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New Era For San Andreas Fault?

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The notorious San Andreas fault may be on the verge of producing a flurry of earthquakes that could rattle Southern California with a strong temblor every few decades or less, a geologist said Wednesday.

A detailed analysis of two periods of past quake activity on a section of the fault suggests a drawn-out period of little seismic activity may be coming to an end, said University of Oregon geologist Ray Weldon.

"Possibly we are at the point of switching from a period of time with a relative paucity of large and frequent earthquakes," Weldon said.

Weldon cautioned the switch could be decades away and that "flurry" is a relative term, since a cluster of quakes can strike over periods lasting 200 to 300 years. However, intervals as brief as 10 years have separated individual quakes in past clusters, he added.

Details were presented Wednesday at the annual meeting of the Seismological Society of America.

Digging into the San Andreas at a location called Wrightwood about 60 miles northeast of downtown Los Angeles, Weldon and his colleagues found evidence of 30 quakes of magnitudes between 7.5 and 8.0 that had struck between 3000 and 1200 B.C. and A.D. 500 and the present.

An earthquake on the southern San Andreas of magnitude-7.5 or larger could kill thousands of people in the greater Los Angeles area and cause damage estimated in the tens of billions of dollars, experts have said.

On average, quakes of that size have struck Wrightwood at roughly 100-year intervals, although the lulls between temblors lasted as little as 10 years and as long as 224 years.

When viewed in series, the quakes appear to have struck in flurries, followed by periods of relative quiet.

The southern portion of the fault last ruptured in 1857. Current levels of strain measured on the San Andreas are at their highest in 1,500 years and suggest the fault is due to begin releasing that pent-up energy as a large earthquake or quakes, Weldon said.

Other seismologists said that the apparent clustering of quakes could be random and warned against inferring they consistently strike in recognizable clusters.

"It's human to look at these things and see patterns," said Geological Survey of Canada seismologist John Adams, who was not connected with the research.

Further excavations and dating of past quakes should expand the chronology of past activity on the 800-mile San Andreas.

Scientists plan a wider campaign of excavations up and down the fault that should broaden their understanding of its past history, said Lucy Jones, of the U.S. Geological Survey.

Seismologists date past quakes by first trenching across a fault and then pinpointing locations in the exposed layers of earth where major earthquakes had ruptured the surface in the past. At the swampy Wrightwood site, the age of each of those ruptures can be determined by carbon dating the peat layers that rapidly built up after each quake, covering the fractures.

By Andrew Bridges