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Quakes pose greater threat to Bay Area.

Quakes Pose Greater Threat to Bay Area

The U.S. Geological Survey is sending a clear message to the San Francisco Bay area: Prepare now.

The federal agency last week released a report estimating a 67 percent chance of a major quake striking the region within the next 30 years, a significant increase over its previous estimate of 50 percent. That warning will hit the streets later this summer when USGS, in an unprecedented action, distributes 2.5 million copies of an informational magazine designed to help people plan for the likely event.

"We hope the main consequence of releasing this updated information is that it will act as another inducement for people in the Bay area, and perhaps other earthquake prone regions of the country, to take the actions now that can make a big difference in how well they fare when an earthquake does happen," says William L. Ellsworth, a seismologist with USGS in Menlo Park, Calif., and a member of the 12-person panel that drafted the new report.

According to calculations by the panel, the Bay area faces 2-to-1 odds -- a 67 percent chance -- that one of the major faults in the region will unleash a magnitude 7 or larger quake in the next 30 years. A 1988 USGS report gave even odds--a 50 percent chance (SN: 7/16/88, p.37).

An earthquake of this size, centered under the populated Bay area, would wreak significantly more damage than last October's Loma Prieta temblor, a magnitude 7.1 shock that originated in the rural Santa Cruz mountains. Comparing the two epicentral regions, Ellsworth says, "instead of 125,000 living in the immediate area where the ground shaking was hardest, there will be 1.2 million people."

The panel produced its estimates by reviewing the available information on major Bay area faults: the San Andreas, Hayward and Rogers Creek faults. The scientists based their calculations on the theory that earthquakes occur when stress on a fault segment builds to a critical level. The probability of a major shock in the near future therefore depends on the date of the last quake on that segment, the rate of stress build-up on the fault, and the average interval between quakes.

The increase in Bay Area risk reflects several new findings that have emerged in the last few years. Most importantly, geologists working north of San Pablo Bay have determined that the Rogers Creek fault caused major tremors in the past and again may be nearing its breaking point (SN: 12/16/89, p.388). In 1988, scientists knew too little about this fault to assess its earthquake risk.

Recent investigations have also determined that stress builds along the Hayward and San Andreas faults faster than previous work had suggested. Moreover, movement along the San Andreas during the Loma Prieta quake slightly increased the stress along the same fault near San Francisco.

Specialists often describe the job of forecasting earthquakes as an inexact science at best. "Everyone realizes that the actual data we have are quite meager," says seismologist Lynn Sykes of the Lamont-Doherty Geological Observatory in Palisades, N.Y.

In grading the reliability of their Bay area estimate, the panel gave it a B on an A to E scale. The group also calculated the chances of an earthquake on each of the individual fault segments in the region, but it had less confidence in these estimates, giving them marks of Cs and Ds.

USGS is preparing to inform millions of Bay area residents about the quake hazard through a magazine supplement to local newspapers. Several nonprofit groups will fund printing costs for the magazine, which will appear in English, Spanish and Chinese.
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Title Annotation:San Francisco Bay Area
Author:Monastersky, Richard
Publication:Science News
Date:Jul 28, 1990
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