Published on July 12th, 2011 | by Joshua S Hill
Ancient Lake Linked to San Andreas Earthquakes
Southern California’s Salton Sea may be one of many factors involved in setting off earthquakes in the region, specifically along the southern San Andreas Fault, and may in fact have already triggered large earthquakes over the past thousand years.
Researchers at Scripps Institution of Oceanography, UC San Diego, the U.S. Geological Survey (USGS) and the University of Nevada, Reno, were able to discover new faults in the Salton Sea near the southern end of the San Andreas Fault by imaging beneath the Salton Sea.
The researchers also found, by examining displacement indicators preserved in sedimentary deposits, that there is coincidental timing between flooding of the ancient Salton Sea and fault rupture in the region.
Ruptures on these newly discovered faults – known as stepover faults – have the potential to end up triggering larger earthquakes on the southern San Andreas Fault.
“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 report lead author Danny 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.”
Brothers notes though, that these discoveries don’t really help in predicting earthquakes, but rather suggest that heightened preparedness for a major earthquake would be sensible in the wake of smaller quakes in the stepover zones.
The San Andreas Fault is already closely monitored as it has been expected to rupture for the past 300 years. “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.”
That reason is the redistribution of stressors in the region with the decrease in the size of the historical Lake Cahuilla which existed in larger form than the current day Salton Sea (as seen in the map to the right). Once one and a half times the size of Lake Tahoe at its maximum, authorities in the early 20th century diverted the Colorado River away from the lake to minimise flooding in the region, thus allowing for the creation of the smaller Salton Sea.
While in its dominance, Lake Cahuilla and its surrounding region experienced five earthquakes of magnitude 7 and above in a period of a thousand years. It has been more than 300 years since the last one took place.
Stress models of the region show that the stepover faults discovered are likely to rupture when the stress above them – in other words, the size and density of the Salton Sea – increases, ie, floods. The diverting of the Colorado River which has decreased the likelihood of flooding could be one possible explanation for why there has been no large quake along the southern San Andreas Fault.
Another worry that scientists have is the propagation of earthquakes north, from the Southern Andreas Fault, into areas like Riverside and Los Angeles.
“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.”