Similarity of Chinese, Calif. Fault Systems Raises Concerns

Scientists don't try to predict earthquakes, but they do produce scenarios and hazard maps. The problem is that the planet doesn't seem to pay close attention to the maps or abide by statistical probabilities. The Great Sumatra-Andaman earthquake on Dec. 26, 2004, which caused a catastrophic tsunami, happened on a known fault, but the fault broke much farther to the north than anyone had anticipated.

The Northridge quake of 1994 took place on an unknown "blind thrust" fault beneath the surface of Southern California. More blind thrust faults have been found since, including the Puente Hills thrust fault running beneath downtown Los Angeles. There's no reason to think the census of such subterranean features is complete.

"We don't really understand the Earth," said Thomas Jordan, director of the Southern California Earthquake Center. "There may be faults we don't even know about."

For decades, the operating assumption in California has been that the Big One will be produced by the San Andreas fault, which marks the meeting point of two enormous tectonic plates: the North American plate and the Pacific plate. They're trying to slide past one another at a rate roughly the speed at which fingernails grow. But they spend most of their time locked tight. Strain builds. An earthquake is a fault's way of releasing that strain.

The San Andreas broke near San Francisco in 1906 -- triggering fires that destroyed much of the city -- and north of Los Angeles in 1857. But the southernmost part of the fault, east of Los Angeles and leading down to the Salton Sea, has not broken since the 17th century. That southern section is widely viewed as ripe for a major rupture.

The government has scheduled a mock San Andreas earthquake this fall. The event, called the Great Southern California Shakeout, will take place at 10 a.m. on Nov. 13. It will give first responders, school officials and government officials the opportunity to see how they would handle a magnitude 7.8 earthquake on the San Andreas.

A recent study forecast 2,000 deaths and $200 billion in damage in such a disaster. Scientists have created a new model showing the earthquake hazards over time in any given part of California. The odds of a 7.5-plus earthquake somewhere in the state over the next 30 years are listed at 46 percent.

The problem is, that may sound rather more precise than the scientific community intends for it to sound. According to Jordan, "That 46 percent number has a very large variation. . . . The uncertainty is 30 to 65 percent."

Susan Hough, scientist-in-charge at the USGS office in Pasadena, is concerned that the public will think that scientists are predicting a 7.8-magnitude San Andreas quake rather than merely running through one scenario among many.

"What worries me is that, yeah, the earthquake takes on a life of its own, and it starts to sound like we're predicting this earthquake," she said. "It's plausible, but there's absolutely no reason to think that it's the scenario that's going to happen. The Earth surprises us, that's what we see over and over."