Similarity of Chinese, Calif. Fault Systems Raises Concerns
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.