The southern end of the San Andreas Fault may be overdue for a large earthquake that could heavily damage 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.
The researchers report finding evidence of many small past quakes that have ruptured along small "stepover" faults, which run at right angles to the fault's southern end. The underground stresses those small quakes have built up could trigger a much bigger one 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.
The scientists' report is published online in the current issue of the journal Nature Geoscience.
They studied ancient sediments around the Salton Sea that reveal many small ruptures along the step-over 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 step-over faults, according to Daniel Brothers, the lead author of the report and a post-doctoral fellow from Scripps now at Woods Hole.
"Now that we've found that those step-over faults around the Salton Sea could trigger a quake on the San Andreas with a magnitude larger than 7, if that earthquake ruptures the ground northward, it could cause heavy damage from shaking across the Los Angeles metropolitan area," Brothers said.
Karen Lutrell, a co-author of the report who is now at the Geological Survey's Volcano Science Center in Menlo Park, Calif., noted that the concept of the Salton Sea region as overdue for another large earthquake remains only tentative at best.
"Our best understanding of those previous large earthquakes is still a subject for more research, and we can't be too specific about the timing," she said. "But because the last large quake was some 300 years ago and intervals between earlier ones were irregular, people like to compare it to an overdue pregnancy with the baby still not born, and feel that another large quake is now long overdue, too.
"But we still know more about babies than we do about earthquakes." she said.