5.4 Aftershock Near Big Bear Jars Southland

A magnitude 5.4 aftershock of last June’s Landers-Big Bear earthquakes shook a broad swath of Southern California Friday, causing a minor rockslide on a mountain highway and scattered damage to homes near the epicenter in the Big Bear Lake area.

Hitting just after 8 a.m. as thousands of early-season skiers prepared to take to the slopes on man-made snow in the resort’s first big winter weekend, the temblor spurred local authorities to issue statements that tourists had nothing to fear.

No injuries resulted from the strong aftershock, centered on an unnamed fault six miles northeast of Big Bear Lake.

Two chimneys toppled in Big Bear, a few new cracks appeared in buildings and one lane of Highway 18 between Lucerne Valley and Big Bear was temporarily covered by the rockslide, San Bernardino County sheriff’s dispatchers said.

At Bear Mountain Ski Resort, a spokesman said that the ski lifts were shut down for about 10 minutes to check for damage, but because of the early hour few skiers were riding them.

“It didn’t affect our business at all,” said Brad Wilson, the resort’s director of marketing. But he added that he was a little worried about the rest of the weekend because “TV was hyping this up as something spectacular.”

At the local Chamber of Commerce, operations director Carole Koenig said, “People are talking about pictures coming off the wall and dishware rattling, but the town is full.

“We’ve been told by Caltech that we can expect these things for two years, and this one just happened to take place on a major weekend,” she said.

At the Big Bear Inn, manager Sheila Moore said, “Our biggest excitement was in the bathrooms where the individual amenities (shampoo and hand lotion containers) rolled around.

“Most of our guests were keeping calm and accepting that these aftershocks will happen,” she said.

Though the earth shook noticeably near the epicenter, the quake was felt only as a gentle swaying in the Los Angeles and Orange County metropolitan areas and was not felt at all by many urban residents. High-rise buildings swayed slightly as far south as San Diego.

Although the aftershock was the most severe since July, it came as no surprise to scientists organized into a task force by the U.S. Geological Survey and the state Office of Emergency Services to assess the probabilities of future seismic events in Southern California.

Recently, the scientists estimated an 85% likelihood of a Landers-Big Bear aftershock of magnitude 5 or greater between Sept. 1, 1992, and Sept. 1, 1993. Friday’s aftershock, coming less than three months into that period, confirmed their prediction.

The scientists also said they believe that other strong aftershocks, possibly greater than magnitude 6, could occur at lesser probabilities during the next three years.

Lucile M. Jones of the Geological Survey said that Friday’s aftershock took place on a 2 1/2-mile-long segment of an unnamed fault that lies perpendicular to the larger Helendale Fault running northwest-southeast through the Big Bear area.

The fault that slid horizontally Friday is parallel to one about 10 miles away that caused the magnitude 6.6 Big Bear earthquake last June 28, three hours after the magnitude 7.5 Landers temblor.

The fault that moved Friday was not involved in the quakes on June 28, Jones said. But the scientists were not surprised by the temblor because stress from large quakes often transfers to nearby faults.

Just 10 days ago, seven earthquake scientists, in separate writings in Nature and Science magazines, reported that stress buildups from the Landers earthquake affected many areas of Southern California, including the San Bernardino and Coachella Valley segments of the mighty San Andreas Fault.

Friday’s aftershock occurred in an area north of Big Bear Lake that the scientists had also calculated to be more highly stressed.

Within six hours after Friday’s quake, more than a dozen smaller aftershocks of magnitude 3.0 or greater occurred in the region near the epicenter, and there were about 100 shocks greater than magnitude 2.

A temblor between magnitude 2 and 3 is usually scarcely felt, if at all.

The Landers-Big Bear earthquake sequence began on April 22 when a magnitude 6.1 temblor shook the High Desert near Joshua Tree. Since then, there have been close to 40,000 related quakes, and scientists say some of them have occurred hundreds of miles to the north, as far as Lassen Peak and Mt. Shasta, and possibly east into Nevada and Utah.

In particular, scientists are puzzled by a cluster of more than 500 quakes ranging up to magnitude 4.7 in a small area six to 10 miles northeast of Barstow. A magnitude 3.9 temblor was felt there Thursday night.

Altogether, the entire sequence has injured more than 400 people, caused nearly $100 million in damage and killed one person, a small boy crushed by a falling chimney on June 28.

Big Bear Aftershock

Friday’s magnitude 5.4 aftershock of the Landers-Big Bear quake sequence occurred at 8 a.m. on an unnamed fault perpendicular to the Helendale Fault, six miles northeast of Big Bear Lake. Although the fault was not directly involved in the June 28 mainshocks of magnitude 7.5 and 6.6, it is common for stress from such large temblors to be transferred to nearby faults, resulting in aftershocks on them.