Cluster of Small Temblors Makes Seismologists Edgy

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.

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.

“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.”

“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.”