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Longer Wait, Larger Quake?The science team, which also includes San Diego State geophysicist Gordon Seitz, who has researched seismic activity at the Salton Sea for several years, began the project s first phase in 2004. One of their initial findings had little to do with earthquakes: There were never hotel rooms available, because contractors erecting scores of area houses always had them booked. For one portion of the fieldwork, the researchers chose to live on a pontoon boat they had specially outfitted to accommodate their sonar CHIRP profiler. They towed the CHIRP behind the boat through opaque waters in two- and three-day stretches, a practice known as capsuling. The CHIRP instrument created images of sediment layers beneath the seafloor that illuminated the location and size of subsurface faults. Seitz has already established some of the history of seismic activity in the Salton Trough. The area is an extensional basin that rests between two tectonic plates. Driscoll likens the movement there to the pulling apart of a Milky Way bar. One plate, the Pacific, moves northwest 50 millimeters (2 inches) a year as it pulls away from the North American plate.  
Driscoll, Kent and Scripps graduate student Jeff Dingler added to the historical knowledge of quakes here during this project by estimating the size of the most recent seismic event. Using a technology called LIDAR (light detection and ranging) to measure deformation patterns in the hills overlooking the sea, they concluded that the quake from 335 years ago produced three meters (10 feet) of slip in one burst. Driscoll said that translates to an earthquake magnitude around 7. The size of that quake gives scientists cause for concern. A magnitude 7 quake anywhere in Southern California is nearly guaranteed to do considerable property damage, but because the quiescent period between this last quake and today is longer than usual, the scientists fear that the extra time could mean an extra buildup of energy and an even larger quake when the energy is finally released. The researchers are still analyzing the seismic CHIRP data collected from their fieldwork, which concluded early this summer. When done, they might be able to produce another key piece of information: what direction will the earthquake rupture travel when the next quake happens? The area has seen quakes on smaller faults trigger secondary quakes near the San Jacinto, most recently in a 1987 event known as the Elmore Ranch Superstition Hills sequence. 
The scientists hope to establish whether a similar event on the Extra Fault in the Salton Sea will trigger an event along the southern San Andreas segment. In such a scenario, the earthquake rupture would radiate northward focusing more energy into the Los Angeles Basin than if it propagated in the opposite direction. If that happens, metropolitan Los Angeles could bear the worst of that energy. Driscoll said the many implications of seismic activity at the Salton Sea argue for much more extensive study. Everyone wants to know how these faults link and communicate deformation, said Driscoll. What we re seeing raises concern about the southern San Andreas Fault and future geohazards. |
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:: © 2009 Scripps Institution of Oceanography, University of California, San Diego. All Rights Reserved.
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