Remnants of 2012's Largest Earthquake

On April 11, a '8.6 magnitude' earthquake shocked Sumatra, off East Indian Ocean. Thankfully, the casualty was not major even at that magnitude. However, analysis from the U.S. Geological Survey (USGS) and University of California, Berkeley confirmed that it did trigger quakes around the world in the week immediately followed that earthquake.

The magnitude says it all – it represents the 10th largest over the last century and, in pattern consistent with other large quakes, it sent seismic waves throughout Earth's crust, especially during the first 3 hours it started to take place.

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A map of the earthquakes triggered around the globe within a week of the April 2012 earthquake off the coast of Sumatra (white star). Credit: Fred Pollitz, USGS

This study rings new alarm. While it shows that some of the faults were not strong enough to cause seismic failure immediately (and the threat usually goes away after 6 days), they have the potential to create havoc on regions that are seismically active, regardless of which corner of the world. This risk is most intense during the first few hours of a major quake. It is definitely not comfort thought for people residing in those regions.

The conventional wisdom has been that distant earthquakes could do little to trigger local quakes. But the new study has presented evidence that this likelihood can never be ruled out. The coauthor of the study, Roland Burgmann earth and planetary science professor at UC Berkeley, says that scientists have always said that there is nothing to worry about distant earthquakes causing local earthquakes and this study reveals that this can happen very rarely, perhaps once every few decades and can actually happen when the right breed of earthquake takes place.

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Some 380 seconds into the greatest earthquake to rupture since 1960, the simulated dynamic Coulomb stress waves (red-blue) shed continuously off the 2004 M=9.2 Sumatra rupture front can be seen sweeping through the Andaman Sea, where faults remarkably shut down for the next five years. Earthquakes since 1964 are shown as black dots, and the Sunda trench along which the 1400-km-long earthquake occurred is the arcuate black line on the left (west). Sumatra is on the right, and Myanmar/Burma is at top. Sevilgen et al (Proc. Nat. Acad. Sci, 2012) find that despite the magnitude of thesedynamic stress waves, the much smaller permanent stresses account for the change in seismicity after the main shock. This graphic accompanies the Sept. 3, 2012 article in Proceedings of the National Academy of Sciences by Volkan Sevilgen, Ross Stein and Fred Pollitz. Credit: U.S. Geological Survey

Prof Burgmann also pointed out that significant local shaking have been detected with the recent Baja California quake of 7.0 and quakes in Japan and Indonesia and it would have been disastrous if those quakes have actually hit an urban locality.

The USGS report from Volkan Sevilgen and Ross S. Stein, Fred F. Pollitz and Burgmann, went online on 26^th day of September ahead of its scheduled publication in journal Nature.

The same team has also put recent major earthquakes under observation including the Sumatra-Andaman quake of 9.2 that occurred in year 2004 and the Tohoku quake of 9.0 killing thousands in 2011 in Japan and found that all seem to point to the fact that there was a very minor increase in the global quake activity level. The quake in East Indian Ocean is considered a "strike-slip" quake. It is capable of causing wave popularly known as Love waves that travel underneath the surface and have just enough energy that is capable of affecting remote fault zones.

The largest threats are those happening at subduction zones. These are the places where the ocean bed goes under a different tectonic plate. This brings to mind the Sumatra-Andaman quake, where a corresponding tsunami claimed 200,000+ lives. Recent 2012 East Indian Ocean Quake included lateral movement known as strike-slip, similar to the pattern of movement along San Andreas Fault of California and was largest of any strike-slip quake recorded in history.

Nevertheless, there were some peculiar observations made about that quake. In term of its strike-slip nature, it was similar to San Francisco earthquake of 1906. But in term of scale, it was humongous and at least 15 times bigger than that of San Francisco.

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Remote earthquakes in the six days preceding (top) and the six days following (bottom) the M=8.6 main shock in the East Indian Ocean on April 11, 2012. The color scale indicates seismic stress, with purple = zero and red = high. This graphic accompanies the September 2012 article in Nature by Fred Pollitz et al. Credit: U.S. Geological Survey

Following the April 11 quake and aftershock, seismologists observed there are 5 times more than the usual number of earthquakes expected in the 6 days. Pollitz ventured that a lower than usual seismic activity during 6-12 days building up to that earthquake of 8.6 magnitude may have contributed to its huge impact probably because of the presence of several faults that were very close to failure and that they were very sensitive to any shock wave that triggered.

One theory put forward for delay caused in action is that earthquake in East Indian Ocean has called into action a mix-bag of smaller and undetectable earthquakes on the fault. Or that a quake strong enough could trigger tremors that are nearly undetectable or may be microquakes that signify slow underground slip.

Burgmann speculated that earthquake in Sumatra quickly triggered slow-slip in several places; perhaps coupled with some tremors that are detectable, and the accumulated effect is this huge earthquake. He also added that certain events of slow slip could take days, weeks or even more for evolving.