San Andreas Fault May Have Many Faces

By Monastersky, Richard | Science News, December 1, 1990 | Go to article overview

San Andreas Fault May Have Many Faces


Monastersky, Richard, Science News


San Andreas fault may have many faces

A close examination of last October's Loma Prieta earthquake and the great San Francisco quake of 1906 has yielded a surprising conclusion: The two shocks appear to have ripped different faces of the San Andreas fault. If so, geoscientists may have underestimated the seismic hazard remaining in the Santa Cruz region after last year's deadly jolt.

"We have to be awfully cautious about saying we understand a certain area and we know what its seismic potential is," says Paul Segall of Stanford University and the U.S. Geological Survey (USGS) in Menlo Park, Calif.

Both the 1989 and 1906 earthquakes broke the San Andreas fault on a segment that runs through the Santa Cruz Mountains. To compare the two events, Segall and Mike Lisowski of the USGS examined surveying data indicating how Loma Prieta Mountain (part of the Santa Cruz range) shifted during each quake.

For last year's quake, scientists gauged ground changes in the region with lasers, satellite signals and even the radio waves from distant quasars. These measurements were then fed into computers.

At the turn of the century, however, surveyors used telescopes to judge the angles between mountaintops and then did all their calculations on paper. Because of these antiquated methods, some scientists view the early calculations with skepticism.

Segall and Lisowski reviewed the old surveying data and plugged them into a computer to recalculate the ground movement during the 1906 event. That quake, with an estimated magnitude of 8.3, moved Loma Prieta Mountain in a direction parallel to the fault, they report in the Nov. 30 SCIENCE. But last year's magnitude 7.1 quake moved the mountain in a markedly different manner, shifting it diagonally toward the fault.

To explain the motion contrast, Segall and Lisowski propose that the two earthquakes ruptured separate faults in the San Andreas system: one that descends vertically and another inclined 20[degrees] from the vertical. …

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