coulomb stress changes from earthquakes may impart (or remove) stress from adjacent crust/faults. I also present their figure where they present seismic observations for the 1983.08.24 M 5.5 earthquake (Rollins and Stein list M 6.3). I place a yellow star in the general location of today’s earthquake.
  • In the upper right corner is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. More about this figure can be found below.


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

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

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

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

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

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

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

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

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:


    References:

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Burgette, R. et al., 2009. Interseismic uplift rates for western Oregon and along-strike variation in locking on the Cascadia subduction zone in Journal of Geophysical Research, v. 114, B01408, doi:10.1029/2008JB005679
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356
  • Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gràcia, E., Enkin, R., Dallimore, A., Dunhill, G., and Vallier, T., 2012. Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper # 1661F. U.S. Geological Survey, Reston, VA, 184 pp.
  • McCrory, P. A., Blair, J. L., Oppenheimer, D. H., and Walter, S. R., 2006. Depth to the Juan de Fuca slab beneath the Cascadia subduction margin; a 3-D model for sorting earthquakes U. S. Geological Survey
  • Nelson, A.R., Kelsey, H.M., and Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone: Quaternary Research, doi:10.1016/j.yqres.2006.02.009, p. 354-365.
  • Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
  • Rollins, J.C., Stein, R.S., 2010. Coulomb Stress Interactions Among M ≥ 5.9 Earthquakes in the Gorda Deformation Zone and on the Mendocino Fault Zone, Cascadia Subduction Zone, and Northern San Andreas Fault. Journal of Geophysical Research 115, 19 pp.
  • USGS Quaternary Fault Database: http://earthquake.usgs.gov/hazards/qfaults/
  • Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D., and Sagiya, T., 2003. A revised dislocation model of interseismic deformation of the Cascadia subduction zone Journal of Geophysical Research, B, Solid Earth and Planets v. 108, no. 1.

Posted in cascadia, earthquake, geology, gorda, strike-slip

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.

Posted in collision, earthquake, education, europe, Extension, geology, plate tectonics, strike-slip, Transform

Earthquake Report: Bering fracture zone: UPDATE #1

Well, based upon a comment on twitter, I thought it prudent to review the seismicity of this region at the westernmost part of the western Aleutian Islands. For many years I considered this region part of the subduction zone that forms the Aleutian Trench. In the past couple of years, there have been a number of earthquakes that reveal this to not be the case. Rather, the Pacific-North America plate boundary in this region is in the form of a shear zone, distributed across several reactivated fracture zones. I noticed a plot from Jascha Polet and found another article that evaluates this plate boundary (Davaille and Lees, 2004).

Here is my initial #EarthquakeReport on this M 7.7 strike-slip earthquake.

Here is a report from the University of Alaska Fairbanks, Alaska Earthquake Center.

Below is my interpretive poster 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 plot moment tensors for the M 7.7 earthquake. Based upon the series of earthquakes and the mapped faults, I interpret this M 7.7 earthquake as a right-lateral strike-slip earthquake related to slip associated with the Bering fracture zone.
  • 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 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 ≥ 5.0. The depths for these earthquakes is represented by color. This is quite revealing when considering whether subduction is occurring in this part of the plate boundary. Note how earthquakes east of Attu Island have hypocentral depths (3-dimensional location of the earthquake focus) that extend to over 150 km. These represent earthquakes in the downgoing Pacific plate. We also observe earthquakes even deeper along the Kamchatka Trench on the west. Also note that the slab contours (Hayes et al., 2012) have decreasing maximum depths from east to west between Bowers Ridge and Attu Island. The same is true for the subduction zone beneath Kamchatka, though the cut-off of these depth contours is more abrupt (due to the tear in the Pacific plate, which marks the edge of the subduction zone beneath Kamchatka). Finally, note how the earthquake depths in the region between Attu Island and Kamchatka are largely less than 33 km.

    I include some inset figures in the poster.

  • In the upper left corner I include a regional tectonic map showing Kamchatka and the westernmost tip of the westernmost Aleutian Islands (Davaille and Lees, 2004).
  • In the lower right corner I show a schematic illustration that depicts how the subduction transitions to strike-slip in the region of the Attu Island (Davaille and Lees, 2004).
  • To the right is a large scale map of this region showing the complicated intersection of strike slip and compressional tectonics as the tip of the Aleutians intersects with Kamchatka (Gaedicke et al., 2000).
  • In the upper right corner is a figure showing seismicity in this region (Davaille and Lees, 2004). There is a map (with earthquakes plotted vs. depth in color) and several cross sections (one parallel to the Kamchatka trench and two normal/perpendicular to the trench). Note how the depths for the earthquakes shallow to the north of the Meiji Seamount, and particularly shallower to the north of the Stellar fault (a.k.a. Bering Kresla Shear zone) and the Bering fault (a.k.a. Bering fracture zone).


  • Here is my original #EarthquakeReport Interpretive Poster and report.

  • Here is the seismicity plot from Davaille and Lees (2000). They suspect that the tear in the Pacific plate is related to the interaction of a mantle plume hotspot and the oceanic lithosphere as it is formed near this hotspot. I include their figure caption below in blockquote.

  • Seismicity in Kamchatka. (a) Mapview showing earthquakes plotted with colors corresponding to depth. Projections of the Bering and Steller faults onto Kamchatka are presented for clarity and reference. (b–d) Vertical cross-sections of seismicity in Kamchatka. Magenta circles are from the Gorbatov et al. [3] catalog and yellow events are from the Engdahl et al. [52] catalog. The projection of the subducting Meiji seamounts is shaded gray.

  • Here is a plot from Jascha Polet, seismologist at Cal Poly Pomona. These data are consistent with the Davaille and Lees (2000) plot, showing shallow seismicity in this region of the plate boundary.

  • Here is the low-angle oblique map from Portnyagin and Manea (2008). This was in my original report, but is a great visualization for this interpretation of the Pacific plate tear (the edge of subduction in Kamchatka). While this is not evidence for the lack of subduction in the region of the Bering Kresla Shear zone, it does suggest that there is no active subduction beneath Kamchatka to the north. If this is the case (and I have no reason to think that it is not the case), there is little evidence for subduction west of Attu Island and east of Kamchatka. I include the figure caption as a blockquote below.

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

  • Here is another great visualization of the subducting slabs in this region, prepared by Lees et al. (2007). This figure shows isosurfaces (surfaces of equal value of some parameter, like seismic velocity, or Vp:Vs ratios; Vp = P-Wave velocity, Vs = S-Wave velocity). These isosurfaces were created using seismic tomography, which is basically just like a CT scan of the earth. CT stands for “computed tomography” of X-Rays. The mathematical solutions used in these methods are the same, even though the source of the data are different (X-Rays vs. seismic waves). Coastlines are in grey outline. Kamchatka is labelled M. The lower panel may be easier to look at first because north is up. Basically, blue is old and cold and red is young and hot. So, the downgoing Pacific plate is in blue and the surrounding mantle is in red. Note how the blue blobs end in the area of interest.

  • Tomographic image of the Kamchatka Subduction zone rendered in three-dimensions. The cut-off perturbation level is 3% with blue regions being high velocity and red lower velocity perturbations. The slab is a clear high velocity zone approximately 100 km thick plunging into the upper mantle at an angle of ~50. The green plane represents the top of the subduction zone seismicity, contoured and rendered along with the tomographic images. Gold cones are active volcanoes along the Kamchatka arc and white squares are stations included for reference to the map in Figure 1. The bars represent length scales of 100 km. Points of interest discussed in the text are marked with letters (I–M).

  • This is a great visualization from IRIS that shows the seismic waves propagating through seismometers operated in the US. First is a screenshot and below is the embedded video. Here is a link to the video file (7 MN mp4). The seismic wave at the bottom of the animation is for the seismometer located in the center of the green circle on the map.

  • Finally of note (unrelated to the previous discussion) is that this M 7.7 had a really long duration (Dr. Polet pointed this out in a tweet). This plot is from IPGP here. The source time function represents how much energy is released over time. Time is on the horizontal axis.

    References:

  • Bassett and Watts, 2015. Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief in Geochemistry, Geophysics, Geosystems, v. 16, doi:10.1002/2014GC005684.
  • Davaille, A. and Lees, J.M., 2004. Thermal modeling of subducted plates: tear and hotspot at the Kamchatka corner in EPSL, v. 226, p. 293-304.
  • Gaedicke, C., Baranov, B., Seliverstov, N., Alexeiev, D., Tsukanov, N., Freitag, R., 2000. Structure of an active arc-continent collision area: the Aleutian-Kamchatka junction. Tectonophysics 325, 63–85
  • 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.
  • Ikuta, R., Mitsui, Y., Kurokawa, Y., and Ando, M., 2015. Evaluation of strain accumulation in global subduction zones from seismicity data in Earth, Planets and Space, v. 67, DOI 10.1186/s40623-015-0361-5
  • Konstantnovskaia, 2001. Arc-continent collision and subduction reversal in the Cenozoic evolution of the Northwest Pacific: an example from Kamchatka (NE Russia) in Tectonophysics, v. 333, p. 75-94.
  • Koulakov, I.Y., Dobretsov, N.L., Bushenkova, N.A., and Yakovlev, A.V., 2011. Slab shape in subduction zones beneath the Kurile–Kamchatka and Aleutian arcs based on regional tomography results in Russian Geology and Geophysics, v. 52, p. 650-667.
  • Krutikov, L., et al., 2008. Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179, doi:10.1029/179GM07
  • Lees, J.M., VanDecar, J., Gordeev, E., Ozerov, A., Brandon, M., Park, J., and Levvin, V, 2007. Three Dimensional Images of the Kamchatka-Pacific Plate Cusp in Volcanism and Subduction: The Kamchatka Region Geophysical Monograph Series 172, DOI: 10.1029/172GM06
  • Portnyagin, M. and Manea, V.C., 2008. Mantle temperature control on composition of arc magmas along the Central Kamchatka Depression in Geology, v. 36, no. 7, p. 519-522.
  • Rhea, S., Tarr, A.C., Hayes, G., Villaseñor, A., Furlong, K.P., and Benz, H.M., 2010. Seismicity of the Earth 1900-2007, Kuril-Kamchatka arc and vicinity: U.S. Geological Survey Open-File Report 2010-1083-C, 1 map sheet, scale 1:5,000,000.

Posted in alaska, earthquake, education, pacific, plate tectonics, strike-slip, subduction, Transform

Earthquake Report: Western Aleutians

We just had an earthquake along the western Aleutian Islands, very close to the international date line. In this region places often have more than two names, depending upon who drew the map.

The majority of the Aleutian Islands are volcanic arc islands formed as a result of the subduction of the Pacific plate beneath the North America plate. To the west, there is another subduction zone along the Kuril and Kamchatka volcanic arcs. These subduction zones form deep sea trenches (the deepest parts of the ocean are in subduction zone trenches). Between these 2 subduction zones is another linear trough, but this does not denote the location of a subduction zone. The plate boundary between the Kamchatka and Aleutian trenches is the Bering Kresla shear zone (BKSZ).

The oceanic basin, Komandorsky Basin, to the north of the BKSZ has been mapped with northwest trending fracture zones, most of which are fossil or inactive. However, due to the oblique convergence west of Bowers Ridge, some of these fossil fracture zones are being reactivated. Based upon offsets in magnetic anomalies in the oceanic crust forming the basement of Komandorsky Basin, these fracture zones show left-lateral offsets. However, the active strike-slip faults (and BKSZ) are right lateral. This is a great example of strike-slip faults reactivating as strike-slip faults.

The mainshock was preceded earlier this day with several foreshocks and occurred very close to a M 6.3 earthquake from 9 months ago (2016.09.05). In addition, there was seismic activity to the east about 6 weeks ago (2017.06.02 M 6.8).

Some place a tear in the downgoing Pacific plate (beneath Kamchatka) in the position of the Bering Kresla Shear zone. This marks the end of the modern Kamchatka subduction zone. However, there was a subduction zone further to the west of the active arc, which extended further to the north and possibly involved subduction of the oceanic crust forming the Komandorsky Basin. The combination of offsets along the right-lateral strike-slip faults to the north of the BKSZ, along with the convergence along the Kamchatka Trench, there is a fold and thrust belt (a zone of compressional tectonics). There was an M 6.6 earthquake earlier this year (2017.03.29), which is somewhat related to this fold and thrust belt.

Finally, of note, is the M 8.3 deep extensional earthquake in the downgoing Pacific plate crust (2013.05.24).

There was a tsunami recorded at the tide gage on Shemya Island, with an observed maximum wave height of 0.3 ft! This is interesting given the strike slip earthquake.

  • Here are the USGS website for today’s earthquakes.
  • 2017-07-17 11:05 M 6.2
  • 2017-07-17 11:23 M 5.1
  • 2017-07-17 23:34 M 7.7
  • Here are my #EarthquakeReport pages for some of these related earthquakes.
  • 2016.09.05 M 6.3
  • 2017.03.29 M 6.6
  • 2017.06.02 M 6.8

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 2007-2017 for magnitudes M ≥ 6.0.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. I plot moment tensors for the M 7.7 earthquake, as well as for the 2013, 2016, and 2017 earthquakes mentioned above. I also include moment tensors for earthquakes in 1999 and 2001 because these are also interesting earthquakes that I had not noticed before. It appears that perhaps the 1999 strike-slip earthquake led to an increased stress on the subduction zone, which slipped in 2001. Based upon the series of earthquakes and the mapped faults, I interpret this M 7.7 earthquake as a right-lateral strike-slip earthquake related to slip associated with the Bering fracture zone.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    I include some inset figures in the poster.

  • In the upper left corner I include a regional tectonic map showing Kamchatka and the westernmost tip of the westernmost Aleutian Islands (Bindeman et a., 2002). Today’s M 7.7 earthquake is just off the map to the east.
  • To the right of the Bindeman map is a map that shows a larger scale view of the faults in this region (Gaedicke et al., 2000). I include an orange star designating the rough location of today’s M 7.7 earthquake.
  • In the lower left corner are the earthquake intensity regression plots for the M 6.2 and M 7.7 earthquakes. These are plots based upon Attenuation Relation equations (formerly called Ground Motion Prediction Equations, GMPEs). Using seismic data from thousands of earthquakes with a range of magnitudes, distances, fault types, etc., people have developed empirical relations between each of these parameters. These plots are based upon these empirical relations and show how earthquake intensity (how strong it shakes) diminishes with distance.
  • In the upper right corner is a series of tectonic maps showing one interpretation of how the plates have moved relative to each other through time (Konstantinovskaia, 2001). The time spans 65, 55, and 37 millions of years into the present. Begin by looking at the present configuration and move backwards in time. These maps show an interesting story (that appears consistent with other published data).


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

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

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

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

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

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

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

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

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


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

  • Here is the low-angle oblique map from Portnyagin and Manea (2008). I include the figure caption as a blockquote below.

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

  • Here is a map that shows the seismicity (1960-2014) for this plate boundary. This is the spatial extent for the videos below.

  • Here is a link to the file to save to your computer.

Posted in alaska, earthquake, education, pacific, plate tectonics, strike-slip, subduction

Earthquake Report: Ecuador

Earlier today there was a moderate sized earthquake (M 6.0) along coast of Ecuador. This earthquake happened in the region of the 2016.04.16 M 7.8 subduction zone earthquake. Based upon the depth and our knowledge of this region, this earthquake may also be on the megathrust. However, the depth is poorly resolved (initially depth ~ 7 km, but now set at 10 km, the default depth). Here is the USGS website for this earthquake.

More information about this earthquake can be found here at earthquake report dot 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 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 poster.

  • In the lower left corner I include a clipping of the map and cross section from the USGS Open File Report for the historic seismicity of this region (Rhea et al., 2010). I include the seismicity cross section in the upper left corner. This cross section shows earthquakes related to the downgoing Nazca plate.
  • Between the map and cross section, I include the MMI intensity maps for both the M 7.8 earthquake and this M 6.0 earthquake.
  • In the upper right corner, I include a map that shows the regional tectonics as published by Gutscher et al. (1999). These authors pose that the Carnegie Ridge exerts a control for the segmentation of the subduction zone.
  • In the lower right corner, I include a figure from Chlieh et al. (2014) that shows their coupling model. This model informs us about how strongly the subduction zone fault is seismogenically “locked” and how this varies spatially. They also plot historical earthquake locations and their “moment rate deficit” calculation (i.e. how much the plate motion rate has been accumulated as tectonic strain, which would presumably lead to earthquake slip). I include blue stars in the general location of these two earthquakes. The M 7.8 lies within the seismic gap hypothesized by Chlieh et al. (2014).


  • Here is the same poster but only with the seismicity from the past month and the MMI contours from the 2016 M 7.8 earthquake. (wait for now to get this done… need to restart software)

  • Here are my two interpretive posters for the 2016 M 7.8 earthquake. Here is my initial report. Here is my update.
  • First is the initial interpretive poster.
  • Here is the updated interpretive poster.

  • Below is the tectonic setting map from Gutscher et al. (1999). I include their figure caption as a blockquote.

  • Tectonic setting of the study area showing major faults, relative plate motions according to GPS data [7] and the NUVEL-1 global kinematic model [8], magnetic anomalies [13] and active volcanoes [50]. Here and in Fig. 4, the locations of the 1906 (Mw D 8:8, very large open circle) and from south to north, the 1953, 1901, 1942, 1958 and 1979 (M  7:8, large open circles) earthquakes are shown. GG D Gulf of Guayaquil; DGM D Dolores–Guayaquil Megashear.

  • Below is a low angle oblique view of the structures in the downgoing Nazca plate, from Gutscher et al. (1999). I include their figure caption as a blockquote.

  • 3-D view of the two-tear model for the Carnegie Ridge collision featuring: a steep ESE-dipping slab beneath central Colombia; a steep NE-dipping slab from 1ºS to 2ºS; the Peru flat slab segment south of 2ºS; a northern tear along the prolongation of the Malpelo fossil spreading center; a southern tear along the Grijalva FZ; a proposed Carnegie flat slab segment (C.F.S.) supported by the prolongation of Carnegie Ridge.

  • The 2016 M 7.8 earthquake is near two historic earthquakes with similar magnitudes. Below I plot a map showing the seismicity from 1900-2016 for earthquakes with magnitudes greater than or equal to M 6.0. Here is the USGS query that I used to make this map.
    • 1906.01.31 M 8.3 occurred ~100 km to the northeast.
    • 1942.05.14 M 7.8 occurred <50 km to the southwest.


  • I prepared an animation that shows the seismicity of this region from 1900-2016 for earthquakes with a magnitude greater than or equal to M 6.0. Here is the kml file that I used to prepare this animation. First I provide a screen shot and then a link and the embedded video.

  • Here is a link to the video file embedded below (4 MB mp4)
  • Here are a couple maps from Chlieh et al. (2014). I include their figure captions below. Chlieh et al. (2014) use GPS data to infer the spatial variation and degree to which the subduction zone megathrust is seismogenically coupled. They consider plate motion rates and estimate the moment (earthquake energy) deficit along this fault (how much strain that plate convergence has imparted upon the fault over time). Then they compare this moment deficit to regions of the fault that have slipped historically.
  • Tectonics and GPS motion rates.

  • Seismotectonic setting of the oceanic Nazca plate, South America Craton (SoAm) and two slivers: the North Andean Sliver (NAS) and the Inca Sliver (IS). The relative Nazca/SoAm plate convergence rate in Ecuador is about 55mm/yr (Kendrick et al., 2003). Black arrows indicate the diverging forearc slivers motions relative to stable SoAm are computed from the pole solutions of Nocquet et al.(2014). The NAS indicates a northeastward long-term rigid motion of about 8.5 ±1mm/yr. The ellipse indicates the approximate rupture of the great 1906 Mw=8.8 Colombia–Ecuador megathrust earthquake. The Carnegie Ridge intersects the trench in central Ecuador and coincides with the southern limit of the great 1906 event. Plate limits (thick red lines) are from Bird(2003). DGFZ =Dolores–Guayaquil Fault Zone; GG =Gulf of Guayaquil; GR =Grijalva Ridge; AR =Alvarado Ridge; SR =Sarmiento Ridge. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

  • GPS velocities along with historic earthquake patches.

  • Interseismic GPS velocity field in the North Andean Sliver reference frame. The relative Nazca/NAS convergence rate is 46 mm/yr. The highest GPS velocity of 26 mm/yr is found on La Plata Island that is the closest point to the trench axis. The GPS network adequately covers the rupture areas of the 1998 Mw=7.1, 1942 Mw=7.8and 1958 Mw=7.7 earthquakes but only 1/4th of the 1979 Mw=8.2 and 2/3rd of the great 1906 Mw=8.8 rupture area. The black star is the epicenter of the great 1906 event and white stars are the epicenters of the Mw>7.01942–1998 seismic sequence. Grey shaded ellipses are the high slip region of the 1942, 1958, 1979 and 1998 seismic sources (Beck and Ruff, 1984;Segovia, 2001; Swenson and Beck, 1996). Red dashed contours are the relocated aftershocks areas of the 1942, 1958 and 1979 events (Mendoza and Dewey, 1984). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

  • Moment deficit along strike and historic earthquake locations (Chlieh et al., 2014). The 2016 M 7.8 earthquake may have occurred in the region marked “gap” in these figures.

  • (A) Along-strike variations of the annual moment deficit for all the interseismic models shown in Fig.5. (B)Maximum ISC model and (C)Minimum ISC model. (A)The blue, green and red lines correspond to the along-strike variation of the annual moment deficit rate respectively for models with smoothing coefficient λ1 =1.0, 0.25 and 0.1. (B) Smoother solution of Fig.5 ith a maximum moment deficit rate of 4.5 ×1018N m/yr. (C)Rougher solution of Fig.5 with a minimum moment deficit rate of 2.5 ×1018N m/yr. Yellow stars are the epicenters of subduction earthquakes with magnitude Mw>6.0 from the last 400 yr catalogue (Beauval et al., 2013). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

References

Posted in Uncategorized

Earthquake Report: Guatemala

This morning we had an earthquake offshore of Guatemala with a magnitude M = 6.8. Here is the USGS website for this earthquake. This earthquake occurred to the east of a sequence from about a week ago. Here is my earthquake report for that sequence.

Offshore of Guatemala is a subduction zone thrust fault, where the Cocos plate dives east beneath the North America (in the north) and Caribbean plates (in the south). Subduction zone faults are capable of generating the largest magnitude earthquakes possible because the fault width is wider than other faults. The seismogenic zone, the region of the crust that can store elastic strain and experience brittle rupture during earthquakes, extends into the earth several tens of kms. Strike-slip faults generally dip vertically, giving them the narrowest fault width. While subduction zones dip at an angle, so their fault width is wider. Earthquake magnitude is a measure of energy released during the earthquake and the moment magnitude (the magnitude most people use) is based on three factors: (1) fault area, (2) fault slip, and (3) shear modulus (how flexible, or rigid, the crust/lithosphere is). Fault area is length times width. Length is the distance on the ground surface that the fault ruptures and width is the distance into the earth that the fault ruptures. Because subduction zone faults dip into the earth at an angle, the distance that they extend before reaching a given depth is larger, owing to a larger possible magnitude.

Today’s M 6.8 earthquake has a USGS hypocentral depth of ~47 km, which is very close to the depth of the subduction zone fault. Also, the fault plane solution (moment tensor, read below) is compressional. Thus, I interpret this earthquake to be a subduction zone earthquake at or near the megathrust. This earthquake is different from the sequence from a week ago. Those earthquakes had two populations: (1) earthquakes in the accretionary prism of the subduction zone and (2) earthquakes in the downgoing Cocos plate. Those earthquakes were not subduction zone earthquakes (though the shallower earthquakes may not have had well located hypocenters, so their depths are suspect… and could have been on the subduction zone). I suspect that this M 6.8 earthquake is related to the earthquakes from last week. I include the moment tensors for 2 of the significant earthquakes from last week. See my report for more on that sequence.

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.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. The hypocentral depth of the M 6.8 plots this close to the location of the fault as mapped by Hayes et al. (2012).

    I include some inset figures in the poster.

  • In the upper right corner, I include a subset of figures from Benz et al. (2011). There is a map that shows USGS epicenters with dots colored by depth and magnitude represented by circle diameter. There is also plotted a cross section that is adjacent (southeast) to this earthquake sequence. Cross section B-B’ shows the earthquake hypocenters along a profile displayed on the map. Note how the subduction zone dip steepens to the northeast. On the map and the cross section, I place a blue stars in the location for the M 6.9 and the 6/14 M 5.5 earthquakes.
  • In the lower right corner is a tectonic map showing the regional tectonics highlighting various study sites from sedimentary, magmatic, metamorphic and ore deposit studies of the subduction zones here (Garcia-Casco et al., 2011).
  • In the lower left corner is a map that shows the plate tectonic setting for this region of Middle America (Symithe et al., 2015). Earthquake epicenters are plotted as circles with color representing depth. Earthquake focal mechanisms are plotted with thrust earthquakes plotted in blue and other mechanisms plotted in red. Today’s earthquake happened just off this map on the left.


  • Here is an explanation from IPGP that helps us visualize the two potential fault planes (the principal and the auxiliary). I interpret this earthquake to have occurred on fault plane 1, the subduction zone.

  • Here is a map that shows today’s earthquake (with MMI contours) in the context of the seismicity from the past 100 years. I plot earthquakes from 1917-2017 for magnitudes M > 6.5.

  • Here is my poster for the earthquake sequence from last week.

  • Here is the tectonic map from Symithe et al. (2015). I include their figure caption below in blockquote.

  • Seismotectonic setting of the Caribbean region. Black lines show the major active plate boundary faults. Colored circles are precisely relocated seismicity [1960–2008, Engdahl et al., 1998] color coded as a function of depth. Earthquake focal mechanism are from the Global CMT Catalog (1976–2014) [Ekstrom et al., 2012], thrust focal mechanisms are shown in blue, others in red. H = Haiti, DR = Dominican Republic, MCS = mid-Cayman spreading center, WP = Windward Passage, EPGF = Enriquillo Plaintain Garden fault.

  • Here is the tectonic map from Garcia-Casco et al. (2011). I include their figure caption below in blockquote.

  • Plate tectonic configuration of the Caribbean region showing the location of the study cases presented in this issue (numbers refer to papers, arranged as in the issue), and other important geological features of the region (compiled from several sources).

  • Here is the Benz et al. (2011) Seismicity of the Earth poster for this region.

References

  • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 Caribbean plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-A, scale 1:8,000,000.
  • Franco, A., C. Lasserre H. Lyon-Caen V. Kostoglodov E. Molina M. Guzman-Speziale D. Monterosso V. Robles C. Figueroa W. Amaya E. Barrier L. Chiquin S. Moran O. Flores J. Romero J. A. Santiago M. Manea V. C. Manea, 2012. Fault kinematics in northern Central America and coupling along the subduction interface of the Cocos Plate, from GPS data in Chiapas (Mexico), Guatemala and El Salvador in Geophysical Journal International., v. 189, no. 3, p. 1223-1236. DOI: https://doi.org/10.1111/j.1365-246X.2012.05390.x
  • Garcia-Casco, A., Projenza, J.A., Iturralde-Vinent, M.A., 2011. Subduction Zones of the Caribbean: the sedimentary, magmatic, metamorphic and ore-deposit records UNESCO/iugs igcp Project 546 Subduction Zones of the Caribbean in Geologica Acta, v. 9, no., 3-4, p. 217-224
  • 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.
  • Symithe, S., E. Calais, J. B. de Chabalier, R. Robertson, and M. Higgins, 2015. Current block motions and strain accumulation on active faults in the Caribbean in J. Geophys. Res. Solid Earth, v. 120, p. 3748–3774, doi:10.1002/2014JB011779.

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