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Studies See Big Quake as More Likely

Magnitude 7 or higher temblor on San Andreas fault in Inland Empire, Palm Springs may occur sooner than thought. Reports stir dispute.

November 17, 2002|Usha Lee McFarling | Times Staff Writer

A new analysis of the San Andreas fault that peers thousands of years into its turbulent past suggests that sections in the Inland Empire and Palm Springs area may be more primed for a major earthquake than previously believed.

The new work -- already the subject of dispute -- also indicates that the San Francisco Bay Area could soon emerge from the relative seismic quiet that has lasted since the magnitude 7.1 Loma Prieta quake in 1989.

The findings are included in 14 reports published today after years of study intended to help scientists forecast the next big shakers -- greater than magnitude 7 -- along the dreaded California landmark. One of the studies looked back 1,500 years at more than a dozen earthquake cycles -- a far richer information trove than the two or three cycles that earthquake archeologists typically find.

The Palm Springs study found that a stretch of the fault there, which once experienced temblors on average every 215 years, has not ruptured in 326 years -- indicating that much stress has accumulated from the movement of plates deep in the Earth. The research team believes this means that a "big one" could be more than a century overdue in that area.

A similar though even less conclusive pattern was seen on the San Bernardino strand of the San Andreas running through the now-highly populated Inland Empire, which ruptured during the 1600s, and not in 1812 as previously assumed.

"We already knew we had a high seismic risk in the Inland Empire," said Sally McGill, a Cal State San Bernardino geologist. "This isn't a call for new alarm. It's just a reminder to stay prepared."

The findings, in the Bulletin of the Seismological Society of America, are the work of 74 paleoseismologists who have spent years -- even decades -- digging deep trenches into the heart of the San Andreas fault and scrutinizing disordered layers of mud, peat and rock.

Those layers provide a record of past earthquakes. The scientists' work is an effort to compile the most detailed record yet of the fault's antics to see if quakes have occurred in regular, predictable patterns.

If they do, many geologists say, there would have been a large earthquake by now in tiny Parkfield in southern Monterey County, where major rumblings seemed to have occurred with striking regularity -- on average every 22 years -- since 1857. The last quake was in 1966. The U.S. Geological Survey has spent tens of millions of dollars there to "trap" the major quake they were sure would strike by 1988. They are still waiting.

"We disproved that [theory] in Parkfield, publicly and with great embarrassment," said Lucy Jones, a seismologist who heads the survey's Pasadena office. She was not involved in the newly reported research.

Jones is among those who think that earthquakes occur randomly and that past activity is not a significant factor.

"We know it's not perfectly periodic," said Ned Field, another seismologist at the survey's Pasadena office, who is leading a project to produce more precise hazard maps. "The question is whether it's periodic at all."

If quakes are random, Field said, they may be triggered by small, unpredictable movements on faults that generate larger quakes, or by movement on neighboring faults.

Time elapsed since past quakes is not factored into the state's earthquake hazard maps, Field said. The official forecast is for an 85% chance of a magnitude 7 or higher Southern California quake in the next 30 years on the San Andreas or one of the smaller faults.

The new findings may help refine estimates of the earthquake risk on segments of the San Andreas. But the results are not expected to dramatically alter the assessment of overall seismic risk in Southern California.

Still other explanations have been offered for why the strands of the San Andreas near Palm Springs and San Bernardino have not ruptured in 300 years.

The pressure that produced them may have changed, Jones said. Shifting tectonic plates under the Earth's crust may be transferring stress to the newer, 1-million-year-old San Jacinto fault, a San Andreas offshoot.

Although the 20-million-year-old San Andreas is the best known and most studied fault on Earth, it remains a mystery. That's because the written record of its earthquakes is just 200 years old and includes only a handful of magnitude 7 or larger quakes.

"We've only been here a few centuries. It's sort of like we just met the San Andreas and we're trying to figure out what our neighbor is going to do," said Lisa Grant, a paleoseismologist at UC Irvine who edited the edition of the journal containing the new research.

To test the idea that earthquakes occur regularly, scientists estimate that they need data on about 30 past quakes on the same stretch of the San Andreas, a record that would probably stretch back 4,000 years. They are nearly halfway there.

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