Boxing Day Earthquake: Sumatra-Andaman subduction zone 2014/12/26

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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., 200Smilie: 8). 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).


  • 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.

Posted in earthquake, education, geology, plate tectonics, subduction, sumatra, tsunami

Earthquake along the New Britain trench (Solomon Isles and Papua New Guinea)

We just had an earthquake swarm along the subduction zone trench offshore of the Solomon Islands. Here is the USGS earthquake page for this Mw 6.8 subduction zone earthquake.

Here is a map of the region with the recent swarm.

This map shows the modeled shaking intensity for this earthquake. Bouganville Island likely experienced MMI IvV. From the USGS page, “Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.”

This map shows the slab contours (an estimate of the subduction zone plate interface). These contours are estimated by Hayes et al., (2012).

This region has been active in the past couple of years. There have been compressional, extensional, and transform earthquakes since 7/7/2013. This map shows the earthquake epicenters (blue dots), their moment tensors, magnitudes and date of rupture. Here is my page regarding the earthquakes from April 2014. This is the USGS tectonic poster for the region that I used as a background. I placed black arrows to show the relative plate motion across the tectonic plate boundaries adjacent to these earthquakes.

This map shows the general plate boundaries on the region (Tregoning et al., 2000).

This map shows the relative age of these oceanic plates of the region (Baldwin et al., 2012).

This map shows plate velocities and euler poles for different blocks. Note the counterclockwise motion of the plate that underlies the Solomon Sea (Baldwin et al., 2012).

Here is a primer for those who would like to understand focal mechanisms better (from the USGS). Normal earthquakes are extensional, reverse earthquakes are compressional, and strike-slip earthquakes are the result of shear.


  • Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012, Tectonics of the New Guinea Region, Annu. Rev. Earth Planet. Sci., v. 40, pp. 495-520.
  • 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.
  • Tregoning, P., McQueen, H., Lambeck, K., Jackson, R. Little, T., Saunders, S., and Rosa, R., 2000. Present-day crustal motion in Papua New Guinea, Earth Planets and Space, v. 52, pp. 727-730.

Posted in College Redwoods, earthquake, education, Extension, geology, plate tectonics, subduction, Transform

Another Aftershock in the Molucca Sea!

This earthquake happened really close to the M 7.1 from earlier (2014/11/15). Here is my page that has more information about the regional tectonic interpretations. Here is the USGS page for this M 6.8 earthquake.

Here is the updated map with moment tensors plotted for four earthquakes.

This map shows the modeled estimate of shaking intensity for this earthquake.

Here is a map from the USGS that shows pre-instrumental seismicity for this region. There was a large magnitude earthquake in this same region in the 19th century.

Posted in College Redwoods, earthquake, education, geology, plate tectonics, subduction, tsunami

Earthquake in Central Honshu!

David N Lindberg liked this post

This morning (my time) there was a large earthquake in central Honshu. Here is the USGS page for this earthquake. The earthquake hypocentral depth is shallow, giving rise to the likelihood of damage to infrastructure and people.

Here is a map of the region, showing the epicenter in orange, near Nangano. This earthquake occurred along the Itoigawa-Shizuoka Tectonic Line, a convergent plate boundary between the Eurasian/Amurian plate and the North America/Okhotsk plates. This earthquake is in the region where this fault zone flips vergence (in the south it dips to the southwest and to the north it dips to the northeast. This is where the Niigata-Kobe fold and thrust belt meets the mobile belt of the east margin of the Japan Sea.

This is the moment tensor, which is interesting in that it is not evidently parallel to the tectonic boundary of this region. The main tectonic boundary is complicated here and changes strike and also incorporates some strike slip faulting.

Here is an illustration showing the subduction zones and how they meet near Honshu. This image is from here.

This map shows the current tectonic configuration of this region, along with some inherited features from the tectonic past (e.g. green lines). This is from NUMO’s report: “Evaluating Site Suitability for a HLW Repository (Scientific Background and Practical Application of NUMO’s Siting Factors), NUMO-TR-04-04.”

Also from the NUMO report, this shows the Niigata-Kobe fold and thrust belt.

Here is the shaking intensity map as modeled using ground motion prediction equations (attenuation models).

Compare the above map with this one, which is generated by surveys from real people, not models.

Here is the attenuation plot, showing how the shaking intensity drops off with distance from the earthquake. Note how the real data (dots) match with the attenuation curve that was developed using seismicity in California, rather than the curve using central and eastern USA seismicity. Think about why you think this is.

Given the shallow depth for this earthquake, we can see that the PAGER estimate of damage is high.

Posted in geology, plate tectonics, subduction

Another Earthquake in the Molucca Sea

Dale Kramer, David N Lindberg liked this post

We just had another large earthquake in this region. Here is the USGS page for this M 6.5 earthquake. This earthquake hypocenter is much deeper than what we might expect for a subduction zone interface earthquake in this region. The M 7.1 earthquake from about a week ago was downdip of this earthquake (deeper along the subduction zone fault) than today’s earthquake. Here is my page on the M 7.1 earthquake.

This is a map of the region, showing the three largest magnitude earthquakes from the last couple of weeks.

Here is a generalized tectonic map showing how these plate boundaries are configured from here.

Here is a more detailed tectonic map with a cross section showing how these two opposing subduction zones are configured. This is from a Geological Society of America Special paper here.

Posted in College Redwoods, earthquake, education, geology, plate tectonics, subduction, tsunami

Earthquake along the Halmahera Arc

David N Lindberg, Dale Kramer liked this post

There was a Mw 7.1 thrust earthquake along the Halmahera Arc (a subduction zone) in the Molucca Sea, Indonesia. Here is the USGS web page for this earthquake. There was a small amplitude tsunami observed in the region of the earthquake: 9 cm at Jailolo, Indonesia (see map below). There was a M 7.5 earthquake to the south of this 2014/11/15 earthquake, also a thrust earthquake here is the USGS page.

This is a map that shows the epicenters on a global scale map.

This map shows the epicenters on a regional scale map.

This is the regional map with Modified Mercalli [shaking] Intensity Contours.

Here is a map I put together that shows the moment tensors for the largest magnitude earthquake and the largest magnitude aftershock. The plate boundaries have relative plate motion vectors. This is a region where there are two opposing ocean-ocean subduction zones. I edited this map since the plate boundaries plotted by USGS are in the incorrect location (the thin lines).

Here is a generalized tectonic map showing how these plate boundaries are configured from here.

Here is a more detailed tectonic map with a cross section showing how these two opposing subduction zones are configured. This is from a Geological Society of America Special paper here.

This earthquake was not located too close to land (i.e. people), so the probability of damage to people and their infrastructure was low (as evidenced by the pager report shown here). This can be downloaded from the USGS web page for this earthquake.

While the above maps of shaking intensity (MMI) are generated using models of ground shaking (GMPEs), this map shows the shaking intensity based upon peoples’ direct observations.

This plot shows the attenuation of ground motion with distance from the epicenter. The Ground Motion Prediction Equation (GMPE) models are plotted in orange and green. The orange model is based upon ground motion attenuation relations for the western US. The green model is based on relations for the eastern US. The western US is mostly accreted terrain, so this model matches the observational data (plotted as dots) probably because the Halmahera region is also composed of accreted terrain (albeit different from the western US).

Posted in geology, plate tectonics, subduction, tsunami

Lassen Earthquake Swarm

There is currently an ongoing earthquake swarm in the region immediately south of Mt. Lassen. The largest magnitude is a M 3.9 (USGS). The California Volcano Observatory (CalVO) is monitoring these earthquakes. The earthquakes do not appear to be related to volcanism, but most likely related to the fault systems that strike through that area. These faults are an extension of the Walker Lane fault system, which is a sister of the San Andreas plate boundary. The Walker Lane fault system is a right lateral (dextral) fault system that extends the east side of the Sierra Nevada Mountains. We do not currently know how this part of the plate boundary ends, if it feeds into Cascadia, etc. We have some ideas, but I will leave this a mystery as many of these ideas are unpublished and I do not want to scoop anyone. There are no USGS active faults mapped in this region, but there has been earthquake activity in the Lake Almanor area in the last year or so (see map below from 5/23/13). This region may be an extension of the Butt Creek fault zone (the red faults directly to the southeast of this swarm.

Here is a map of the epicenters in the region of Mt. Lassen.

The CAVO is monitoring these earthquakes, but they clearly state this swarm is related to plate tectonics in the region and not volcanism. Here is a plot of seismicity for this region.

Here is the Did You Feel It map for the M 3.9 earthquake.

The M 3.9 earthquake has a focal mechanism that shows NE-SW extension.

Here is a primer for those who would like to understand focal mechanisms better (from the USGS.

Here is a map I made about some of these Almanor earthquakes.

Posted in College Redwoods, earthquake, education, geology, plate tectonics, San Andreas

Earthquake Swarm in NW Nevada!

There has been an earthquake swarm in northwest Nevada. There was also a swarm in this region last August. The largest magnitude EQ so far is a magnitude 4.6. Here is a magnitude 4.0 EQ that initiated my interest in posting about this swarm. These epicenters plot along the Warner Valley and Guano valley fault zones.

This map shows the recent EQs in orange, yellow, and white, in increasing age. The grey circles are part of the August 2014 swarm. The faults are from the USGS active fault and fold database.

Here is the extensional moment tensor for the magnitude 4.6 EQ:

Here is the extensional moment tensor for the magnitude 4.0 EQ:

Here is the shake map for the M 4.6 EQ:

Here is the shake map for the M 4.0 EQ:

Here is the USGS 2% probability of exceedance in 50 years map of peak ground acceleration. One can observe that this region of Nevada has a low probability of ground acceleration.

I will post more on this swarm later today. check back!

Posted in College Redwoods, earthquake, education, geology, plate tectonics

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

Posted in College Redwoods, earthquake, education, geology, plate tectonics, subduction

Earthquake offshore of El Salvador!

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We just had a large magnitude earthquake offshore of El Salvador. Here is the USGS web page for the earthquake. Based on the magnitude and depth, it is not expected to have triggered a tsunami that would reach the coast of CA, OR, nor WA.

Here is a map that shows the region. The epicenter is plotted in green (the initial depth was set at 70 km, so it plotted in green. the depth was updated to 40 km, so will plot in yellow in other maps.).

This map shows the historic earthquakes as grey circles. These earthquakes are largely associated with the subduction zone here, where the Cocos plate is subducting northeastwardly beneath the Carribean plate to form the Middle America trench.

This map shows the modeled intensity using the Modified Mercali Intensity Scale (MMI, a measure of the ground shaking). The MMI has a scale of I – XII.

This is the same map, zoomed in for a larger scale view of the area.

Here is a view of how the intensity attenuates (diminishes as the energy is absorbed by the earth’s crust) with distance from the earthquake. The blue dots are real data, with medians plotted as brown dots. One can see that there is considerable variation in the intensity for individual measurements. This can be due to a variety of reasons (variation in the earth’s crust, site conditions, etc.). The green and orange lines are model estimates of intensity attenuation given two models from Atkinson and Wald, 2007 (using Ground Motion Prediction Equations, GMPEs, based on regression through data from earthquakes in California and Central-Eastern US, respectively). The California regression seems to fit the data better than the Central/Eastern US model.

This map has the intensity plotted as contours, which allows one to see how the intensity varies across the region in relation to the geography and place names.

Based on the location, depth, and moment tensor shown below, I interpret this to be a normal earthquake. This earthquake is the result of extension in the downgoing plate, possibly from the bending (i.e. a bending moment normal fault).

This map shows the depth contours to the slab (the top of the downgoing Cocos plate). These slab contours were developed by Gavin Hayes who works at the USGS. Here is a web page that has links to the other slabs Hayes and his colleagues have developed. They are largely based on their interpretations of historic seismicity.

This is the pager (version 1) that shows modeled estimates of human and infrastructural damage from this earthquake. These pager estimates are revised as more seismological information is analyzed. The loss estimates assist aid agencies make plans on how much assustance they may need to send to the region (in the form of food, medical, money, etc.).

Here is the Did You Feel It map, showing observations based upon the DYFI survey tool. The results are plotted using the same color range for the MMI scale.

Here is the USGS poster for the tectonics of the region:

Here is a profile showing Ranero et al. (2003) interpretation of the structure of the subduction zone here. Look at the upper panel and note how the top of the slab bends downward. This causes extension that may be responsible for today’s earthquake. This is figure 8.14 from Mann et al., 2007.

Here is a map that shows preliminary tsunami wave height estimates from Here.

Here is a table of estimates for the tsunami wave height.

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.
Mann, Paul, Robert D. Rogers, and Lisa Gahagan. “Overview of plate tectonic history and its unresolved tectonic problems.” Central America: Geology, resources and hazards 1 (2007): 201-237.
Ranero, C., Morgan, P., McIntosh, K. & Reichert, C.: Bending-related faulting and mantle serpentinization at the Middle America trench. Nature 425 (2003), pp.367–373.

Posted in College Redwoods, earthquake, education, geology, plate tectonics, subduction, tsunami