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San Andreas fault may have many faces.

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. Seismic evidence collected during the 1989 quake shows that it occurred along a titled surface that forced the southwest side of the fault to ride up over the northeast side. The recalculated surveying data, along with other evidence, suggest the 1906 event occurred on a vertical fault plane. In this earthquake, the two sides of the fault slid past each other horizontally, with very little movement up or down.

Purely horizontal slippage is characteristic of the San Andreas fault, which absorbs the motion between the Pacific and North American tectonic plates. In fact, seismologists had never seen significant vertical motion during a San Andreas earthquake before 1989. But until Segall and Lisowski reexamined the data for 1906, geoscientists could not rule out the possibility that the earlier quake also included some vertical motion.

Seismologist Hiroo Kanamori of the California Institute of Technology in Pasadena says the suggestion of multiple faults makes sense because of the San Andreas' complex geometry near Santa Cruz. The fault bends there and does not align exactly with the motion of the Pacific and North American plates.

The USGS geophysicists say their analysis raises some critical questions about the "recurrence model" used by the federal government to forecast the probability of earthquakes for specific regions. This model is based on the assumption that the next large earthquake in an area will resemble previous ones. But if the San Andreas near Santa Cruz includes several fault planes, then the recurrence model might be inappropriate.

Seismologists using that model have said they do not expect another strong quake soon along the Santa Cruz Mountain section of the San Andreas. But Segall and Lisowski conclude that the 1989 shock on the titled fault could have increased stress on a shallow part of a vertical fault. If so, they argue, "the present earthquake hazard in the Santa Cruz Mountains is not negligible."
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Title Annotation:potential for further earthquakes in the San Francisco Bay Area
Author:Monastersky, Richard
Publication:Science News
Date:Dec 1, 1990
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