Small quakes rattle California

LOS ANGELES — 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 centred near the town of Brawley close to the state’s inland Salton Sea, said Jeanne Hardebeck, research seismologist for the US 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.

Threat from larger quake

After that kind of an earthquake of magnitude 5.5 or above, there is a five per cent 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. (Reuters)

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