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10/11/94

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Lessons from Loma Prieta quake

STANFORD -- If there¹s one thing that major quakes like Loma Prieta reveal, it¹s that each one holds its own surprise² says Greg Beroza, an associate professor of geophysics at Stanford.

In the case of the Oct. 17, 1989, Loma Prieta quake, there were some surprises - the fault slipped in a fundamentally different way than it did in previous major earthquakes, such as the 1906 quake, and the damage it caused led scientists to look for new and different fault structures underlying the Bay Area.

Using data gathered during the quake and in studies performed over the past five years, geophysicists have learned that the Bay Area is riddled with a much more complex web of earthquake faults than previously understood. While experts probably will be debating for years the detailed interpretation of recent studies, geophysicists from Stanford and elsewhere have made several key findings:

  • The ¹89 quake actually may not have occurred on the San Andreas, but rather on a subsidiary fault. Its motion included substantial lifting and sinking rather than the pure horizontal sliding motion normally seen on strike-slip faults like the San Andreas. Beroza analyzed seismic waves generated during the quake, and Stanford colleague Paul Segall made accurate measurements of the change in position of certain ³benchmarks² on the earth¹s surface to show that much of the motion during the Loma Prieta quake was vertical motion typical of reverse or ³thrust² faults.

³We are realizing that reverse [thrust] faults are very common in California - many of our most damaging earthquakes are not on major strike-slip faults, but on subsidiary faults,² Segall said. The 1994 Northridge earthquake in Los Angeles was such a quake - it occurred on a ³blind² thrust fault whose existence was previously unknown.

  • Segall¹s data show that stress transfer from the Loma Prieta quake activated other thrust faults known to exist to the east of the Santa Cruz Mountains. The increased slip of these faults has caused the peak of Loma Prieta to rise about 1-1/2 inches since the quake, regaining about one-third of the height it lost due to horizontal ground motions during the quake itself.
  • Uneven patterns of destruction observed after Loma Prieta helped point to the existence of previously unknown fault structures in the Bay Area. Damage to the Cypress elevated freeway structure in Oakland was so much greater than expected at that distance from the epicenter that it led some scientists to speculate that seismic energy was reflected off horizontal rock layers or faults deep in the earth¹s crust.

Simon Klemperer, an associate professor of geophysics at Stanford, and scientists from the U.S. Geological Survey recently found evidence for such a horizontal break 9 miles below the earth¹s surface. The break undercuts San Francisco Bay and possibly links the San Andreas fault and the Hayward fault in the East Bay.

According to Tom Brocher, a USGS scientist involved in the study, the finding means that the major strike-slip faults like the San Andreas and Hayward are ³superficial² features that cut only the top 9 miles of a 60-mile-thick lithosphere. Previously, it had been assumed that these major faults cut all the way through the earth¹s crust.

While they each contribute a different piece to the complex puzzle, these studies all point to one conclusion - there will be more destructive earthquakes to come in the Bay Area.

³We were relatively lucky with Loma Prieta,² said Beroza. ³There are many more quakes in our future and they will shake buildings and bridges more strongly than Loma Prieta because they will likely be closer.²

A working group formed after the 1989 quake concluded that there is a two-thirds probability of a magnitude 7.0 or greater earthquake within the next 30 years. This estimate is based on analysis of four major fault segments in the Bay Area - the San Andreas, the North and South segments of the Hayward fault, and the Rodgers Creek fault. While it was based on consensus at the time, there are many geophysicists who feel this estimate should be revised upward, particularly because of evidence that lesser known faults like the offshore San Gregorio have slipped more frequently and more violently than previously thought.

Even if only the major faults are considered, the risk has not subsided since the Loma Prieta quake. Segall said that ³for the overall strain budget on the San Andreas system, Loma Prieta was a relatively small actor in the whole play."

-jh-

Jennifer Howard is a science writing intern with the Stanford News Service.

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