Seismologists drawing to San Andreas fault
"It's an amazing place," said Barrows, director of Southern California's Coachella Valley Preserve east of Palm Springs. But the 20,000-acre sanctuary owes its splendors to the San Andreas fault.
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.
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, on the eve of the 10th anniversary of the Northridge temblor, 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 California State University, 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 drilling 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
In the Coachella preserve's Thousand Palms Oasis, Barrows walks in the jungle shadows of 70-foot trees. Wooden planks cover the path, and 80-degree springs gurgle underfoot.
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 dunes.
Every one of the phenomena is a sign of looming calamity, said Sally McGill, a geologist at California State University, 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. "I do worry about it. This is a dangerous place."
The San Andreas last slipped in the region about 190 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 the cracks — give or take 50 years — that quakes opened to the sunlight and that flood sediments filled later.
The technique 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.
"Over the past 10 years, we've put a lot of effort into the L.A. Basin," McGill said. "Now it's time to put more attention on the San Andreas."
"The San Andreas is the only one capable of producing a magnitude-8 earthquake," said McGill. "I would expect a 7 here, or a 7.5. A 7.9 is possible."
A 7.9 would release as much as 100 times the energy of the 6.7 Northridge quake, which killed 57 people and caused $40 billion in property and economic losses.
That doesn't mean a 7.9 would be a 100 times more destructive. Magnitude is a deceptive measurement. It denotes the physical length of a quake: The more miles of splintering on the fault, the higher the magnitude. The Northridge quake struck along a 10-mile fault. The San Andreas can break for 200-plus miles, as it did in the 1906 quake, a 7.8 that devastated San Francisco.
There is "good news" about the fault, experts like to say. Most of the San Andreas is remote from major cities in Southern and Central California. And scientists generally believe the 1906 cataclysm vented enough stress to spare fault-straddling San Francisco another mammoth quake for decades to come.
Then again, the San Andreas runs through the fast-growing Inland Empire and Antelope Valley, which last felt the fault's fury in 1812 and 1857.
In a magnitude-8 temblor, streets could be rendered quicksand by liquefaction as the quake briefly scrambled saturated soils. Buildings could come crashing down.
In Parkfield, 350 scientists from the U.S. Geological Survey, Stanford University and other institutions have been gearing up to drive a seismometer-lined shaft into and across the San Andreas. A 1.4-mile pilot hole already has been sunk near the fault to test equipment.
Digging of the main "well" in Parkfield — the geologists are borrowing oil technology — will begin in the summer. The probe will target the "creeping section" of the San Andreas, which has delivered the most regular pattern of moderate quakes on the fault: six in the magnitude-6 range since 1857, the last in 1966.
That record of punctuality led seismic forecasters in the 1980s to predict another quake within several years. It has yet to strike, but scientists remain convinced that Parkfield has secrets to spill.
"We're going to be able to instrument a fault right where a quake is happening," said Andy Snyder, a U.S. Geological Survey geologist.
Nowhere else on the planet is a fault so intently watched.
"This is where you could capture an earthquake," Snyder said. He was standing in a weedy field beside the pilot hole's wellhead, a green hunk of metal that resembles a large fire hydrant. A nearby shed shelters banks of computers that clock every shudder in the hole.
In the 2.4-mile-deep shaft, the coin-size seismometers will track shaking and tilting while devices called transducers gauge fluid pressures. Geologists theorize that increases in day-to-day fault deformations and changes in subterranean fluids precede a quake.
Tom Taylor, a geophysics scholar from Duke University, talked more about the next step. "Right now it's a voodoo science," he said. "I want to find out if you can predict them."
So does Robert Uhrhammer, a researcher at the University of California, Berkeley's seismological laboratory. He is working on the Plate Boundary Observatory, part of the National Science Foundation's EarthScope program, which includes the Parkfield experiments.
"In the last 10 years, there have been considerable advances in paleoseismic studies and global position systems," he said. "Earthquake probabilities are much more formal now and much more reasonable."
Uhrhammer said the observatory would help scientists determine where the San Andreas might be lurching toward a 1906-strength quake.