Category Archives: GIS

Earthquake Report: Nicobar Isles and Sumatra!

This past 24 hours include two large earthquakes in the region of the Sumatra-Andaman subduction zone offshore of Sumatra. Here is a map using the USGS online GIS interface.

    Here are the two large earthquakes posted on the USGS websites:

  • 2015.11.08 M 6.4
  • 2015.11.08 M 6.1

Below is a map that I prepared with the seismicity from the past week, as well as the seismicity since 1900 with earthquakes of magnitude greater than M = 7.5. I plot the slab contours (these show the depth where we think that the subduction zone fault is located; Hayes et al., 2012). I plot the moment tensors (read more below about mt data) for these two earthquakes, along with the moment tensors from four significant earthquakes since ~2004. The 2004.12.26 and 2005.03.28 earthquakes are subduction zone earthquakes. The 2012.04.11 earthquakes are the largest strike slip earthquakes ever recorded on modern seismometers. While the tectonic fabric in the India plate is dominated by north-south fracture zones (see the blue line to the right of the label “Sunda trench”), these two M~8+ earthquakes ruptured east-west faults. The oceanic crust is very thick in the region of the Ninteyeast Ridge (thought to have thickened as the crust traveled over a hot spot), possibly contributing to the depth of these earthquakes, and their large magnitude.

I placed a moment tensor / focal mechanism legend in the upper right corner of the map. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.

The interesting things about these two earthquakes is that they are not on the subduction zone fault interface. The M = 6.4 earthquake is shallow (USGS depth = 7.7 km). Note how the subduction zone is mapped to ~120-140 km depth near the M 6.4 earthquake. The Andaman Sea is a region of backarc spreading and forearc sliver faulting. Due to oblique convergence along the Sunda trench, the strain is partitioned between the subduction zone fault and the forearc sliver Sumatra fault. In the Andaman Sea, there is a series of en echelon strike-slip/spreading ridges. The M 6.4 earthquake appears to have slipped along one of these strike-slip faults. I interpret this earthquake to be a right lateral strike-slip earthquake, based upon the faults mapped in this region. The smaller earthquakes align in a west-southwest orientation. These may be earthquakes along the spreading center, or all of these earthquakes may be left lateral strike slip faults aligned with a spreading ridge. More analyses would need to be conducted to really know.

In May 2015, there was another Andaman Sea earthquake. Here is my report for that M 5.8 earthquake. Below is a map of the region. The M 5.8 did not have a moment tensor nor focal mechanism calculated, but I placed a generic strike-slip focal mechanism in an orientation that aligns with transform plate boundary that likely ruptured during this earthquake. The M 5.8 epicenter is depicted by a red and black star. I pose that this was a right-lateral strike slip earthquake along a transform plate boundary (shown in blue). Note that I have also added updated fault locations in this area, based upon seismicity (the USGS located plate boundaries are not quite correct; mine are imperfect too, but are consistent with the seismicity). The USGS fault lines have a stepped appearance and the ones that I drew look more smooth.

I placed moment tensors for some of the largest earthquakes in this region. The 2009 and 2008 earthquakes in the northwest are extensional, so are probably in the downgoing India plate (extension from bending of the plate or slab pull). The 2010 and 2005 earthquakes in the southwest are strike-slip and may be due to the oblique subduction (strain partitioning).

Here is a graphic that depicts how a sliver fault accommodates the strain partitioning from oblique subduction.

The M = 6.1 earthquake is a deep earthquake, based upon its USGS hypocentral depth of 75 km. The slab depth in this region is about 70 km, so this is close to the megathrust. However, these slab contours are mostly based upon seismicity, so there is considerable uncertainty regarding the precise location of the fault (Hayes et al., 2012).

Here is a map that I just put together that shows the historic earthquakes along the Suamtra-Andaman subduction zone. Compiled multiband single beam bathymetry and Shuttle Radar Topography Mission (SRTM) topography is in shaded relief and colored vs. depth (Smith and Sandwell, 1997, Graindorge et al., 2008; Ladage et al., 2006).The India-Australia plate subducts northeastward beneath the Sunda plate (part of Eurasia; sz–subduction zone). Orange vectors plot India plate movement relative to Sunda, and black vectors plot Australia relative to Sunda (global positioning system velocity based on Nuvel-1A; Bock et al., 2003; Subarya et al., 2006). Historic ruptures (Bilham, 2005; Malik et al., 2011) are plotted in grey, calendar years are in white. The 2004 and 2005 slip contours are shown orange and green, respectively (Chlieh et al., 2007, fig. 11 therein; Chlieh et al., 2008, figure 20 therein). Bengal and Nicobar fans cover structures of the India-Australia plate in the northern part of the map; are dashed black lines delimit their southern boundaries (Stow et al., 1990). The 2004 and 2005 earthquake focal mechanisms are plotted.

This figure from Meltzner et al. (2010) shows measurements of vertical deformation collected from coral microatolls (which are sensitive to the tides, basically, they cannot survive above a certain level of tidal elevation. Read his and related papers to learn more about this method.). These are observations that are independent of GPS data. I include this figure because it shows the complicated tectonic setting. Note all the different strike-slip and extensional faulting north of Sumatra. These faults are from Curray (2005).

Here is a map where I plot the USGS Modified Mercalli Intensity (MMI) contours for these two M 6.4 and 6.1 earthquakes. These are estimates of ground shaking based upon Ground Motion Prediction Equations, empirical relations between shaking intensity and distance to the earthquake.

Here is one example of the MMI scale from the wiki site.

    These are the two “Did You Feel It?” (DYFI) maps for these two earthquakes. The DYFI maps are based on real observations, not models. Compare these maps with the above MMI Contour map.

  • M 6.4 strike-slip

  • M 6.1 normal

    Here are the attenuation plots comparing the DYFI and MMI model based estimates of ground shaking.

  • M 6.4 strike-slip

  • M 6.1 normal

Here are the USGS web pages for the earthquakes I will discuss below:

Here are a couple posts I put together regarding the initial instigator to this entire series of earthquakes (Mw = 9.15) and then the two largest strike slip earthquakes ever recorded (Mw = 82 and 8.6).

Here is a map showing moment tensors for the largest earthquakes since the 26 December 2004 Mw = 9.15 Megathrust Great Sumatra-Andaman subduction zone (SASZ) earthquake. Below is a map showing the earthquake slip contours. The beginning of this series started with the Mw 9.15 and Mw = 8.7 Nias earthquakes. There were some other earthquakes along the Mentawaii patch to the south (Mw = 8.5, 7.9, and 7.0). These were also subduction zone earthquakes, but failed to release the strain that had accumulated since the last large magnitude earthquakes to have slipped in this region in 1797 and 1833. In 2012 we had two strike slip earthquakes in the outer rise, where the India-Australia plate flexes in response to the subduction. At first I interpreted these to be earthquakes on northeast striking faults since those the orientation of the predominant faulting in the region. The I-A plate has many of these N-S striking fracture zones, most notably the Investigator fracture zone (the most easterly faults shown in this map as a pair of strike slip faults that head directly for the epicenter of yesterday’s earthquake). However, considering the aftershocks and a large number of different analyses, these two earthquakes (the two largest strike slip earthquakes EVER recorded!) were deemed to have ruptured northwest striking faults. We called these off fault earthquakes, since the main structural grain is those N-S striking fracture zones. Also of note is the focal depth of these two large earthquakes (Mw 8.2 & 8.6). These earthquakes ruptured well into the mantle. Before the 2004 SASZ earthquake and the 2011 Tohoku-Oki earthquake (which also probably ruptured into the mantle), we would not have expected earthquakes in the mantle.

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

  • link to the embedded video below. (45 mb mp4)
  • YT link to the embedded video below.
    • References:

    • Bilham, R., 2005. Partial and Complete Rupture of the Indo-Andaman Plate Boundary 1847 – 2004: Seismological Research Letters, v. 76, p. 299-311.
    • 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.
    • Curray, J. R., 2005. Tectonics and history of the Andaman Sea region, J. Asian Earth Sci., 25, 187–232, doi:10.1016/j.jseaes.2004.09.001.
    • 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.
    • Graindorge , D., Klingelhoefer, F., Sibuet, J.-C., McNeill , L., Henstock, T.J., Dean, S., Gutscher, M.-A., Dessa, J.X., Permana, H., Singh, S.C., Leau, H., White, N., Carton, H., Malod, J.A., Rangin, C., Aryawan, K.G., Chaubey, A.K., Chauhan, A., Galih, D.R., Greenroyd, C.J., Laesanpura, A., Prihantono, J., Royle, G., Shankar, U., 2008. Impact of lower plate structure on upper plate deformation at the NW Sumatran convergent margin from seafloor morphology: Earth and Planetary Science Letters, v. 275, p. 201-210.
    • 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.
    • 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.
    • 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.
    • Patton, J.R., Goldfinger, C., Morey, A.E., Ikehara, K., Romsos, C., Stoner, J., Djadjadihardja, Y., Udrekh, Ardhyastuti, S., Gaffar, E.Z., and Vizcaino, A., 2015. A 6500 year earthquake history in the region of the 2004 Sumatra-Andaman subduction zone earthquake: Geosphere, v. 11, no. 6, p. 1–62, doi:10.1130/GES01066.1.
    • 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
    • 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
    • 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.
    • 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.
    • Stow, D.A.V., et al., 1990. Sediment facies and processes on the distal Bengal Fan, Leg 116, ODP Texas & M University College Station; UK distributors IPOD Committee NERC Swindon, p. 377-396.
    • Tolstoy, M., Bohnenstiehl, D.R., 2006. Hydroacoustic contributions to understanding the December 26th 2004 great Sumatra–Andaman Earthquake. Survey of Geophysics 27, 633-646.

10/23/2015 Inaugural Cascadia GeoSciences Present

You’re invited to come network with old friends and colleagues, meet new ones, enjoy a locally-made beverage, maybe learn something and share your local knowledge with others.

Cascadia GeoSciences presents:
A Research Presentation by Todd B. Williams and Dr. Jason R. Patton
Unraveling tectonic and eustatic factors of sea level rise in northern California, Humboldt Bay.

WHEN: Friday October 23rd 5:30-8 pm
WHERE: Arcata D Street Neighborhood Center
1301 D St, Arcata, CA 95521
(see map below)
Hope to see you there!

Future Presents will be posted online here.

Plate Tectonics: 200 Ma

Gibbons and others (2015) have put together a suite of geologic data (e.g. ages of geologic units, fossils), plate motion data (geometry of plates and ocean ridge spreading rates), and plate tectonic data (initiation and cessation of subduction or collision, obduction of ophiolites) to create a global plate tectonic map that spans the past 200 million years (Ma). Here is the facebook post that I first saw to include this video.

Here is a view of their tectonic map at a specific time.

Gibbons et al. (2015) created several animations using their plate tectonic model. I have embedded both of these videos below. Other versions of these files are placed on the NOAA Science on a Sphere Program website.

They also compare their model with P-wave tomographic analytical results. P-Wave tomography works similar to CT-scans. CT-scans are the the result of integrating X-Ray data, from many 3-D orientations, to model the 3-D spatial variations in density. “CT” is an acronym for “computed tomography.” Both are kinds of tomography. Here is a book about seismic tomography. Here is a paper from Goes et al. (2002) that discusses their model of the thermal structure of the uppermost mantle in North America as inferred from seismic tomography.

Here is an illustration from the wiki page that that attempts to help us visualize what tomography is.

P-Wave tomography uses Seismic P-Waves to model the 3-D spatial variation of Earth’s internal structure. P-Wave tomography is similar to Computed-Tomography of X-Rays because the P-wave sources are also in different spatial locations. For CT-scans, the variation in density is inferred with the model. For P-Wave tomography, the variation in seismic velocity. Typically, when seismic waves travel faster, they are travelling through old, cold, and more dense crust/lithosphere/mantle. Likewise, when seismic waves travel slower, they are travelling through relatively young, hot, and less dense crust/lithosphere/mantle.

Regions of Earth’s interior that have faster seismic velocities are often plotted in blue. Regions that have slower velocities are often plotted in red.

Here are their plots showing the velocity perturbation (faster or slower). I include the figure caption below the image.

Plate reconstructions superimposed on age-coded depth slices from P-wave seismic tomography (Li et al., 2008) using first-order assumptions of near-vertical slab sinking, with a) 3.0 and 1.2 cm/yr constant sinking rates in the upper and lower mantle, respectively, following Zahirovic et al. (2012), and b) 5.0 and 2.0 cm/yr upper and lower mantle sinking rates, respectively, following Replumaz et al. (2004). Both end-member sinking rates indicate bands of slab material (blue, S1–S2) offset southward from the Andean-style subduction zone along southern Lhasa, consistentwith the interpretations of Tethyan subducted slabs by Hafkenscheid et al. (2006). However, although the P-wave tomography provides higher resolution than S-wave tomography, the amplitude of the velocity perturbation is significantly lower in oceanic regions (e.g., S2) and the southern hemisphere due to continental sampling biases. Orthographic projection centered on 0°N, 90°E.

Earthquake in the Fiji Region!

There was a large earthquake in the Fiji region yesterday (Saturday my time). The epicenter is far from the major convergent plate boundary. Here is the USGS page for this earthquake.

Here is a map at the global scale. The epicenter is marked by a red circle, just South of Samoa and East of Fiji. This earthquake is related to the subduction zone associated with the Tonga Trench. The moment tensor shows an east-northeast striking compressional solution, due to principal axis compression in the n-nw/s-se direction. Prior to looking at the moment tensor, I was expecting to see an extensional solution at this depth. The cross section of seismicity is sourced from earthquakes designated by the purple line.

Here is a regional scale map showing the plate configuration.

This is a local scaled map showing the complexity of the spreading ridges and transform faults to the east of this earthquake swarm. The Modified Mercali Intensity Contour 3.5 is plotted as a light blue circle. Epicenters are plotted by color in relation to depth.

Here is the regional map showing the slab contours. The depth of this earthquake (434 km) is close to, but above, the slab depth (500 km). If one looks at the cross section of historic seismicity, it appears that the slab is possibly bending upwards. Perhaps there is some compression in the upper plate here, causing the compressional moment tensor.

Here is another view of the slab, generated using P-wave tomography. Doug Weins discusses his work in this region. “Red and blue colors denote slow and fast velocities, respectively, and the velocity perturbation scale is shown at the bottom.”

Interestingly, deep focus earthquakes take up ~66% of the deep earthquakes globally. From this paper, we can see that the slab contour may change strike in the region of yesterday’s earthquake.

Richards et al., 2011 also show bends in the downgoing slab. There is some controversy about the configuration of the slab in this region. They show a detached slab just above the main port (more Star Wars), above the main slab.

The New Hebrides subduction zone dips to the east and turns into a transform fault just west of yesterdays earthquake. This map shows the profile for the above cross section from Richards et al. (2102)

This figure shows Richards et al. Figure 4, that displays their interpretation of how the plates came to be configured here. The Australia plate detached and collided with the Pacific slab about 4 million years ago.

Here is the USGS Open File poster for the region (Benz et al., 2010). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.

As one might expect, an earthquake at this depth, given this magnitude, would not generate strong ground motions at the surface. The pager, an estimate of human and infrastructural losses, reflects this low likelihood of damage.

Here is a primer about Focal Mechanisms from the USGS.


  • 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.
  • Benz, H.M., Herman, Matthew, Tarr, A.C., Furlong, K.P., Hayes, G.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011, Seismicity of the Earth 1900–2010 eastern margin of the Australia plate: U.S. Geological Survey Open-File Report 2010–1083-I, scale 1:8,000,000.
  • Richards, S., Holm., R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region, Geology, v. 39, pp. 787-790.
  • Yu, W. and Wen, L., 2012. Deep-Focus Repeating Earthquakes in the Tonga–Fiji Subduction Zone, BSSA, v. 102, no. 4, pp. 1829-1849

Economic Geography in the USA

An article in The Atlantic, written by Derek Thompson, discusses the results from a non peer-reviewed paper from the National Bureau of Economic Research. This working paper is circulated for discussion and comment purposes.

Here is the report: Where is the Land of Opportunity? The Geography of Intergenerational Mobility in the United States. Written BY Raj Chetty, Nathaniel Hendren, Patrick Kline, and Emmanuel Saez. Working Paper 19843.

The Atlantic article uses to figures from the NBER paper to help us visualize the regional variation in upward mobility. Absolute upward mobility measures how children stack up to their parents. Relative mobility measures their chances of moving up or down the income ladder relative to their peers. Lighter colors suggest higher mobility.

Absolute Upward Mobility:
Absolute Upward Mobility

Relative Upward Mobility:
Relative Upward Mobility

Here are some observations made in the appendix and highlighted by Thompson:

  1. The most upwardly mobile region is the Great Plains (followed by the West Coast and the Northeast)
  2. The most upwardly mobile cities are Salt Lake City, for moving into the middle class, and San Jose, Ca., for moving into the top quintile. Here are the top ten cities for the American Dream, ranked by absolute (child vs. parent) upward mobility.
  3. The losers are the Southeast and Rust Belt (the entire regions, basically) and Charlotte, North Carolina. Here are the ten worst cities for upward mobility.
  4. Although the map looks like it’s colored by region, there are some huge differences between cities and areas that are just miles away from each other. For example, many Texas areas have high rates of upward mobility, unlike the entire South. Pennsylvania has many upwardly mobile cities, but Ohio fares poorly. Ohio has much lower rates of upward mobility than Pennsylvania.
  5. The race composition of your area seems to matter—not just the race of your family. “Both blacks and whites living in areas with large African-American populations have lower rates of upward income mobility,” the researchers write.