Warning Downgraded : Earthquake Experts Say Chance of ‘Big One’ Less

Earthquake experts had good news Thursday for San Diegans who have been holding their breath this week anxiously anticipating The Big One. The chances of a powerful quake hitting the city have lessened since the first temblors were felt Sunday.

Even though another minor quake rattled San Diego Thursday morning--the 11th in four days --seismologists at California Institute of Technology in Pasadena downgraded their earlier warnings that the area stood a 5% chance of a major earthquake through Saturday.

All 11 quakes have been of Richter magnitude 2.5 or greater. Approximately 20 aftershocks of lesser power also have been recorded by seismic sensors.

“The danger that we talked about initially was that this swarm of quakes could be foreshocks to a larger quake because about one in 20 swarms turn out to be foreshocks,” said Kate Hutton, a Caltech seismologist. “But the crucial time period appears to be ending, so in all probability they were not foreshocks.”

Thursday’s jolt, recorded at 4:56 a.m., was centered about four miles southeast of downtown San Diego and registered 2.9 on the Richter scale, Hutton said.

There were no reports of damage as a result of Thursday’s temblor, nor from any of the others that preceded it this week.

Also Thursday, Caltech announced that there were six tremors in Monday’s initial quake series. Previously, only three had been reported.

Seismologists also revised the magnitude of the quakes in Monday’s series. The largest actually measured 4.2--up from 4.0--while two temblors that were reported to be 3.9 each in magnitude were revised to 4.0. Three milder jolts ranging from 2.5 to 3.3 also were recorded.

On the same night, another temblor, this one measuring 3.9, rattled the Barstow area east of Los Angeles, Hutton said. That quake caused no damage and was unrelated to San Diego’s seismic activity.

Caltech seismologists based their initial warnings of a slightly increased earthquake possibility in San Diego on a U.S. Geological Survey study of Southern California’s earthquake history. The study showed a 3% chance that a 4.0 quake will be followed within five days by a large quake measuring 5.0 or more.

A quake of Richter magnitude 6 would be roughly equivalent to the 1971 San Fernando earthquake that killed 65 people and caused more than $500 million in property damage.

Because of Monday’s three temblors in the 4.0 range, seismologists estimated a 5% chance that a larger quake would occur in San Diego by Saturday evening.

“But the risk diminishes with time, and the highest risk was on Monday and Tuesday,” Hutton said. “I imagine it (the earthquake swarm) will start dying off now.”

The exact location of the fault that caused this week’s temblors has not been determined.

Marian Wright, operations officer for the San Diego County Office of Disaster Preparedness, said that her office has received dozens of calls since Monday from area residents concerned about what to do in case of an earthquake. The office installed a tape-recorded message on three telephone lines to handle the calls that streamed in.