California overdue for The Big One

LOS ANGELES — Southern California is long overdue for a major earthquake along the San Andreas fault, according to a study of historic seismic activity released Friday.

The study, produced after years of field studies in an area about 100 miles northwest of Los Angeles, found that earthquakes along the San Andreas fault have occurred far more often than previously believed.

For years, scientists have said major earthquakes occurred every 250 to 450 years along this part of the San Andreas. The new study found big temblors on the fault every 88 years, on average.

The last massive earthquake on that part of the fault was in 1857, leading scientists to warn that another such temblor is likely.

“The next earthquake could be sooner than later,” said Lisa Grant Ludwig, a University of California, Irvine, earthquake expert and co-author of the study, which was published online in the journal Geology. “It was thought that we weren’t at risk of having another large one any time soon. Well, now, it might be ready to rupture.”

Other seismic experts described the revelation as a major change in the way they think about earthquake risks along the southern San Andreas fault.

Thomas Jordan, director of the Southern California Earthquake Center, said the fault is “locked and loaded. It’s been a long time since an earthquake has occurred on that fault — over 150 years.”

To reach the new conclusion, scientists dug deep trenches and used imaging technology known as lidar to find signs of earth movements. They were able to detect earthquakes dating back to the 15th century, creating a far more complete record than had previously been known.

Scientists now estimate that earthquakes occurred on that section of the fault in 1417, 1462, 1565, 1614 and 1713. The finding adds weight to the view that a major rupture is now likely.