Giant quake near the Salton Sea in the San Di...July 12, 2011

salton

The southern end of the San Andreas Fault may be overdue for a large earthquake that could cause heavy damage to the Los Angeles area, scientists have concluded after studying a record of ancient quakes and flooding around the seismically active region of the Salton Sea.

By David Perlman, Chronicle Science Editor | July 3, 2011

The researchers report finding evidence of many small past quakes that have ruptured along small “stepover” faults which run at right angles to the southern end of the San Andreas. The underground stresses those small quakes have built up could trigger a much bigger one in the future on the dominant San Andreas, they say.

A report from a group at the U.S. Geological Survey in Woods Hole, Mass., the Scripps Institution of Oceanography in San Diego and the Nevada Seismological Laboratory in Reno suggests that a major temblor in the Salton Sea region could reach a magnitude greater than 7 – significantly larger than the 6.9-magnitude Loma Prieta quake of 1989 in the Bay Area.

They studied ancient sediments around the Salton Sea that reveal many small ruptures along the stepover faults in the past, as well as records of flooding from the Colorado River into what was once a much larger lake where the Salton Sea lies now. They also examined the record of five major earthquakes that have hit the southern San Andreas in the past 1,000 years.

Those quakes struck at intervals of roughly 180 years, and it has been more than 300 years since the last one occurred – suggesting that another big temblor might well be overdue, the researchers said.

The Salton Sea was created in 1905 when the Colorado River overflowed and flooded the much larger area of the Salton Basin once known as Lake Cahuilla, which in prehistoric times was nearly double the size of Lake Tahoe.

According to the scientists, the ancient lake alternately flooded and dried up over the centuries, and that cycle most probably created seismic stresses that were responsible for the many small earthquakes that occurred on the stepover faults, according to Daniel Brothers, the lead author of the report and a post-doctoral fellow from Scripps now at the USGS Science Center in Woods Hole.