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01/08/92

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Models of earthquake aftershocks probably wrong, Stanford geophysicists say

STANFORD -- Aftershocks following earthquakes do not appear to move in predictable directions, two Stanford geophysicists report in the current issue of Science magazine.

Gregory C. Beroza, assistant professor of geophysics and Mark D. Zoback, professor, examined the unprecedented amount of data collected after the 1989 Loma Prieta earthquake. They found that the myriad aftershocks that current models say should have moved in the same direction as the main quake did not.

Models predicting aftershock motion are probably wrong for most quakes, particularly those along zones where tectonic plates meet, they believe.

"Aftershocks seem to occur not because the main shock increases the stress [on a fault line] but because you lower the strength of that particular line," Zoback said in an interview.

According to Beroza, most geophysicists have agreed that the motion of aftershocks followed the same motion as the larger, main shock. They have used two models to explain what might happen, models predicated on theory, not experimentation.

One model, the so-called asperity model, suggested that stress along an earthquake zone is heterogeneous before a quake, and because of the quake becomes more homogeneous. The second model, called the barrier model, claims the reverse: the zone is homogeneous to start with and becomes more diverse after the main quake.

However, the Loma Prieta quake, which happened along an area where the Pacific tectonic plate meets the North American plate - and in an area heavily instrumented by geoscientists, disputed these models. It showed that the motion of its aftershocks was not in the same direction as the main quake.

"There was reverse slip in which the Pacific plate moved up with respect to North America but many aftershocks occurred in which the Pacific plate moved down," Zoback said. "In addition to the Pacific plate moving north with respect to North America, which is the motion one expects, there were aftershocks that moved south."

Using an analogy, think of bending a pencil almost but not quite to the point at which it would break. If the pencil eventually snaps, it is because either you somehow added just a little more pressure or because the stress weakened it enough that it could not withstand the same pressure. The latter is what appears to have happened with Loma Prieta aftershocks, he said.

"In terms of the mechanics of the interactions, the aftershock fault planes should have had more stress on them before the main shock than they did afterward. If the strength of the fault planes didn't change, they shouldn't have moved.

"What we infer from this is that the main shock, or some process associated with the main shock, made the aftershock fault planes weaker and that is what caused the aftershocks to happen." The main shock didn't build up stress, it just weakened the planes on which the aftershocks happened.

"It's a new way of thinking about aftershocks," Zoback said. "Instead of an aftershock occurring because the stress at that point got higher . . . it turns out not to be caused by the stress at all. It turns out it may well be that the fault on which the aftershock occurs is weaker as a result of the main shock.

"This may hold important clues about the physics of faulting."

Zoback and a student have found similar activity after an earthquake offshore of Honshu island in Japan. Beroza said the same thing may have explained the aftershocks in last year's Landers earthquake in southern California.

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