Earthquake swarm gives Californians the jitters repeated


Published: Sunday, December 1, 2002 at 6:01 a.m.
Last Modified: Saturday, November 30, 2002 at 10:15 p.m.
SAN RAMON, Calif. - The ground started shaking last Sunday morning beneath this bedroom community about 35 miles east of San Francisco, but there was little concern. The quake measured 3.9 on the Richter scale, and left barely a picture on the wall askew. People in California tend to save their worry for the Big One - something catastrophic, like the quake that destroyed much of San Francisco in 1906.
But when another small quake struck here Sunday night, and another Monday morning, and another and another and another - by Friday, the United States Geological Survey had registered what it called a "swarm" of more than 120 quakes - nervousness set in.
"Earthquakes are primal-fear things," said Stephanie J. Hanna, the Geological Survey's communications chief for the Western region. "They don't kill as many people as you think, but when terra firma isn't anymore, it scares people down to the deepest level."
Technically, the San Ramon earthquakes are regarded as "background seismicity" because they did not occur on a main fault. But San Ramon is not far from the Calaveras fault, one of three major faults in the San Francisco Bay area that scientists consider most likely to produce a big earthquake.
More significantly, the San Ramon quakes have happened on a small, previously unknown fault that crosses the Calaveras seven miles beneath the surface, potentially triggering quakes on the Calaveras as well.
"I grew up in San Ramon, and normally you have an earthquake and it passes," said Parshaw Vaziri, the outreach director at the Pacific Earthquake Engineering Research Center in Berkeley. "To have it be repetitive for so many days has people thinking. There is a fear that this is leading to something bigger."
Seismologists say swarms like San Ramon's are not uncommon, but when they strike in heavily populated places known for seismic volatility, they can create unusual anxiety.
The last significant swarm near San Ramon occurred several miles to the north in Alamo, Calif., in 1990, when there were 350 earthquakes over six weeks. In nearby Danville in 1970, there were 353 earthquakes over one month.
David P. Schwartz, chief of the Geological Survey's San Francisco Bay Area Earthquake Hazards Project, said the previous swarms did not set off any major earthquakes. But he and other earthquake experts do not know whether this swarm will be harmless.

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