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 200 temblors since Saturday -- including several that were felt Wednesday. But this time, scientists had sophisticated instruments in the ground to record the activity, helping them to better understand the swarms and how they can affect seismic risk elsewhere.
Scientists have noticed that the quakes appear to have a pattern, moving southeast as the days progress. But a bigger question remains: 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 meter on the San Andreas just north of the Salton Sea area, operated by the University of Colorado, found a 0.002-inch slip on the fault right after the largest earthquake in the swarm -- a magnitude 4.8 on Tuesday.
Experts say that's a tiny slip for a fault so large, but the novelty of having that kind of data is tantalizing for scientists.
"If you look at the statistics, they say the odds of something bigger happening is on the order of 1%," said Susan Hough of the U.S. Geological Survey in Pasadena. "It raises your blood pressure as a seismologist, but we're trying to read the tea leaves."
Seismologists have long suspected that quakes in the Salton Sea area can trigger movement on 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 magnitude 6.6 quake 12 hours later on the Superstition Hills fault to the south.
Experts are still trying to figure out why the hard-scrabble landscape of desert expanse and small struggling towns around the sea is so seismically active.
The sea sits atop a very thin crust that is being constantly stretched as the North American and Pacific plates grind against each other. The area is also veined by dozens of fault lines that run parallel to and criss-cross one another.
The area experienced regular quake swarms until 1979, when a large temblor just south of the U.S.-Mexico border seemed to curb activity, said Doug Given, a U.S. Geological Survey scientist.
But this quiet period appears to have ended.
