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USGS: Small earthquakes rattling California not related

7:56 AM, Sep 10, 2012   |    comments
U.S. Geological Survey
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The small earthquakes that rattled Central and Southern California late last week aren't related to each other, nor are they predictors of larger, more dangerous quakes, scientists say.

"As far as we know, they aren't related," says Paul Caruso, a geophysicist with the U.S. Geological Survey in Golden, Colo.

Two quakes - magnitude 4.0 and 4.1 - struck Fresno County on Friday morning. And about six hours earlier, a magnitude 3.4 quake was felt by thousands of people in Beverly Hills.

Kate Hutton, a seismologist at the California Institute of Technology, said Sunday that she doesn't see how an earthquake as small as the magnitude-3.4 temblor in Beverly Hills could trigger a quake as far away as Fresno County. "That's too far," she said. However, she points out that Beverly Hills was struck by a magnitude-3.2 quake last Monday, and that was probably a foreshock of the 3.4 quake Friday.

Neither the Fresno quakes nor the Beverly Hills quakes were on the famed San Andreas Fault, which runs through California.

"California has quakes all the time," says Caruso. On average, Southern California has about 10,000 earthquakes each year, many so small they cannot be felt, the U.S. Geological Survey reports.

No damage or injuries were reported in Friday's earthquakes. Caruso says significant damage usually occurs only at magnitude 5.5 and above. There haven't been any significant quakes in Southern California since Friday, Hutton said.

Additionally, while a small earthquake may temporarily ease stress on a fault line, it does not prevent a larger quake, according to the California Geological Survey.

Nor are the small quakes indicators of bigger events, says John Vidale, a seismologist at the University of Washington in Seattle.

Could the quakes have been triggered by Thursday's big, magnitude-7.6 quake in Costa Rica?

"There might be some controversy about whether they are related, but actually we are too far away from Costa Rica for one quake to influence another that much," Hutton said.

Vidale concurs: There wasn't enough energy in the Costa Rica quake to trigger other quakes as far away as California, he says. Costa Rica and Southern California are about 2,700 miles apart.

Also, on Aug. 25, an earthquake "swarm" was reported in the California desert near Brawley, a few miles north of the Mexican border. Only minor damage was reported. Again, that swarm can't be tied into Friday's quakes, Caruso says.

USA Today

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