Ripples From the Hayward

Berkeley, Livermore Scientists Simulate Precise Patterns of Quake Ground Motion

Scientists at Berkeley and Lawrence Livermore National Laboratory have announced results of a study that models the localized severity of ground motion during a major earthquake along the Hayward fault. The fault runs beneath a densely populated section of the eastern San Francisco Bay Area and is expected to experience a magnitude 7 earthquake within the next 30 years.

"The Hayward fault is the most dangerous fault in the Bay Area," said Livermore geophysicist and computer scientist Shawn Larsen. "You need credible ground motion analysis to...predict how structures such as the Bay Bridge will respond during an earthquake, and this study should help provide it."

Using one of the world's most powerful supercomputers at Lawrence Livermore, Larsen, teamed with Berkeley seismologists Mike Antolik and Doug Dreger, simulated expected ground motions for a magnitude 7.1 earthquake along the Hayward fault beginning south of Fremont and rupturing 50 miles northwest to San Pablo Bay.

The researchers developed a computer movie based on this scenario that shows seismic waves propagating throughout the Bay Area and impacting different regions with varying degrees of severity. The simulations predict that the most severe ground shaking will occur in East Bay cities along the Hay-ward fault. Parts of the San Francisco peninsula and southern Marin county will also experience significant motion. But the strongest shaking is centered around San Pablo Bay.

"San Pablo Bay is a deep sedimentary basin and this has the effect of magnifying the amplitudes and durations of shaking," explains Dreger, an assistant professor of geology and geophysics and a member of Berkeley's Seismographic Station. Dreger and Antolik also simulated a 4.8 earthquake that occurred on the Calaveras fault east of San Jose on May 21, 1996, and a magnitude 4.2 earthquake which occurred near Berkeley on June 26, 1994.

According to Larsen, ground motion will be different if the fault breaks from the northwest to the southeast. Indeed, a second computer simulation done at Livermore reveals that in such an earthquake the Santa Clara valley will also experience significant ground motion. Future efforts by the researchers will focus on different earthquake scenarios along the Hayward fault. In addition, they hope to perform similar simulations for other active faults in the San Francisco area, including the San Andreas and Rogers Creek faults.

Currently engineers trying to predict earthquake ground motion for a localized area or single structure, such as the Bay Bridge, have to rely on imprecise measurements such as data from smaller earthquakes that are extrapolated in somewhat linear fashion to larger magnitudes.

"Ultimately it's our goal that those responsible for earthquake retrofit and new structure design can take our findings and combine them with current approaches to make better engineering decisions," said Antolik.

The researchers announced their findings at the Seismological Society of America annual meeting, held in Honolulu.

Their simulations use a sophisticated computer code developed at Livermore and a seismic velocity model developed at Berkeley.

The ground motion study was performed under the auspices of Livermore's Advanced Computer Initiative in Science and Engineering and the Campus-Laboratory Collaboration program, an initiative that joins the unique strengths of Lawrence Livermore and UC campuses in an effort to solve scientific and technical problems of societal importance.



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