Calif. Quake Moved Earth Up To 15 Feet

LOS ANGELES - The Hector Mine earthquake in Southern California on Saturday ripped a 25-mile gash in the Mojave Desert along an obscure fault that had been considered inactive, not even worth naming on state hazard maps.

On a wild helicopter ride across the rugged, bomb-blasted terrain of the Marine base at Twenty Nine Palms late Saturday, federal earthquake experts discovered that the earthquake - the fourth-largest in Southern California this century - exerted so much muscle that it displaced the desert floor on either side of the strike-slip fault up to 15 feet.

The magnitude of the quake was put today at 7.1. It had been listed as 7.0, but the reading was upgraded after a review of data, said Robert Tindol, spokesman for seismologists at the California Institute of Technology.

The temblor etched a spider's web of new cracks that ran for miles through ancient lava flows, creek beds and ridges.

The rupture appeared to be centered on the dry bed of Lavic Lake, earthquake experts said yesterday.

When it struck at 2:46 a.m. Saturday, it caught thousands of U.S. Marines in the middle of three weeks of live-fire combat maneuvers in this remote region of the Mojave Desert, about 120 miles northeast of Los Angeles. The quake tossed heavy artillery pieces like toys and bounced automatic weapons helter-skelter across the desert hardscrabble.

Kenneth Hudnut, a geophysicist at the U.S. Geological Survey who led the inspection team through the quake zone, said the surface rupture was beyond his "wildest expectations" - twice as extensive as he would have expected from a quake of this magnitude.

"Suddenly all at once we saw this big set of fractures running across the desert. All of us realized we were right on it and it was big," he said.

The source of Saturday's major quake - now unofficially christened the Lavic Lake fault - was a sobering discovery, several earthquake experts said.

It means that for the second time in less than a decade a major earthquake in Southern California has occurred along faults that were overlooked or thought to pose no danger.

The 1994 Northridge earthquake that measured 6.7 and killed 57 people occurred on a hidden fault that had not entered into official hazard estimates.

On Saturday, an officially inactive fault suddenly came to life.

"We have to consider faults to be a potential hazard even if it appears to us to be inactive," Hudnut said.

Based on a preliminary analysis, the fault appears to be much younger than many of the fractures that help the mammoth San Andreas fault vent the titanic tectonic stresses building up in Southern California. It showed almost no evidence of activity in the past several thousand years.

"Here we have a minor fault producing a major quake, which is disturbing," said USGS seismologist Ross Stein in Menlo Park.

Thomas Henyey, director of the Southern California Earthquake Center at the University of Southern California, said, "It is not a fault one would have suspected. This one at this magnitude in this area was a bit surprising," in part because it was in the same region rocked by the powerful Landers earthquake in 1992.

It has some scientists wondering about the pattern of major Southern California earthquakes in recent years. Some of the largest quakes appear to cluster together as if - across many miles and many years - they may trigger each other.

Like major quakes at Big Bear and Joshua Tree in 1992, the Hector Mine quake struck in a zone of elevated seismic stress caused by the 7.3 Landers earthquake. It is not considered an aftershock of the Landers quake, however, but a separate temblor that is triggering a cascade of aftershocks on its own.

"This triggering from one earthquake to the next is important in understanding earthquake behavior," Stein said. "Seven years after Landers, another quake pops off and it pops off in a region brought closer to failure by the Landers quake."

Meanwhile, California Institute of Technology seismologists said hundreds of aftershocks continued to shake the California desert. So far, there have been 25 aftershocks of magnitude 4.0 or more, including one of magnitude 5.8.

Information from The Associated Press is included in this report.