Southern California earthquake mystery shakes out of a slumber

Los Angeles: It's one of the great mysteries of Southern California seismology: Every couple of years, the remote desert area around the Salton Sea is shaken by swarms of small to moderate earthquakes that often last several days.

The swarms returned this week, with the area recording more than 160 temblors since Saturday - including several that were felt on Wednesday. But this time, scientists had sophisticated instruments in the ground to record the activities and are learning more than ever before about the swarms and how they affect the seismic risk elsewhere.

For one thing, scientists have noticed the quakes appear to have a pattern, moving southeast as the days progress. But the bigger question remains to be answered: Can the quakes trigger larger - and potentially more destructive - quakes along the San Andreas Fault, which terminates at the shore of the Salton Sea.

A creep metre on the San Andreas Fault just north of the Salton Sea area, operated by the University of Colorado, found a 0.002-inch slip on the San Andreas right after the largest earthquake in the swarm - a magnitude 4.8 quake on Tuesday.

This leaves scientists tantalised. "If you look at the statistics, they say the odds of something bigger happening is on the order of 1 per cent," said Susan Hough, scientist in charge at the US Geological Survey in Pasadena, California. "It raises your blood pressure as a seismologist, but we're trying to read the tea leaves."

Seismologists have long suspected quakes in the Salton Sea area can trigger other nearby faults, including the San Andreas. The strongest evidence of this occurred in 1987, when a magnitude 6.2 earthquake near the Salton Sea triggered a 6.6 quake 12 hours later on the Superstition Hills fault to the south. The San Andreas fault is north of these crossing faults, and the geometry is similar, experts said.

Experts aren't exactly sure why the area around the Salton Sea, with its desert expanse and small struggling towns, is so seismically active.