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STANFORD -- If there's one thing that major quakes like Loma Prieta reveal, it's that each one holds its own surprise” 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:
“We 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,” Segall said. The 1994 Northridge earthquake in Los Angeles was such a quake - it occurred on a “blind” thrust fault whose existence was previously unknown.
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 earth's 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 “superficial” 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 earth's 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.
“We were relatively lucky with Loma Prieta,” said Beroza. “There 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.”
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 “for the overall strain budget on the San Andreas system, Loma Prieta was a relatively small actor in the whole play."
Jennifer Howard is a science writing intern with the Stanford News Service.
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