5.2 Aftershock Jolts Big Bear

Another moderate aftershock of the Landers earthquake struck the Big Bear area Thursday night, and a Caltech scientist said it was on a fault that had not been active since before the magnitude 7.5 temblor on June 28.

The magnitude 5.2 earthquake occurred at 6:08 p.m. and was felt in downtown Los Angeles as well as the San Fernando Valley, Hollywood and cities in Orange, Riverside, San Bernardino and San Diego counties. It was centered three miles north of the epicenter of last Friday’s 5.4 aftershock, about six miles north-northwest of Big Bear City.

At 9:25 p.m., Caltech and the San Bernardino County sheriff’s station in Big Bear reported another sizable aftershock--magnitude 4.5--and said there had been numerous smaller jolts.

No damage or injuries were reported by Big Bear authorities, although some residents said the 5.2 quake lasted longer and caused more shaking than last week’s temblor.

“It was a good one, more rocky in motion than what I remembered from Friday,” said Carole Koenig, operations director of the local Chamber of Commerce. “They’re not pleasant to go through, but there’s no damage typically, and we’re OK. We don’t want people to be afraid to come to Big Bear because of them.”

The aftershock in the North Frontal Thrust Zone of the San Bernardino Mountains had been seismically quiet in recent years.

It is not unusual in quake sequences such as Landers for stress to be transferred to nearby faults. This phenomenon has happened many times since June 28, and related quakes have occurred as far away as Mt. Shasta in Northern California.

Egill Hauksson, a Caltech seismologist, said the 5.2 jolt was different than last Friday’s because it was a vertical thrust quake at a very shallow depth, only about a mile under the surface, and it had “scratched the surface of a major new fault zone.” In a vertical thrust quake, the motion comes from below rather than sliding horizontally, as did last Friday’s temblor.

“There could be further activity,” Hauksson said, “but there’s no way of telling if that’s more magnitude 3 earthquakes or something else. We’ve seen quite a bit of activity in the general area since Friday, and we’re watching it carefully.”

Within three hours of Thursday night’s aftershock, there were numerous secondary aftershocks, including six over magnitude 3.0 and 33 others between 2.0 and 3.0. Then the 4.5 shock was felt.

A U.S. Geological Survey scientist, Ken Hudnut, said seismologists were intrigued by Thursday’s moderate aftershocks because they occurred in a region of the San Bernardino Mountains that was moderately active in the mid-1970s.

The latest seismic activity was away from the San Bernardino Mountains segment of the San Andreas Fault, pinpointed by a scientific panel Monday as a possible site for a major earthquake within the next five years.

The same scientific panel said there was an 87% probability that a magnitude 5 aftershock would occur by next September.

When such a quake hit Friday, it then said there was a 67% likelihood another one would occur in the next year. It took just six days.