Super-shear quakes pack extra wallop.

• Position: Seismology

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...