Photo Â© Jenny E. Ross/www.jennyross.com Danny Brothers (left), lead author of a new study of earthquakes in the Salton Sea, and Rob Baskin from the U.S. Geological Survey deploy a sidescan sonar instrument in the Salton Sea. Photo Â© Jenny E. Ross/www.jennyross.com When its natural dimensions were in place, Lake Cahuilla and its surrounding region experienced in a 1,000-year period five earthquakes on the Southern San Andreas that are believed to have been larger than magnitude 7. The temblors occurred about 180 years apart. It's been more than 300 years since the last one. Diversion of the Colorado River and the lack of flooding events in the local basin known as the Salton Trough may be one possible explanation. The researchers studied the sediments deposited over several millennia on the lake floor and found coincident timing between several flooding events and rupture of step-over faults, which in turn, may have loaded the San Andreas. Stress models showed that the predominantly normal faults with vertical displacement in the Salton Sea are more vulnerable to sudden increases in vertical loads caused by lake filling. Those failures may have triggered the movement of California's primary fault in several instances, the researchers said. No such sequence has taken place since the lake assumed its current dimensions. "We've been baffled as to why the Southern San Andreas hasn't gone. It's been compared to a woman who is 15 months pregnant," said Scripps seismologist Debi Kilb, a report co-author. "Now this paper offers one explanation why." The researchers cautioned that failure of the stepover faults is ultimately driven by tectonic forces and could still set off a major rupture of the San Andreas Fault independently of any lake level fluctuations. Other research teams have estimated that stress buildup in the area is still great enough to produce a quake between magnitude 7 and 8. The idea that the San Andreas is triggered by stress loading in the Salton Sea supports the assumption by many scientists that a future quake sequence could propagate northward and potentially cause significant damage in the Los Angeles area. Then-Scripps graduate student Danny Brothers surveys sediments near Salton Sea in 2007. Then-Scripps graduate student Danny Brothers surveys sediments near Salton Sea in 2007. "Earthquake simulations reveal that shaking of large metropolitan areas such as Riverside and Los Angeles will be larger if the earthquake propagates from south to north - our research suggests that the Salton Sea stepover zone may provide a trigger for such a propagation direction," said Scripps geologist Neal Driscoll, a report co-author. Brothers said that one of the most immediate applications of the research is as a guide to development in the Salton Sea region, which has been the subject of environmental restoration efforts in recent years. "Large earthquakes on the southern San Andreas most likely will be accompanied by liquefaction in the Imperial Valley. In addition to ground shaking, the liquefaction will cause damage to water conveyance systems and existing infrastructure in the region and is likely to affect Salton Sea restoration efforts," he said. "Not only were we able to address seismic hazards issues along the San Andreas Fault, but this research also highlights the broader use and capabilities of new techniques and technologies to study hazards under bodies of water," added Graham Kent, director of the Nevada Seismological Laboratory at the University of Nevada, Reno and a co-author of the report. "This can have application for other regions where the presence of water has left problems undetected."
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