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San Jose Quake Study Shakes Earth to Identify Fault Lines

October 08, 2003|Kenneth Reich | Times Staff Writer

A 12-day seismological experiment in San Jose designed to identify dangerous faults subsidiary to the nearby San Andreas is now complete.

But the results of the tests, which involved creating vibrations in the subsurface along a five-mile section of residential property in the San Jose area, won't be known for months.

Robert Williams, the U.S. Geological Survey geophysicist who directed the experiment in Saratoga, San Jose and Cupertino, said it would take that long to process the material and draw a better map of the subsurface to as deep as 4,000 feet.

Researchers used a specially equipped "vibroseis" truck to make the ground vibrate and then listened to the echoes.

The seven-ton truck was driven slowly along the survey line and stopped every 30 feet to vibrate. A 4-foot-wide pad was pressed against the ground beneath the truck and vibrated about 10 seconds several times. Then the truck was moved forward another 30 feet, and the process was repeated.

This is not the only way to track fault lines. Williams said that using small quantities of explosives to send out the vibrations -- as is currently being done beside Interstate 215 in San Bernardino -- produces a more precise image. But it is not as practical in residential areas. Williams said the line of the experiment had been chosen to avoid heavy truck traffic, which can interfere with the soundings.

"We've been looking at three different features of interest," the scientist said in a phone interview from his offices in Golden, Colo. "These are two faults and the third was a line of minor damage associated with the 1906 San Francisco quake.

The San Francisco quake, now frequently assessed at about magnitude 7.8, ruptured the San Andreas for 200 miles in Northern and Central California. Like the 1994 Northridge earthquake, it appears to have caused damage along certain adjacent fault lines.

Williams said the aim of the San Jose study is to gather information on the layering, stiffness and faulting of the sediments to achieve a better understanding of how the ground will shake in temblors. This, in turn, can be used to develop safer construction codes to protect property.

The cost of the study was about $150,000, including the truck, the technology and the labor, Williams said.

Similar experiments were conducted near San Jose in 2002 and in San Bernardino in 1999.

This kind of work is designed for areas that could be subject to violent shaking. San Jose is near three major faults -- the San Andreas, the Hayward and the Calaveras -- while San Bernardino is near two, the San Andreas and the San Jacinto.

But experience has shown that small faults close to these major ones also can cause considerable damage.

This was true of the magnitude 7.1 Loma Prieta quake in the Bay Area on Oct. 17, 1989. First reports put this quake on the San Andreas, but further investigation showed it was on a hitherto unknown fault close by.

Williams said it was still too early to determine how successful the San Jose experiment has been. But, he said, the 1999 San Bernardino study had identified previously unknown faults and its results were published in the Bulletin of the Seismological Society of America.

He indicated that such study is more useful in the case of horizontal strike-slip, rather than the kind of thrust fault that caused the Northridge quake.

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