India study hints California faults may be severe
LOS ANGELES – Faults hidden in the crust of California may produce larger earthquakes than previously thought, a team of researchers who studied the recent 7.7-magnitude temblor in India said Tuesday.
The type of fault that produced the Jan. 26 Gujarat state earthquake, called a blind-thrust fault, is found in California as well, including directly beneath the skyscrapers in downtown Los Angeles.
However, members of a working group that assesses the state’s earthquake probabilities previously assumed the blind-thrust faults in Southern California were not capable of producing earthquakes greater than magnitude 7.3.
The group’s recent reconnaissance trip to India, where the Gujarat earthquake killed more than 19,500, has left some questioning that upper limit. The difference is significant: A magnitude 7.7 earthquake releases about 2.5 times more energy than a 7.3 quake.
”It sort of raises the bar for what the maximum earthquake detection threshold is for blind-thrust faults,” said William Lettis, an earthquake hazard consultant and member of the research team sponsored by the National Science Foundation and the Earthquake Engineering Research Institute. ”As a geologist can you go out and easily find a fault that will produce a future 7.7 earthquake? The answer is no and that’s an important lesson.”
By their definition, blind-thrust faults remain buried and do not break the surface of the ground, making them more difficult to map. The India earthquake, for example, ruptured at a depth of about 12 miles before breaking downward deeper into the Earth’s crust, Lettis said.
Preliminary findings from India suggests that similarly large earthquakes could occur in areas of California where unmapped blind-thrust faults exist – and that those types of earthquakes could be much larger than anticipated.
Seismologists already know that multiple blind-thrust faults crisscross Southern California, including those under Los Angeles.
”The question here is, could an earthquake like this happen in California? And if it could – and I am not saying it can – if it could, the fact that it did not break the surface could potentially be of great concern,” said Jonathan Stewart, an assistant professor of civil engineering at the University of California, Los Angeles, who was in the group that traveled to India.
Lettis cautioned that the jury is still out on the nature of the India earthquake and there may not be a direct analog in the United States to its tectonic setting. In fact, the temblor may more closely resemble the large earthquakes that occurred along the New Madrid fault in Missouri in 1811 and 1812 than any quake in California.
Regardless, the blind-thrust faults known to exist remain a risk for California, said Thomas Heaton, a professor of engineering seismology at the California Institute of Technology who is not affiliated with the India group.
”I don’t think we really know what the largest earthquake (is) that could happen on a blind-thrust fault under Los Angeles,” Heaton said. ”The good news is those earthquakes don’t happen very often so there’s a good chance we’ll be dead by the time it does.”
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