Seismic experiments provide new clues

In recent years, seismologists thought they were getting a handle on how an earthquake tends to rupture in a preferred direction along big strike-slip faults like the San Andreas. The direction of rupture has a profound influence on the distribution of ground shaking. But a new study could undermine their confidence a bit.

In the April 29 issue of the journal Science, researchers from Caltech and Harvard University discuss new laboratory experiments using dissimilar polymer plates to mimic Earth’s crusts. The results show that the direction of rupture that controls the pattern of destruction is less predictable than recently thought.

These findings explain puzzling results from last year’s Parkfield earthquake, in which a northwestward rupture occurred. A southeastward rupture had been predicted on the basis of the two past earthquakes in the area.

The phenomenon has to do with the basic ways rupture fronts are propagated along a boundary between two materials with different wave speeds.

In the experiment, von Kármán Professor of Aeronautics and Mechanical Engineering Ares Rosakis and Smits Professor of Geophysics Hiroo Kanamori, both of Caltech; Professor James Rice of Harvard University; and Caltech grad student Kaiwen Xia, prepared polymer plates to mimic the effects of major strike-slip faults. These are faults in which two plates are rammed against each other by forces coming in at an angle, and which then spontaneously snap (or slide) to move sideways.

The team fixed two clear polymer plates made of two different materials so that force was applied to them at an acute angle relative to the “fault” between them. The researchers then set off a small plasma explosion with a wire running to the center of the plates (the “hypocenter”), causing the plates to quickly slide apart just as two tectonic plates would slide apart during an earthquake.

What’s more, if the rupture fronts are super-shear, i.e., faster than the shear speed in the plates, they produce a shock-wave pattern that looks something like the Mach cone of a jet fighter breaking the sound barrier.

“Previously, it was generally thought that, if there is a velocity contrast, the rupture preferentially goes toward the direction of the slip in the low-velocity medium,” says Kanamori. In other words, if the lower-velocity medium is the plate shifting to the west, then the preferred direction of rupture would typically be to the west.

“What we see, when the force is small and the angle is small, is that we simultaneously generate ruptures to the west and to the east, and that the rupture fronts in both sides go with sub-shear speed,” Rosakis says. “But as the pressure increases substantially, the westward direction stays the same, but the other, eastward direction, becomes super-shear. This super-shear rupture speed is very close to the p-wave speed of the slower of the two materials.”

The results also showed that, when the experiment is done at forces below those required for super-shear, the directionality of the rupture is unpredictable. Both waves are at sub-shear speed, but waves in either direction can be devastating.

This explains why the Parkfield earthquake last year ruptured in the direction opposite to that of past events. The experiment also strongly suggests that, if the earthquake had been sufficiently large, the super-shear waves would have traveled northwest, even though the preferred direction was southeast.

But the question remains whether super-shear is necessarily a bad thing, Kanamori says. “It’s scientifically an interesting result, but I can’t say what the exact implications are. It could also mean that earthquake ruptures are less predictable than ever,” he adds.

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