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A Megaquake becomes a Megapuzzle
The magnitude 8.6 earthquake, which struck off the coast of Sumatra last week, was one of those rare megaquakes. It belongs to the group of the ten or so strongest quakes ever measured. As explained in the previous blog entry, scientists were surprised - and certainly breathed a deep sigh of relief - that this quake did not cause a significant tsunami. In fact, hardly anybody was even seriously hurt when the extremely strong seismic waves caused by this quake hit the Indonesian islands closest to the epicenter. But once the immediate danger of a tidal wave was over, seismologists began to realize that this quake and its aftershock of 8.2 two hours later pose an immensely interesting, multifaceted scientific puzzle. Here is why:
These two quakes occurred in the immediate vicinity of the Sumatra subduction zone, where the Indian Ocean plate dives under fragments of the Eurasian plate. The Great Sumatra Quake of December 26th, 2004 had its origin in the same general area. But while this quake, which produced the devastating tsunami seven and a half years ago, had a focal mechanism expected for subduction zones, last week's quakes did not. They were earthquakes of the California type, that is the two flanks of the rupture slid past each other horizontally instead of slipping vertically against each other as in most subduction temblors. Such horizontal strike slip quakes have almost never been observed on the seaward side of a subduction zone. How and why they occurred off Sumatra is puzzle number 1.
The other unexpected feature of this quake should concern everybody in California. Most quakes along the San Andreas and its sister faults also shift in a strike slip fashion. Seismologists have always assumed that such strike slip events would be able to generate strong temblors, which are likely to cause a lot of damage. However, all the scientific evidence collected here in California and in other strike slip regimes around the world suggests that strike slip quakes cannot grow into really devastating megaquakes with magnitudes of 8 and greater. It seems that the horizontal slip fault zones are too weak to accommodate enough tectonic stress to delay rupturing until the breaking results in a megaquake. Well, since last Wednesday researchers have such an unexpected strike slip megaquake in their collection. And now they are scratching their heads as to how to fit these two unexpected aspects of the puzzle into current earthquake theory.
One possible solution was pointed out at an earthquake conference in Memphis last week by Lori Dengler, a Professor of Geology at Humboldt State University and California's foremost expert on tsunamis. She noted that the Great Sumatra Earthquake released a large amount of tectonic stress along its extremely large rupture area. As a consequence the mechanical stress along the fringes of the rupture area grew and put the surrounding rock under an immense strain, with two forces acting in opposite directions. This pattern is similar to the forces acting on the San Andreas Fault. When the rocks finally gave way last week, the result was a strike slip earthquake. And why was it so large? Dengler thinks that the rocks in the hypocentral area were virgin. They had never been broken in an earthquake before - in contrast to the rocks along the San Andreas fault, which have ruptured many times during the past eons. As a consequence, the oceanic crust off Sumatra was able to accumulate an enormous amount of stress. When the rocks finally gave way, the result was last week's megaquake. (hra077)