Los Angeles basin at risk of major quake

Posted by Emma Woollacott

More great news for Californians. Just a couple of weeks ago, they were told that they were at serious risk of a superstorm - and now they need to prepare for a major quake.

Ray Weldon, professor and head of geological sciences at the University of Oregon, was researching whether or not changes in ancient lakes at the southern end of the San Andreas Fault could have caused past quakes.

His results seemed to nix that theory. But what his research did uncover was evidence that this portion of the fault is way overdue for a major temblor that would strongly shake the Los Angeles Basin.

And the data was drawn from samples from an area that's now rapidly being built up and populated, just north of the Salton Sea. It shows 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 of the San Andreas Fault, which is the only piece of the fault that hasn't ruptured in historical times," says Weldon.

"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."

The seven earthquake events, including the two possible temblors, were placed between 905-961 AD, 959-1015 (possible), 1090-1152, 1275-1347, 1320-1489 (possible), 1588-1662 and 1657-1713, based on analyses of seismic structures preserved in the sediment in three trenches and 82 radiocarbon dates drawn from 61 samples of organic material.

"At some point, this area will get kicked by shaking from one of the many quakes that happen south of the San Andreas Fault," says Weldon. "It will rupture northward along the fault. When it comes into the San Bernardino Valley, seismic energy will be directed by a series of basins, including the Los Angeles Basin, into the most highly populated part of Southern California."