August 29, 2012 / 1:30 AM / 7 years ago

Unusual earthquake swarm shakes Southern California

LOS ANGELES (Reuters) - An unusual swarm of hundreds of mostly small earthquakes has struck Southern California over the last three days and shaken the nerves of quake-hardy residents, but scientists say the cluster is not a sign a larger temblor is imminent.

The earthquakes, the largest of which measured magnitude 5.5, began on Saturday evening and have been centered near the town of Brawley close to the state’s inland Salton Sea, said Jeanne Hardebeck, research seismologist for the U.S. Geological Survey.

Scientists were monitoring the earthquake cluster, which continued on Tuesday, to see if it approaches the Imperial Fault, about three miles away. A destructive and deadly earthquake of magnitude 7.0 struck on that fault in 1940, she said.

“We don’t have any reason to believe that the (earthquake) storm is going to trigger on the Imperial Fault, but there’s a minute possibility that it could,” Hardebeck said, adding that the swarm of quakes was not moving closer to that fault.

The Brawley quake cluster, which is caused by hot fluid moving around in the Earth’s crust, is different than a typical earthquake, in which two blocks of earth slip past each other along a tectonic fault line.

After that kind of an earthquake of magnitude 5.5 or above, there is a 5 percent chance a larger quake will follow, Hardebeck said. But she added the same kinds of probability estimates were not possible with earthquake clusters caused by the movement of hot fluid.

“We understand them even less than we understand normal earthquakes,” Hardebeck said, adding that scientists do not know why a cluster of earthquakes will occur at one time rather than another.

The swarm led to jangled nerves in Brawley, a town of about 25,000 residents 170 miles southeast of Los Angeles near the border with Mexico.

“It’s pretty bad. We had to evacuate the hotel just for safety,” Rowena Rapoza, office manager of a local Best Western Hotel, said on Sunday.

There were two earthquakes on Sunday afternoon, one with a 5.5 magnitude and one measuring 5.3, Hardebeck said. Those were the largest quakes in the cluster amid hundreds of others, she said.

In the past, earthquake clusters have gone on for as long as two weeks, Hardebeck said. Before this recent cluster in Brawley, the last swarm of this size to hit the area was in 1981, she said.

Earlier this month, a pair of moderate-sized earthquakes both registering a magnitude 4.5 struck the California town of Yorba Linda within 10 hours of each other, but no damage was reported. Yorba Linda, the birthplace of the late President Richard Nixon, is 145 miles northwest of Brawley.

Reporting By Alex Dobuzinskis; Editing by Cynthia Johnston and Philip Barbara

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