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Cluster of Small Temblors Makes Seismologists Edgy

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TIMES STAFF WRITER

A cluster of magnitude-3 and -4 earthquakes along the San Jacinto Fault, which runs from the San Bernardino area southeast to the Mexico border, has local seismologists wondering if a much larger quake may be approaching. But although the pattern is unusual, they add, they are not making outright predictions of a major jolt any time soon and see little cause for alarm.

“It was the topic at coffee this morning, but we haven’t had a conference call or a meeting about it,” said Lucile Jones of the U.S. Geological Survey.

In December, six small- to-moderate quakes shook the San Jacinto Fault or its tributary faults, including three over the last two days that were centered seven miles northwest of San Bernardino.

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A 4.3-magnitude quake at 1:41 a.m. Thursday jarred people awake over a 100-mile area from Los Angeles to Palm Springs. That temblor was preceded by a 3.2-magnitude foreshock Wednesday, and was followed by a 3.3-magnitude aftershock at 2 a.m. Thursday.

No injuries or damage were reported. Quakes of that intensity generally cause little more than cracks in walls or items knocked off shelves.

But combined with three Riverside County quakes--one a 4.0 near Anza on Dec. 2, a 3.4 near Hemet on Dec. 6 and a 3.4 south of Anza on Dec. 22--the series represented a doubling of normal activity of that intensity in a given month, Jones said.

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“Threes are so common that you can’t get excited about them, but the fours made us take notice,” said Kate Hutton, a Caltech seismologist.

“Generally, the small quakes are more symptomatic than they are tension-releasing, because they’re too small to let off a lot.”

The problem, she added, is that “we don’t know what they’re symptomatic of.”

A July, 1988, forecast by the U.S. Geological Survey showed a 50% chance of a magnitude-7 quake along the San Jacinto Fault by the year 2018. “That’s about the same size earthquake as San Francisco’s (7.1-magnitude) earthquake in October,” Jones said. “And the distance from San Bernardino to Los Angeles is about the same as from Santa Cruz to San Francisco.”

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“If you’re getting the fours more often, that would suggest you’re going to get a six or seven more often, too,” Jones said.

She pointed out, however, that scientists noticed a similar cluster of moderate quakes in early 1988 “and nothing happened.”

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