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More earthquake faults discovered at the Salton Sea

Sound waves bounced off the sea floor reveal the shifting blocks of crust, leading to a new theory of how the ground is sinking and stretching near the infamous San Andreas fault.

July 28, 2009|Jia-Rui Chong

By bouncing sound waves off the floor of the Salton Sea, researchers have discovered more than a dozen previously unknown earthquake faults, leading to a new theory of how the ground is sinking and stretching near the infamous San Andreas fault.

Danny Brothers, lead author of a study published Sunday, said the new understanding of the area's seismic mechanics does not appear to suggest that a massive quake on the San Andreas is more imminent than previously believed. Earthquake scientists have been interested in the region, about 140 miles east of Los Angeles, because the southernmost end of the San Andreas disappears at the banks of the Salton Sea.

"By all reports, the San Andreas is considered overdue," Brothers, a geophysics graduate student at UC San Diego, said Monday. "What this does is gives us more information to assess it. Now we can start to run some scenarios on how earthquakes beneath the Salton Sea might affect the state of stress on the San Andreas and vice versa."

Scientists have not had very detailed maps of the crust under the Salton Sea, in part because underwater conditions have made it difficult to employ traditional techniques for studying faults. For years, scientists inferred fault locations there by studying earthquake data. The recordings led scientists to suggest that blocks of crust were swiveling side to side, but generally moving horizontally.

For the new study, published Sunday in the journal Nature Geoscience, Brothers and colleagues crisscrossed the Salton Sea with a sonar-like instrument. Measuring the reflections of sound waves gave the researchers images of sediment layers down to about 200 feet below the lake bed.

They discovered about 15 to 20 relatively short faults angled toward the San Andreas.

Their data led them to propose that the crust in the area is being pulled apart by the San Andreas and the nearby Imperial fault. When the crust pulls apart, it subsides and dips into a basin, rather than just shifts horizontally, Brothers said.

Craig Nicholson, a research geophysicist at UC Santa Barbara who was not involved in the Nature paper, said the more detailed picture of the lake bed raises new questions about the region's seismic complexity.

"It's not clear what the connection is between the features they're seeing and the faults at [greater] depth," where earthquakes are thought to originate, Nicholson said. "Maybe the earthquakes are reflecting a slightly older pattern, and the sediment cover is telling us more about what's happened more recently."

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jia-rui.chong@latimes.com

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