EMBARGOED BY Nature Geoscience
FOR RELEASE ON Sunday, June 26, 2011 10:00 AM PDT
Monday, June 27, 2011
Flooding of Ancient Salton Sea Linked to San Andreas Earthquakes
Study finds that faults beneath the Salton Sea ruptured during Colorado River floods and may have triggered large earthquakes on the southern San Andreas Fault
Scripps Institution of Oceanography / University of California, San Diego
Southern California's Salton Sea, once a large natural lake fed by the Colorado River, may play an important role in the earthquake cycle of the southern San Andreas Fault and may have triggered large earthquakes in the past.Researchers at Scripps Institution of Oceanography, UC San Diego, the U.S. Geological Survey (USGS) and the University of Nevada, Reno, discovered new faults in the Salton Sea near the southern end of the San Andreas Fault. By examining displacement indicators preserved in pristine sedimentary deposits, the team reconstructed their earthquake history and found evidence for coincident timing between flooding of the ancient Salton Sea and fault rupture. Rupture on these newly discovered "stepover" faults has the potential to trigger large earthquakes on the southern San Andreas Fault.
Map of the current Salton Sea boundaries and outline of Lake Cahuilla at its peak size as well as locations of major area faults.
The Salton Sea covers a structural boundary at the southern end of the San Andreas Fault where it takes a southwestward step to the Imperial Fault. The region is closely monitored because the last large earthquake on this section of the San Andreas occurred approximately 300 years ago and the fault is considered by many experts to be overdue for another.
By imaging beneath the Salton Sea, the study identified the key role of stepover faults that run at an angle to the San Andreas Fault. The smaller faults rupture relatively frequently and, at times, they ruptured in concert with Colorado River flooding of the Salton Trough. Report lead author Danny Brothers said that this research does not improve the ability to predict such a quake but suggests that heightened preparedness for a major quake immediately following smaller quakes in the stepover zone is warranted.
"To fully understand the hazards and rupture scenarios associated with the southern San Andreas Fault, we can't limit our study to the San Andreas Fault itself," said Brothers, a researcher now at the USGS who conducted most of the research while a graduate student at Scripps. "These stepover zones really need to be considered when assessing earthquake hazards and need to be examined as potential triggers for destructive earthquakes on the larger faults."
The current dimensions of the Salton Sea located in California's Imperial Valley are but a fraction of the natural lake that preceded it. Through cycles of flooding and evaporation, the historical Lake Cahuilla was once one and a half times the size of Lake Tahoe at its maximum. What is left since the beginning of the 20th Century - when local authorities redirected the Colorado River away from the lake - is less than 1/25th that size.
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 © Jenny E. Ross/www.jennyross.com
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.
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