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California gets more reminders of quake potential. Experts say temblors that hit the lower Imperial Valley and Mexicali, Mexico, don't mean more are likely to follow soon

By Scott ArmstrongStaff writer of The Christian Science Monitor / November 25, 1987



Los Angeles

The sequence of earthquakes that shook a rural section of southern California this week was considered geologically significant. But experts said the quakes did not necessarily herald an unusual period of seismic activity. Nevertheless, they provided an upsetting reminder to residents in the region that they cannot grow complacent about earthquakes.

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The latest sequence, which began Monday night and stretched into Tuesday morning, included two temblors that were larger than the 5.9-magnitude earthquake that struck the Los Angeles suburb of Whittier Oct. 1 and caused more than $213 million in damage.

One shock Monday night registered 6.0 on the Richter scale, while Tuesday's biggest shaker was estimated to range between 6.3 and 6.8. But because they were centered in the sparsely populated Imperial Valley, damage was relatively moderate.

Hardest hit appeared to be Mexicali, Mexico, just south of the border. At this writing, two people were reported to have been killed there Monday night in an auto accident said to have been earthquake related. There were some fires in Mexicali, and structural damage to a number of buildings.

In California, damage was limited to buckled roads, power outages, broken glass, and ruptured water mains. Some minor injuries were reported.

The quakes appeared to occur along a fault near the tiny desert community of Westmorland, near the southern tip of the Salton Sea. They were felt as far away as San Diego, 100 miles to the southwest, and Los Angeles, about 200 miles to the north.

Scientists say the recent spate of activity is not unusual for fault-veined California.

``We get a magnitude 6 or greater [earthquake] on average about once every two years,'' says Dr. Robert Uhrhammer, a seismologist at the University of California, Berkeley.

``But to have two sequences within a couple months is not all that unusual,'' Dr. Uhrhammer said.

The experts see little likelihood of another big quake happening in the area in the next few days, although aftershocks will continue to reverberate throughout the region.

The latest temblors were slightly different from the one that occurred on Oct. 1. That quake was caused by one side of the fault riding over the top of the other.

The movements this time involved what is termed a ``strike-slip'' fault, where one side of a vertical fault slides past the other.

The region is seismically active because it lies on the boundary between the North American and Pacific plates.

These plates are creeping past each other at about 3 to 5 centimeters a year.

In 1979, a 6.4-magnitude earthquake struck the area near El Centro. Damage in that quake was estimated at $30 million.

The quake was a reminder to area residents that the terra is not always firma here. Chris Tyner, a Massachusetts resident who is staying in San Diego, was sleeping when the Tuesday morning quake hit.

``It was pretty hefty,'' he says. ``It really rolled around for about 20 or 30 seconds. I watched my curtains go flying back and forth.''

In El Centro, closer to the center of the quake, plants bounced off desks and filing cabinets flew open at the Imperial County Sheriff's office. ``It was a shaker,'' says a sheriff's spokesman. ``It was bad.''

Another El Centro resident, Betty Germani, said Tuesday's temblor felt like an 11 on the 10-point Richter scale. ``A wave came out of the pool a yard high and hit the house,'' she said. ``The pool is now two feet down. There is not a dry spot in the backyard.''

The latest quakes come as residents of the suburban Los Angeles community of Whittier are still cleaning up after the Oct. 1 temblor.

Earlier this month, the California Legislature approved a $91 million package of emergency financial aid for the victims.

It includes $10,000 grants for homeowners, tax breaks, and low-interest loans to rehabilitate damaged dwellings.

Staff writer Peter N. Spotts contributed to this report.