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'Pregnant' San Andreas Could Be Ready to Deliver

Other faults have set off recent large quakes, but Coachella Valley is ripe for disaster, experts say.

December 30, 2003|Paul Pringle | Times Staff Writer

Scary beauty surrounds Cameron Barrows. He works in lush groves of fan palms that erupt like mirages from moonscape terrain. Hot springs bubble beneath them. Sand dunes drift nearby.

"It's an amazing place," said Barrows, director of the Coachella Valley Preserve east of Palm Springs. The 20,000-acre sanctuary owes its splendors to the San Andreas fault, the frightening part of the bargain.

Many scientists say the Coachella Valley is where the 750-mile San Andreas seems most prone for an epic earthquake, a monster that would be enormously more powerful than the recent temblors in San Simeon, Calif., and Bam, Iran.

"There's not a lot we can do about that," Barrows said with a resigned smile. He was outside the preserve's visitors center, a 1930s log cabin that sits directly above the fault. "I'm not a worrying type person."

As the possible generator of the feared Big One, the San Andreas once dominated the quake worries of Californians. But that was before "subsidiary faults" in locales such as Loma Prieta and, especially, Northridge reordered popular anxieties. They flattened buildings and buckled interstates while the San Andreas remained relatively quiet, as it has since the great San Francisco quake of 1906.

Now, with the 10th anniversary of the Northridge temblor approaching, and after much study of those second-tier faults, scientists again are highlighting the San Andreas as the rupture without rival -- a slumbering beast napping on borrowed time.

"The primary fault in California -- the big dog -- is the San Andreas, and it's important for people to remember that," said Doug Yule, a geologist at Cal State Northridge, which was badly damaged in the Jan. 17, 1994, disaster. "The San Andreas will produce the largest earthquakes."

Yule and his colleagues have dug trenches along the southern section of the fault to carbon-date its buried fissures in hopes of determining just how "pregnant" it is. Their best guess: The San Andreas, from the Salton Sea to San Bernardino, is at term.

Two hundred fifty miles to the north -- and 35 miles from the epicenter of the magnitude 6.5 San Simeon quake Dec. 22 -- investigators in Parkfield are preparing to drill a $20.5-million hole into the San Andreas.

The idea behind the 2.4-mile-deep probe, the first of its kind, is to capture a live quake on a battery of monitors -- and perhaps advance the balky business of quake predictions.

The San Andreas also is a principal subject of the new Plate Boundary Observatory, a $100-million array of global positioning stations, strain meters and ground-motion detectors.

The unfolding project will chart inching shifts in the landscape that result from the grinding collision of the Pacific and North American tectonic plates, the crash that created the fault.

All those efforts have returned the San Andreas to the pages of science journals, quake probability reports, even travel publications -- as a foreboding curiosity that helped shape California in more ways than one.

"The tourists play golf, then they come out here to see the San Andreas," Barrows said.

He was walking through the preserve's Thousand Palms Oasis, in the jungle shadows of 70-foot trees. Wooden planks cover the path and 80-degree springs gurgle underfoot. The water nurtures the fans, California's only native palm.

The springs are propelled upward by the San Andreas, which is otherwise undiscernible to the untrained eye.

"People are always disappointed it isn't this huge chasm in the ground," Barrows said.

Instead, the San Andreas reveals itself in geological magic tricks: gullies that turn gravity-defying corners, and abrupt changes in the desert floor, with uplifted bedrock yielding to gravels that are blown into Arabian-style dunes.

Every one of the phenomena is a sign of looming calamity, said Sally McGill, a geologist at Cal State San Bernardino. Her classrooms are within a mile of the San Andreas.

"This is as close at it gets to a heavily populated area in Southern California," McGill said.

She was setting out on a short hike to the fault, whose presence is marked by an unlikely picket of trees thriving along arid foothills. "I do worry about it. This is a dangerous place."

The San Andreas last slipped in the region 191 years ago. That is 40 years beyond the average interval for the southern segment, based on estimates that stretch back 12 centuries.

Geologists arrived at the calculations through paleoseismology, a fairly new technique that dates prehistoric quakes. Scientists dig into the fault to look for layers of peat and sand.

The strata time-stamp cracks -- give or take 50 years -- that quakes opened to the sunlight and that flood sediments filled later.

It is one of the disciplines that has made strides since Northridge, although the fault responsible for that quake resists paleo-detective work because it never broke the surface. The San Andreas is a proven crust-buster.

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