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Similarity of Chinese, Calif. Fault Systems Raises Concerns

By Joel Achenbach
Washington Post Staff Writer
Wednesday, June 11, 2008

Kenneth Hudnut sees trouble out his window. He works in Pasadena, Calif., in a sunny valley of palm trees, historic bungalows, gourmet coffee shops and elite institutions of higher learning and space technology. But Hudnut, a geophysicist with the U.S. Geological Survey, knows that it also is home to something called the Sierra Madre fault, which is adjacent to something called the Cucamonga fault.

That, in turn, is not far from the fabled San Andreas fault. What worries Hudnut is the possibility of the geological equivalent of dominos: What if an earthquake on one fault causes a chain reaction?

That, he believes, is what happened in China last month in the earthquake that has so far been blamed for more than 69,000 deaths.

"The fault system that ruptured is a lot like the one right out my window here," Hudnut said.

Heightening his anxiety is the fact that many scientists were caught by surprise by the magnitude of the China earthquake, estimated at 7.9 by U.S. scientists. Sichuan province has a history of earthquakes, but none so devastating. It was not near the top of anyone's list of the most likely locations for a great quake. The data from satellites, which can track the motion of vast plates of the Earth's crust, suggested a relatively moderate amount of strain building up in the rugged mountain front along the edge of the Sichuan basin.

"The lesson that one gets from this Sichuan earthquake is that we don't yet fully understand where all the hazard is," said Eric Kirby, a Penn State geologist who has extensively studied faults in that part of China. "We knew this was an active mountain belt, but we didn't quite realize what it was capable of."

"This did not look like a very active region," agreed Peter Molnar, professor of geological sciences at the University of Colorado.

Chen Ji, a University of California at Santa Barbara geologist, said of last month's China earthquake, "It teaches us to be more cautious about the argument we make based on today's data, today's observation."

Hudnut and his colleagues say they believe, based on preliminary data, that at least three different faults ruptured in succession. Rarely has such a cascading event been documented.

"Now that we see the cascading behavior, we get even more nervous. We see the potential here" in the Los Angeles area "for an earthquake that's larger than what we thought it was capable of. We could get our comeuppance from Mother Nature any day here," Hudnut said.

James Dolan, a University of Southern California geologist, has put together a map that shows faults in the Los Angeles area butting up against one another like passengers on a subway at rush hour. "Some of these faults could link up in ways we had never anticipated, which could lead to larger events," Dolan said.

For years, scientists have debated whether earthquakes are in any way predictable. There's a divide between those who see them as innately chaotic and those who think there are discernible patterns, even precursors, that would make it possible in theory to know when the Big One is coming.

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."

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