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Looking inside the San Andreas

A new earthquake study could provide the best idea yet of how the Big One will affect the Southland.

March 23, 2011|Thomas Curwen

THE SALTON SEA — Three days after the earthquake and tsunami devastated northeastern Japan, Gary Fuis walked across the San Andreas fault under a moonlit sky. The desert was quiet. A breeze fanned through the creosote. To the west, he could see the Salton Sea, and to the east, the headlamps of the night crew taking up their positions.

In a little more than an hour, they would start detonating their explosives, generating seismic waves that would be recorded by seismometers buried throughout these sandy hills and positioned on the floor of the Salton Sea.

A geophysicist with the United States Geological Survey, Fuis is overseeing an ambitious project to create an underground image of one of the most seismically active and geologically complex regions of the country, a triangle of land extending from Palm Springs to the Mexico border.

This work, he believes, will change current assumptions about the earthquakes that originate here, especially the Big One expected on the San Andreas fault. For nearly three weeks his teams have worked night and day to cover hundreds of miles and position thousands of instruments.

Fuis, 67, sat on the top of a ridge and took out his dinner, a ham and jalapeno sandwich. From here, he would be able to stay in touch with the crew by two-way radio and cellphone in case any problems or confusion arose during the night.

A voice broke over the radio.

"Train south."

A freight speeding along the shore of the lake would interfere with the readings from the detonations, and they'd have to wait until it passed.

Fuis looked south toward Bombay Beach, a community of small homes and double-wide trailers on the edge of the Salton Sea, where the San Andreas fault begins its jagged 800-mile course toward the Mendocino coast.

Three years ago, seismologists imagined the effect of a magnitude 7.8 earthquake with an epicenter less than a mile from where he sat. Their scenario had the full force of the temblor reaching the L.A. Basin in less than two minutes. The shaking would extend as far north as Ventura.

The released energy would be approximately 30 times less than the Japanese earthquake. Still, landslides, fires, collapsed buildings and roadways, severed communication lines, cracked runways, derailed trains, broken aqueducts and dams were projected, along with nearly 2,000 deaths, 50,000 injuries and $200 billion in damage.

The model was based on the last rupture of the San Andreas in this region, dated more than 300 years ago by recent geological studies. Because this stretch of the fault -- from Bombay Beach to the Cajon Pass -- has not moved since then, it is considered especially vulnerable to a major earthquake.

Fuis describes the fault with dispassionate conviction. It is "near failure," he says, though he believes the seismologists' predictions may not be accurate. Whether the destruction will be worse or not, he's not certain. He just knows that some conclusions have been drawn without enough information.

"Neither the shape of the San Andreas fault nor the sedimentary basins that the cities have been built upon are well enough understood to provide accurate calculations of the shaking," he said.

The chatter on his radio picked up.

He checked the time -- 21:59:07 -- less than a minute before the first blast.

Earlier that morning, just as the sun was rising, the day crew gathered at a warehouse in El Centro.

John Hole paced with clipboard and pen in hand. Hole, 48, is an associate professor of geophysics at Virginia Tech and is managing the study along with Fuis and Joann Stock, 51, professor of geology and geophysics at Caltech.

The Salton Seismic Imaging Project is funded with a $1.2-million grant from the National Science Foundation. Additional money comes from the U.S. Geological Survey. The first findings of the study will be released in September.

Similar research in the 1990s looked at the Los Angeles Basin and the San Andreas north of the Cajon Pass. The results showed a number of faults lying deep beneath Los Angeles that are capable of producing dangerous earthquakes like the one that caused the Northridge earthquake in 1994.

By detonating explosives and measuring the speed of the seismic waves as they move horizontally and vertically underground, seismologists can assemble images of the crust of the Earth, capturing structures like fault lines.

The project was three years in the planning, and the fieldwork got underway in late February. During public hearings, Fuis had fielded concerns from residents and local officials who worried that the explosions might set off earthquakes. Rock quarries, he told them, conduct similar blasts without any consequence.

Then less than 24 hours after the earthquake in Japan, the fears arose again. Residents worried about earthquakes, damaged buildings and tainted ground water. An aide to Rep. Mary Bono Mack, whose district includes the Coachella and Imperial valleys, called the U.S. Geological Survey for reassurance.

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