LOS ANGELES -- Southern California is overdue for the next "Big One," say geologists who have uncovered a detailed history of more than a dozen major earthquakes in the past 1,500 years on a stretch of the San Andreas fault.
Digging into layers of peat and debris that drape a section of the fault near Wrightwood, a small town in the mountains that divide the Los Angeles basin from the Mojave Desert, geologists dated 14 earthquakes that each measured an estimated magnitude of 7.5.
Since 534, those quakes recurred every 105 years on average. The interval has been as short as 62 years and as long as 192.
The last was in 1857 -- 145 years ago.
The chronology, the longest for any one fault in the United States and probably the world, is detailed in several studies appearing this month in a special issue of the Bulletin of the Seismological Society of America devoted to paleoseismology, or the study of ancient earthquakes, and to the 800-mile-long San Andreas.
"As far back as we look, we see these earthquakes happening, so I'd say there's a very high probability of it happening again in the lifetimes of most people living in Southern California right now," said Tom Fumal, a research geologist with the U.S. Geological Survey and a co-author of several of the issue's 14 studies.
Similar results from other San Andreas sites southeast of Wrightwood reinforce the suggestion that the southernmost 120-mile section of the fault is near failure.
"When I am stuck in traffic and I am listening to the news about homeland security and trying to prevent terrorist attacks, I am thinking, What about the 'terrorist' that is going to strike no matter what we do?" said Lisa Grant, a University of California, Irvine, geologist and lead editor of the special issue.
Based on recurrence models, there is about a 30 percent probability that a quake nearly as large as the magnitude-7.9 that rocked Alaska on Nov. 3 will strike the southern San Andreas in the next 30 years. That number is in line with previous estimates.
"Basically, the overarching significance to the general public is we are overdue," said Kerry Sieh, a California Institute of Technology geologist, a paleoseismic expert who was not connected with the latest research.
Were a major quake to strike the region, home of nearly 13 million people, it would cause an estimated $50 billion in damage and kill a few thousand people, said Guy Morrow, vice president of engineering for Risk Management Solutions, a San Francisco Bay Area risk modeling company.
Geologists cautioned that earthquakes, like the stock market, can defy prediction and don't necessarily strike at consistent intervals.
"It's a very hotly contested concept in the field. There is no consensus. There is a group that tends to believe in randomness, and there's a group that believes in the characteristic earthquake model," said Lucy Jones, scientist in charge of the USGS office in Pasadena.
The scientists studying San Andreas activity near Wrightwood dug a series of trenches across the fault, then pinpointed locations in the trench walls where major earthquakes had clearly ruptured the surface in past earthquakes. They analyzed the organic material that had been buried over time and used carbon dating to determine the age of each rupture.
Geologists are now working to extend the record of earthquakes at Wrightwood back an additional 2,500 years.
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On the Net:
Seismological Society of America: http://www.seismosoc.org/
San Andreas Fault primer: http://pubs.usgs.gov/gip/earthq3/
Southern Calif quake map: http://www.scecdc.scec.org/