Rumors of the 'Big One' may be exaggerated, scientists say

A computer-generated 3D relief image of the San Francisco Bay area, showing major faults in red. Click on the image to see a larger version

By Environmental News Network staff

Before the 1906 earthquake that leveled much of San Francisco, it was known as "the Big One."

In 1868, the southern Hayward Fault in northern California ruptured from Fremont to Berkeley, causing an earthquake estimated at magnitude 7.0.

Since that time, the Hayward Fault has been firmly locked at depth and creeping slowly at the surface. And Californians, rocked by quakes up and down the state throughout the 20th century, have lived in fear of "the next Big One."

Now comes a study published today in Science magazine that concludes the likelihood of a major earthquake on the northern Hayward Fault might be less than previously predicted.

"If you add up the slippage of all the faults in the region, from the San Andreas in the west to the Greenville Fault on the edge of the Central Valley, there is a deficit in the amount the northern Hayward Fault is slipping," says Robert Nadeau, a researcher in the Seismological Laboratory at the University of California at Berkeley.

Researchers from the University of California, the Department of Energy's Lawrence Berkeley National Laboratory, NASA's Jet Propulsion Laboratory and the University of California at Davis used data from outer space and deep inside Earth to measure earthquake potential along the fault, which runs through the major cities on the east side of San Francisco Bay.

The new study modifies the assumption of the Working Group on California Earthquake Probabilities, which as recently as 1990 proposed that the odds are 1-to-1 that, in 30 years or less, an earthquake of magnitude 7 or larger will occur along the Hayward Fault. Such an event could claim thousands of lives and cost billions of dollars.

The strain that builds up along major faults can be relieved by creep, which can't be felt but can be scientifically detected or, if the fault locks up until strain grows too great, by large earthquakes.

Geological evidence suggests that north of Berkeley the last Big One occurred much earlier, sometime between the mid-1600s and the arrival of Spanish colonists in the Bay Area in 1776. Whether or not the northern section of the fault is locked has much to do with when and where the next big quake is likely to occur and how powerful it is likely to be.

Some seismologists believe the Hayward Fault has stored an enormous amount of energy and is on the verge of catastrophic release.

"This is a 'worst case' estimate," says Nadeau, "based on a model that assumes the fault is creeping on the surface but locked at depth. But the data previously available couldn't discriminate between that model and one in which the fault is creeping all the way down."

Researchers used measurements taken in space of different time periods to monitor interseismic deformation along the Hayward Fault in northern California

To improve the data, Roland Burgmann, an assistant professor of geology at UC Berkeley, decided to integrate traditional measurements with space-based measurements such as interferometric synthetic aperture radar from the Jet Propulsion Laboratory.

Nadeau and Thomas McEvilly of Berkeley Lab's earth sciences Division offered to provide Burgmann with a different perspective on the problem one that opens a window on fault activity kilometers beneath the surface.

For many years, McEvilly, Nadeau and colleagues have studied the San Andreas Fault near Parkfield, a tiny ranching community 165 miles south of San Francisco. There they developed a technique of comparing the timing of identical repeating "microquakes" with slippage events deep underground, whose foci and energy could be accurately pinpointed by a network of seismometers at the bottom of boreholes.

On the northern Hayward Fault, the researchers identified three major clusters of identical repeating small quakes along the northern fault zone, representing centers of slippage up to 7 millimeters per year at from 4 to 10 kilometers depth.

When data from all sources was collated, a remarkably detailed picture of the Hayward Fault emerged. Shallow creep falls off markedly where the fault dives under the San Francisco Bay at Point Pinole. Creep is at a maximum at El Cerrito, with indications of rapid slippage continuing to great depths. Surface creep diminishes south of Berkeley over the locked portion of the fault that broke in 1868.

If the potential for a huge, near-term quake centered under the northern Hayward Fault seems less than before, the researchers caution that one need only remember the 62 lives lost and billions of dollars of damage caused by the magnitude 7.1 Loma Prieta quake of 1989, centered 60 miles constant preparedness.

"Our research shows no evidence of locking at any depth, which means the threat from one of our worst hazards, right in our backyard, is much reduced," said Burgmann. "However, other hazards, from the southern Hayward fault, the San Andreas fault and other nearby faults, leave the need to build reinforced homes and the need to be prepared just as high as before."

Copyright 2000, Environmental News Network, All Rights Reserved

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