June 20, 2006 11:08 AM PDT

California primed for a big quake near L.A.

When will California experience its next big quake along the San Andreas Fault? It's hard to say, but conditions are ripe for a big one near Los Angeles and San Diego.

The southern section of the fault, which crosses through Palm Springs and San Bernardino, has not experienced a major earthquake in at least 300 years, according to Yuri Fialko of the Scripps Institution of Oceanography at the University of California, San Diego.

As a result, the fault has been stressed to a level where it could unleash an earthquake with a magnitude of seven or greater. The fault marks the intersection where the Pacific plate meets the North American plate. The fault-slip rate, or pace of the plate movement at the fault, is about one inch per year. But pressure buildup indicates that the southern region of the fault has accumulated 6 to 8 meters of slip deficit--that is, movement that should have occurred but hasn't because the plates have temporarily become stuck against each other--which inevitably will be released in future earthquakes.

If the accumulated deficit were released in a single event, it would result in an earthquake about the same size as the San Francisco earthquake of 1906. Such a quake would affect many of the coastal cities in southern California.

"In the earthquake business, the past is a key to understanding the present and, by comparing ancient observations of the fault with what we have measured over the last 10 years, we can say with some certainty that the fault is approaching the end of its loading period," Fialko said in a statement. "All these data suggest that the fault is ready for the next big earthquake, but exactly when the triggering will happen and when the earthquake will occur, we cannot tell. It could be tomorrow, or it could be 10 years or more from now."

Quakes occurred along the central part of the fault in 1857 and 1906 in the northern part of the fault.

Predicting earthquakes, however, is difficult. Scientists generally believe there is a 70 percent chance of a large quake along the San Andreas in the next 30 years. Researchers, though, concede it could happen tomorrow or in 50 years.

Some scientists, however, have begun to conduct research that indicates the Earth gives off early warning signs through increased magnetic signals and other phenomena.

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