Quake scientists focus on the southern San Andreas fault
Saturday, March 28, 2009
THERMAL, Calif. -- An arid expanse of California desert at the southern end of the notorious San Andreas fault is being wired with high-tech sensors that scientists hope will tell them when the state's sleeping giant could awaken.
The effort could not be more timely. A swarm of more than 250 earthquakes has jiggled the desert for a week where the first new seismic instruments were installed earlier this year.
"Even the smallest little bump shows up there," said Bill Curtis, demonstrating the sensitivity of the new monitors with a foot-stomp that created telltale squiggles on a connected laptop.
Curtis, a field technician with the U.S. Geological Survey, was satisfied that the newly installed instruments work.
Of the more than 300 known earthquake faults that crisscross California like a maze, the San Andreas is the premier fault line. The nearly 800-mile fault runs from a peninsula just north of San Francisco to the Salton Sea, 125 miles southeast of Los Angeles.
In the first large-scale monitoring upgrade to one of the Earth's most infamous cracks, scientists are showering unprecedented attention to the forgotten southernmost section, which has not ruptured in more than three centuries and is thought to be the most ripe to break.
Crews are hopscotching the southern San Andreas, burying motion sensors within three miles of the fault in hopes of catching the next "Big One."
"We're data poor here," said Ken Hudnut, a USGS geophysicist. "We don't have the past record to tell us what's going to happen."
The USGS currently maintains a sprawling network of 300 seismic stations across the state. They automatically estimate a temblor's size when the ground heaves and beam real-time data to a central hub in Pasadena, alerting that an earthquake has hit.
Since the southern San Andreas poses a significant danger to the roughly 20 million residents living near it, scientists decided to pepper the fault with 11 new seismic stations and update six old ones.
Each San Andreas station contains two sensors to measure speed and acceleration as well as a data-logger and a minicomputer that can process seismic signals without having to rely on the central hub.
The entire southern end upgrade will cost more than $500,000, which is financed by a USGS grant. The instruments cost $40,000 plus an extra $15,000 for solar panels, antennas, construction and labor.
On a recent afternoon, USGS seismologist Doug Given and a team of technicians tested seismic equipment near the fast-growing Coachella Valley, a desert playground for golfers and sun-worshippers. Surrounding them were the Mecca Hills, a maze of eroded canyons and badlands lifted up by the nonstop geologic bump and grind of the North American and Pacific plates.
The new sensor was installed near a dry creek bed on federal property next to a private mining company. The location had a line of sight to a communication tower, but it was remote enough that four-wheel drive was needed.
"It's kind of like parking a Cadillac Escalade out here and leaving it," said Given of the costly seismic equipment.
By clustering sensors along the southern edge, scientists hope to get faster readings of when quakes occur on the San Andreas and to better understand the science behind how faults break. Eventually, they hope the sensors could be incorporated into an early warning system in Southern California -- a goal that is still years away.
"What we're after is measuring the ground motion," Given said. "The closer you are to the source, the sooner you know an earthquake has begun."
Any time a small fault next to a bigger one breaks, it slightly increases the chance of a larger quake, but that risk rapidly decreases with time.
"That doesn't mean this section of the San Andreas is the next to go," said Hudnut. "The San Andreas likes to demonstrate how irregular its behavior can be."
The work recently paid off. Newly installed seismic stations along the eastern shores of the Salton Sea were among the first to pick up the recent swarm, which broke a fault perpendicular to the San Andreas. The sensors have been sending back valuable data that scientists are only beginning to analyze. Scientists noted the rate of quakes had declined Friday.
"We're obviously watching it because there is a possibility that something bigger can happen," said Susan Hough, scientist-in-charge of the USGS Pasadena office. "We're really talking about a chance, but a very small one."
California faces a 99.7 percent chance of experiencing a magnitude-6.7 quake or larger in the next 30 years, according to the USGS. A temblor that size would be on par with the 1994 Northridge disaster, which killed 72 people and caused $25 billion in damage.
The geological survey put the odds of a "Big One" -- a magnitude-7.5 or larger -- during the same period at 46 percent. Of all the faults in the state, the southern San Andreas is the most ready to go, scientists say.
On the Net:
U.S. Geological Survey: http://www.usgs.gov