Super-shear quakes pack extra wallop

As if people living in earthquake country do not already have enough to worry about, scientists have identified another rupture phenomenon that can occur during certain types of largo earthquakes.

California Institute of Technology, Pasadena, geophysics graduate student Kaiwen Xia, aeronautics and mechanical engineering professor Ares Rosakis, and geophysics professor Hiroo Kanamori have demonstrated for the first time that a very fast, spontaneously generated rupture known as "super-shear" can take place on large strike-slip faults like the San Andreas. They base their claims on a laboratory experiment designed to simulate a fault rupture.

The researchers forced two plates of a special polymer material together under pressure and then initiated an "earthquake" by inserting a tiny wire into the interface, which is turned into an expanding plasma by the sudden discharge of an electrical pulse. By means of high-speed photography and laser light, the scientists photographed the rupture and the stress waves as they propagated through the material. The data shows that, under the right conditions, the rupture occurs much faster than the shear speed in the plates, producing a shock-wave pattern, something like the Mach cone of a jet fighter breaking the sound barrier.

The split-second photography also demonstrates that such ruptures may travel at about twice the tale of a "regular" one. However, they do not reach super-shear speeds until they have propagated a certain distance from the point of origin. Based on the experiments, a theoretical model was developed by the researchers to predict the length of travel before the transition to super-shear.

In the case of a strike-slip fault like the San Andreas, the lab results indicate that the rupture needs to rip along for about 100 kilometers and the magnitude must be about 7.5 or se before the rupture becomes super shear. Large earthquakes along the San Andreas tend to be at least this large if not larger, typically involving rupture lengths of about 300 to 400 kilometers.

Several seismologists believe that super-shear was exhibited in Tibet in 2001 and Alaska in 2002. Both earthquakes were located in a remote region and had little, if any, human impact, but analysis of the evidence shows that the fault rupture propagated much taster than normally would be expected, thus, implying super-shear.

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