‘Near Miss’ Will Offer Insights

Foreshadowed by a trio of minor temblors, the 7.0 Hector Mine earthquake Saturday--three times stronger than the 1994 Northridge earthquake--is among the four strongest earthquakes to strike Southern California this century.

But unlike more powerful quakes that severely damaged Turkey and Taiwan in recent months, this desert temblor struck so far from densely settled urban areas that its most lasting impact will be from its scientific reverberations and the insights it may offer into the region’s abiding seismic hazards, experts said.

As quickly as millions of shaken residents reached for telephones to reassure worried relatives around the country--and swimming pools stopped sloshing--many geologists around Southern California reached for their field boots and topographical maps. They headed out the door before dawn to search for any surface scars left by the powerful quake.

As a damaging earthquake, it was a near miss.

A similar-sized quake on any one of half a dozen faults in the densely populated Los Angeles Basin could cause upward of $200 billion in damage and thousands of deaths, experts estimated. On the Hayward fault in the East Bay, such a powerful temblor could wreak similar havoc in Oakland and Berkeley.

Hitting about 120 miles east of Los Angeles at 2:46 a.m. Saturday, the earthquake originated in the vicinity of a little-known strike-slip fault called the Pisgah fault within the boundaries of the Marine base at Twentynine Palms.

The fault is in a highly active seismic area in the Mojave Desert, northeast of the San Andreas fault and not far from the fault that triggered the 7.3 Landers earthquake in 1992.

Rolling shock waves were triggered in an instant as fractured rocks about four miles underground slipped against each other explosively, earthquake experts said. It was considered a new event and not an aftershock of the Landers quake.

It is likely that more than one fault might have been involved in the earthquake, as was the case in the Landers quake, Caltech scientists said.

In the first hours after the jolt, earthquake experts and emergency response officials were briefly worried that it might herald a more serious quake on the huge San Andreas fault. But as two clusters of aftershocks near the San Andreas diminished, that concern--and the possibility of an official quake warning--faded quickly.

Within hours of the main quake, the desert barrens were rocked by three aftershocks of magnitude 5.0 or greater and at least 17 earthquakes of magnitude 4.0. Three or four more aftershocks of 5.0 or greater could occur in the next week.

“We have about a 5% chance of an even bigger earthquake in the next week, which is a 95% chance we won’t,” said Lucy Jones, chief seismologist at the U.S. Geological Survey’s office in Pasadena.

“This is an earthquake larger than Northridge, but the good news is almost nobody lives out there. This happened about as far as you can get from anywhere,” Jones said.

The maximum ground shaking probably exceeded 50% of the force of gravity--strong enough to derail a speeding train, crack highways and snap gas lines, water mains and electrical transmission wires in the immediate quake zone.

Earthquake expert Kate Hutton at Caltech said the quake was preceded by three foreshocks, all occurring exactly at the subsequent epicenter of the Hector Mine quake. A magnitude 3.8 quake occurred at 7:41 p.m. Friday, followed by a 2.5 foreshock at 10:07 p.m. and a magnitude 1.9 at 10:33 p.m.

Striking one day shy of a decade after the 1989 Loma Prieta earthquake, the Hector Mine quake is the most recent in a series of major quakes striking at or near inhabited areas around the world, but experts said it was a normal expression of the violent seismic tensions within the Earth.

“We seem to be seeing a global cluster of earthquakes,” said Caltech seismologist Egill Hauksson. “So far as we can tell at the moment, this is normal. All of these have occurred in or near urban areas. Most of the time, [large quakes] occur in places where people don’t live.”

No place in California is completely immune from the possibility of a serious earthquake.

Several researchers have suggested that an earthquake of magnitude 7.0 or greater should shake the metropolitan Los Angeles region every century or so. But no quake of that size has occurred in the city since it was founded in 1781.

On Thursday, federal officials reassessed the earthquake hazard facing the San Francisco Bay Area. There is a 70% probability that one or more damaging earthquakes of magnitude 6.7 or larger will strike the region in the next 30 years, they concluded.

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California Earthquakes

Notable Quakes on Land

Source: Earthshock, by Andrew Robinson, Thames & Hudson, c. 1993

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Strongest Quakes in Southern California This Century

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Name Magnitude Date Losses Tehachapi 7.7 July 21, 1952 12 dead/$50 million in damages Landers 7.3 June 28, 1992 1 dead/$91 million in damages El Centro 7.1 May 18, 1940 9 dead/$6 million in damages Hector Mine 7.0 Oct. 16, 1999 0 dead/damage total unknown

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Deadliest Quakes in Southern California This Century

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Name Magnitude Date Losses Long Beach 6.3 March 10. 1933 115 dead/$50 million in damages Sylmar 6.5 Feb. 9, 1971 58 dead/$511 million in damages Northridge 6.7 Jan. 17, 1994 57 dead/$48 billion in damages

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Source: Times Research