‘Inactive’ Fault Blamed for 7.0 Earthquake

On Saturday, the Marine Corps suspended its combat exercises just long enough for the scientists to make a three-hour helicopter survey of the immediate quake zone, but the Marines refused to allow a plane carrying Jet Propulsion Laboratory earthquake experts to fly over the base.

Two Marine ordinance experts accompanied Hudnut’s USGS team to ensure that when they landed periodically to scramble on foot along the new rupture, they did not step on mines or unexploded shells.

Before being allowed to begin their survey, Hudnut, USGS research geologist Katherine Kendrick and Tom Rockwell, a San Diego State University earthquake expert, were briefed about the unusual man-made hazards in the quake zone, like materials that could look like rocks but blow off a hand.

Their flight--often skimming only a hundred yards above the rupture as it snaked across alluvial fans and then swooping up to several thousand feet to clear the nearby Bullion Mountains--was guided by base air traffic control.

“We had tremendous help from the Marines,” Hudnut said.

But they forgot the daunting survival warnings in the excitement of spotting the rupture from the air.

The 7.0 Hector Mine temblor Saturday ripped a gash in the Mojave Desert almost 25 miles long on an obscure fault that had been universally considered inactive, not even worth naming on state hazard maps.

On a wild helicopter ride across the rugged, bomb-blasted terrain of the Twentynine Palms Marine Base late Saturday, federal earthquake experts discovered that the quake exerted so much muscle that it displaced the desert floor on either side of the strike-slip fault up to 15 feet. The fourth largest earthquake in Southern California this century etched a spider’s web of new cracks that ran for miles through ancient lava flows, creekbeds and ridges.

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

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. 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 Sunday the surface rupture was beyond his “wildest expectations”--twice as extensive as he would have anticipated 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,” Hudnut said.

The fault--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 or near the metropolitan Los Angeles region has occurred along faults that were overlooked or thought to pose no danger.

The 1994 Northridge earthquake occurred on a hidden blind thrust fault that had not entered into official hazard estimates until it revealed itself in a brutal 6.7 shock that killed 57 people and caused $40 billion in damages.

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.

Younger Fault Than Most

Based on a preliminary analysis, the fault appears to be much younger than many of the more well-known 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. “But it occurred on a relatively young fault. This has got to be a very rare earthquake.”

Thomas Henyey, director of the Southern California Earthquake Center at USC, 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.

Effects of Landers Quake

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,” said Stein. “Seven years after Landers, another quake pops off and it pops off in a region brought closer to failure by the Landers quake.”

The Hector Mine quake was about three times smaller than the 7.3 Landers quake but three times more powerful than the 6.7 Northridge temblor.

Don Helmberger, director of the Caltech Seismological Laboratory, said that researchers were especially interested in studying the very slow, distinctive rolling ground motions the quake generated all over the Southwest and their effect on the physical integrity of buildings.

Meanwhile, Caltech seismologists said hundreds of aftershocks continued to rock the California desert Sunday. So far, there have been 25 aftershocks of magnitude 4.0 or more, including shakers of 5.8, 5.3 and 5.0 magnitude. Aftershocks may continue for a decade.

Caltech experts, however, said there were very few quakes of any size in areas--which had been the scene of two clusters of aftershocks on Saturday--near the San Andreas fault.

Certainly, field geologists in the pursuit of evidence of earthquakes are no strangers to desolate terrain or exotic locales.

But in some respects, the Marine base posed special challenges for those seeking to investigate the Hector Mine quake.

Seeing How Earth Acts

Rockwell said he first realized the displacement was bigger than anything he expected, when from the air he saw road tracks that suddenly jogged to the right as if the truck had been picked up and then set down in another spot to continue on its way.

“We were flying along, mapping the rupture and then we saw road tracks offset by a good 13 feet, maybe 15 or 16,” Rockwell said. “We were looking at the actual earthquake.”

The scientists saw multiple cracks across the rupture. They saw creekbeds that had moved as much as three feet. At one point the earth was mounded up as if some snuffling, prehistoric behemoth had been burrowing in the dirt.

They saw “sympathetic” cracking along the Bullion fault and no evidence of ruptures along the nearby Pisgah fault, which briefly was considered the most likely source for the earthquake.

At dawn this morning, before the Marine gunfire and bombing runs were scheduled to resume, an aircraft specially outfitted with wing cameras was expected to fly the length of the rupture to obtain continuous 3-D footage.

The Southern California Earthquake Center also is arranging for special satellite radar scans of the entire region as well. Four teams of experts are being organized to conduct additional satellite surveys and field mapping of the quake zone in the next few weeks.

“Our experiments are earthquakes,” said Henyey. “This is our opportunity to study how the earth behaves.”

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Too Much Stress

The Hector Mine quake struck in an area of pent-up seismic strain (shown in red) in which stresses were heightened after the 7.3 Landers earthquake in 1992. Fault lines with recent quakes are shown in white, epicenters are shown as stars. *

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Sources: Ross Stein et al, USGS Menlo Park

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High and Low Intensity

This Intensity map of the Hector Mine earthquake reflects more detailed, Southland-wide information from the Trinet network of seismic stations developed by Caltech, the U.S. Geological Survey and the California Division of Mines and Geology. Intensity maps such as this are available at www.trinet.org/shake.html.

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Sources: David J. Wald, Caltech and U.S. Geological Survey, California Division of Mines and Geology