Here is an update to my post earlier on the 2004 Sumatra-Andaman subduction zone earthquake of 2004/12/26 (about a decade ago).
This series of plots show the large variation in non unique slip models for this earthquake. The authors divide the fault into hypothetical fault segments and impart an amount of slip on each segment (typically a rectangle shaped region). Then they use their models (elastic half space, basically, a 3-D model of the earth that is assumed to have some material properties that are elastic, plastic, and viscous). The motion of the earth is propagated from the fault upwards to the ground surface. The result of this ‘half space’ modeling is a map that shows vertical (or horizontal) deformation. The authors then compare their model results with direct observations of vertical deformation from measurements like those collected at GPS sites. These slip model results can also be compared with other observations, like tsunami wave heights, to better resolve the resolution of slip on the fault (either the amount of slip, the variation of slip (like with different sized rectangle fault segments, the material properties of the earth, and/or a combination of all of these). This figure is from Shearer and Burgman (2010).

Here is a figure that shows the wave height observations from satellites that happened to be passing over the Indian Ocean as the tsunami crossed towards India and Sri Lanka (Shearer and Burgman 2010).


These two figures show coseismic vertical and horizontal motions from the 2004 and 2005 earthquakes (Prawirodirdjo et al., 2010). The first figure shows the sense of motion vertically and the sense and amount of motion horizontally as measured at GPS sites.


This second figure from Prawirodirdjo et al. (2010) shows the amount of horizontal coseismic and postseismic deformation for the 2004 earthquake and the amount of coseismic vertical and horizontal deformation during the 2005 earthquake. These are the types of data that people use to validate their slip models. The slip models in the above figure show that there is large variation in their solutions, demonstrating that these results are largely equivocal.


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.


The next series of figures show how people have estimated the amount of energy released during the 2004 earthquake. These plots are often called “source time” functions. They may be plotted as energy versus time or latitude. The larger the amount of slip on the fault, the more energy is released. This first plot is from Chlieh et al. (2007).


Here is the source time function from Ishi et al. (2005). Note the similarity between this plot and the above one from Chlieh et al. (2007). These results are more comparable that the slip models we saw earlier.


This is from Subarya et al. (2006), an earlier plot, but still similar to Chlieh et al. (2007) and Ishii et al. (2007).


This is another estimate published also in 2006 (Tolstoy and Bohnenstiehl, 2006), again showing similarities with the other plots (though this is the most different). There are a number of other examples as well (e.g. Okal).


These next two figures from Singh et al. (2008) show a map and cross section at the location of the earthquake. The 2004 SASZ earthquake ruptured very deep in a location previously thought to not harbor strain to be accumulated and released during an earthquake.


Here is the cross section showing where the earthquake hypocenter is compared to where we think the mantle exists. We have not been here, so nobody actually knows… These interpretations are based on industry deep seismic data.


Finally today we look at the ground shaking during the 2004 SASZ earthquake (Sorensen et al., 2007). These first plots show how the ground shaking intensity attenuates (diminishes) with distance from the earthquake. This makes sense, that the further one is away from an earthquake, the less shaking one would feel. These plots show direct observations and the model results that are also plotted in the map below.


Here is the map showing the modeled ground shaking intensity for the 2004 SASZ earthquake. These models were adjusted to fit the observations in the above plot.


References:

• 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.
• 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.
• 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.
• 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.
• 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.
• Tolstoy, M., Bohnenstiehl, D.R., 2006. Hydroacoustic contributions to understanding the December 26th 2004 great Sumatra–Andaman Earthquake. Survey of Geophysics 27, 633-646.

The Decadal Anniversary is coming up and I will be posting some material regarding the earthquake and tsunami that changed the lives of millions. This Mw 9.15 earthquake, slightly smaller in magnitude than the M 9.2 Good Friday Earthquake of 1964/3/27, was devastating as over 200,000 people lost their lives. We hope that their lives were not lost in vain as we attempt to learn many lessons from the results of our observations following this earthquake and tsunami. This post will cover the initial observations and I will follow up with material that was found later.
In 2004 many people had the belief that the historical seismicity for a fault related directly to the seismic potential of that fault. Not everyone held this belief, especially those that recognized that many large fault systems had recurrence intervals (average time between earthquakes of a given magnitude) that were longer than the record of seismicity for those fault systems. Seismologic records (records from seismometers) only extend back about a century. Large plate boundary faults, those with the possibility of generating large magnitude earthquakes, the possibility of generating large ground shaking, and the potential to exert significant hazard to people and their possessions (e.g. bridges, schools, and hospitals) have recurrence intervals ranging from centuries to millenia. In addition, these large plate boundary systems seem to also have larger strain cycles that result in a variation of earthquake magnitude through time (aka “supercycles,” Sieh et al., 2008). In other words, there may be a series of smaller large magnitude earthquakes in-between larger large magnitude earthquakes. This cycling of magnitude variation may produce a series of earthquakes, each separated by the mean recurrence time of centuries, with magnitudes in series: Mw 8, Mw 7.5, Mw 8.2, Mw 9.2, Mw 8, Mw 7.8, Mw 7.6, Mw 8, Mw 7.5, Mw 9.5. Here, of 11 earthquakes, only 2 are larger than Mw 9, but each are separated by centuries. Given such a short instrumental record of earthquakes, we can all now recognize that we do not know enough about plate boundary earthquakes to effectively evaluate their hazard (unless we might be a seismologist, though many of them are coming around).
Prior to December 2004, this region of the Sunda subduction zone had only had earthquakes of magnitude less than M 8. Therefore, people did not expect a larger earthquake there. They were incorrect (I did not know any better either, but was not thinking of this subduction zone at this time in my life. I worked primarily on Cascadia, which also has not had an historic large magnitude earthquake.).
Here is a map showing the historic earthquake regions. Earthquake slip contours are shown for the 2004 and 2005 earthquakes. Some references for these earthquake sources include: Newcomb and McMann, 1987; Rivera et al., 2002; Abercrombie et al., 2003; Natawidjaja et al., 2006; Konca et al., 2008; Bothara, 2010; Kanamori et al., 2010; Philibosian et al., 2012.


This map shows the magnitude of these historic earthquakes overlain upon a map showing the magnetic anomalies. I will discuss the tectonic significance of these anomalies later.


This map also includes the plate motion vectors, regional plate boundaries, and the extent of the Bengal and Nicobar fans.


Here is the USGS poster for this earthquake. These results were put out very soon after the earthquake and later reports made more refined analyses. For example, there are over a dozen earthquake slip models for this earthquake, most all are better than this initial USGS version.


This is the modeled shaking intensity map generated by the USGS, using the Modified Mercalli Intensity Scale. This map is based on numerical modeling of the fault slip and is preliminary. Compare it with the next map, which is based upon observations from real people. I will write a little more about the MMI scale in the coming week.


Here is the Did You Feel It map, which is based on felt reports by people who could submit their observations via the USGS website.


Here is some eye candy to get us rolling. This is a figure from Chlieh et al. (2007). These figures show different versions of their slip model as they relate to observations (black vectors) and models (red vectors) of uplift and subsidence. Vectors are arrows that show sense of motion (in this case, up or down) and some magnitude (in this case, amount of vertical motion).


References:

• Abercrombie, R.E., Antolik, M., Ekstrom, G., 2003. The June 2000 Mw 7.9 earthquakes south of Sumatra: Deformation in the India–Australia Plate. Journal of Geophysical Research 108, 16.
• Bothara, J., Beetham, R.D., Brunston, D., Stannard, M., Brown, R., Hyland, C., Lewis, W., Miller, S., Sanders, R., Sulistio, Y., 2010. General observations of effects of the 30th September 2009 Padang earthquake, Indonesia. Bulletin of the New Zealand Society for Earthquake Engineering 43, 143-173.
• Chlieh, M., Avouac, J.-P., Hjorleifsdottir, V., Song, T.-R.A., Ji, C., Sieh, K., Sladen, A., Hebert, H., Prawirodirdjo, L., Bock, Y., Galetzka, J., 2007. Coseismic Slip and Afterslip of the Great (Mw 9.15) Sumatra-Andaman Earthquake of 2004. Bulletin of the Seismological Society of America 97, S152-S173.
• 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.
• 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.
• Rivera, L., Sieh, K., Helmberger, D., Natawidjaja, D.H., 2002. A Comparative Study of the Sumatran Subduction-Zone Earthquakes of 1935 and 1984. BSSA 92, 1721-1736.
• Sieh, K., Natawidjaja, D.H., Meltzner, A.J., Shen, C., Cheng, H., Li, K., Suwargadi, B.W., Galetzka, J., Philobosian, B., Edwards, R.L., 2008. Earthquake Supercycles Inferred from Sea-Level Changes Recorded in the Corals of West Sumatra. Science 322, 1674-1678.