A chronology of 1,000 years of earthquakes at the southern end of the San Andreas Fault nixes the idea that lake changes in the now-dry region caused past quakes. However, researchers say, the timeline pulled from sediment in three deep trenches confirms that this portion of the fault is long past the expected time for a major temblor that would strongly shake the Los Angeles Basin.
The new study, appearing in the February issue of the Bulletin of the Seismological Society of America, doesn't change existing thinking about the threat of a major quake -- potentially measuring 7.0 to 8.0 on the Richter scale -- for southern California. It does, however, provide the first published documentation of much-discussed data that have emerged in the last three decades from an area that is now rapidly being built up and populated, just north of the Salton Sea.
Earthquake Map with Weldon study, 2011
Credit: Ray Weldon, University of Oregon
Projections of such a quake in recent years led to the nation's largest-ever drill, the Great Southern California ShakeOut, last year. The 2011 ShakeOut is set for Oct. 20. There's even a video projection of the quake's probable route created by the Southern California Earthquake Center. The last earthquake to originate from the area occurred in about 1690.
The new study, said co-author Ray Weldon, professor and head of the department of geological sciences at the University of Oregon, documents that the south end of the San Andreas fault has gone perhaps 140 years longer than the average 180 years between quakes.
"We have dated the last five to seven prehistoric earthquakes of the southernmost 100 kilometers (about 60 miles) of the San Andreas Fault, which is the only piece of the fault that hasn't ruptured in historical times," Weldon said. "If you were there in about 1690, when the last earthquake occurred, the odds of getting to 2010 without an earthquake would have been 20 percent or less."...MORE