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Slow-moving plates appear to lower odds / When they lurch past one another, the Earth rumbles

U.S. Focus Studying the Faults

Friday, August 18, 2000

Times-Dispatch Staff Writer

Plates in the Earth's crust along a major fault line in the San Francisco Bay area appear to be creeping past each other rather than sticking and jerking, according to scientists.

The slow movement could lower the chances of a major earthquake originating along the fault line.

Earthquakes are caused by the movement of large plates that fit together like a puzzle. When the pieces lurch past each other along fault lines in abrupt, stick-and-slip motions, major earthquakes ensue. But the plates can also creep slowly and continuously along fault lines, relieving strain along the fault so that tensions don't build up and explode into a large earthquakes, according to a study published in today's Science.

This aseismic creep of about a quarter-inch a year is seen along the northern portion of the 60-mile Hayward fault line, the researchers said.

"The likelihood of a large earthquake originating and centering on the northern Hayward fault alone is rather low," said the study's lead author, Roland Burgmann of the University of California in Berkeley.

Instead, the gradual creeping results in cracked pavement, distorted buildings and crooked fence lines in Oakland, Berkeley and other cities along the Hayward fault.

The Hayward fault line is one of the major branches of the well-known San Andreas Fault system. Scientists have previously predicted that a major earthquake could hit the northern part of the fault in the next 30 years and threaten the lives of people in the San Francisco Bay area because the scientists thought the creeping happened only at the surface, not in the deeper parts of the plates, increasing the chances of a major earthquake.

But scientists didn't know just how far down into the Earth the plates were creeping.

Using satellite technology to monitor changes in the Earth's surface and data from microearthquakes deep in the fault, the researchers came up with a computer model that suggests the plates along the northern Hayward fault slide freely past each other all the way down to the point where rock is so hot it turns into a thick fluid -- about 7.5 miles.

This constant creeping could explain why there's no record of major earthquakes originating from this area, said Burgmann.

Earlier versions of the study's data were used by the 1999 Working Group on Earthquake Probabilities in the San Francisco Bay region to lower the estimated probability for an earthquake in the northern Hayward fault in the next 30 years from 28 percent to 16 percent.

The Hayward fault is only one of several faults in California. Most of them, including the southern half of the Hayward, are still at higher risk for earthquakes because the plates are locked together and trap tension in the rocks, Burgmann said. Only the central portion of the San Andreas fault is also known for its deep creeping. If an earthquake originated along these other locked lines, their effects could still be felt in the northern Hayward area, he added.

Burgmann and his colleagues hope to look at other faults and learn how and why their behaviors differ. What they learn could eventually lead to improved estimates on where and when the next big earthquake will hit, he said.

Contact Corie Lok at (804) 649-6204 or clok@timesdispatch.com

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