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Major Earthquake Due to Hit Southern California, Study Says

John Roach
for National Geographic News
June 21, 2006
 
Get ready for the Big One.

About 300 years of pent-up stress in southern California is sufficient to trigger a catastrophic earthquake on the San Andreas Fault system, according to a new study.

The San Andreas Fault marks the boundary between the North American and Pacific tectonic plates. Tectonic plates are pieces of the Earth's outer crust that jostle about like constantly moving pieces of a jigsaw puzzle.

The fault is notorious for major earthquakes, including the 1906 earthquake that reduced the San Francisco Bay Area to piles of smoldering rubble.

(See photos of the 1906 San Francisco earthquake.)

But the 100-mile (160-kilometer) southern section of the fault, which runs south from San Bernardino to the east of Los Angeles and San Diego, has remained eerily quiet for nearly three centuries.

(See California map.)

Now, scientists believe, the fault is ready to rumble.

"It is fully charged for the next big event," said Yuri Fialko, a geophysicist at the Scripps Institution of Oceanography in La Jolla, California.

"When the event will occur, we cannot tell," he continued.

"It could be tomorrow or 20 years from now, but it appears unlikely the fault can take another few hundred years of slow strain accumulation."

Fialko reached this conclusion after studying the fault system with radar-equipped satellites and global positioning systems (GPS).

He reports the finding in tomorrow's issue of the journal Nature.

Mary Lou Zoback is a research geologist with the U.S. Geological Survey in Menlo Park, California.

She said Fialko "really exploits the true power" of modern satellite radar and GPS equipment to reach a conclusion with a familiar message.

"The conclusion … seems to concur with previous interpretations that the southern San Andreas Fault is in a late stage of its earthquake cycle—meaning it's likely to go," she said.

San Andreas Fault

Fialko used a technique called interferometric aperture radar (InSAR), a set of satellite-based radar systems that are able to detect 1-millimeter (0.04-inch) displacements in the Earth's crust.

The satellite data, combined with ground-based GPS measurements, allowed Fialko to calculate how much the North American and Pacific plates slip past each other on the San Andreas Fault—about 1 inch (25 millimeters) a year.

But the plates are locked together along the southern section of the fault. As a result, the plate motion builds up underneath the crust as stress.

(See an interactive feature on how earthquakes form.)

Over the past 300 years, energy equivalent to 20 to 26 feet (6 to 8 meters) of slip has accumulated below the locked part of the southern San Andreas Fault, according to Fialko.

"This is an indication the fault is storing significant elastic strain, and the amount of strain is equivalent to a major earthquake," he said.

According to Zoback, the amount of strain Fialko's calculations show is at the high end of estimates obtained from other research using different methods.

"That's like being ten months pregnant—past all reasonable estimates of when it should go," she said.

Warning Signs?

Zoback adds that an increase in earthquake activity in southern California over the past several decades may indicate the main fault is ready to rupture.

A similar increase in earthquake activity occurred on the northern section of the fault in the 70 years prior to the 1906 San Francisco earthquake, she notes.

"We may be seeing something similar," she said.

Moderate temblors—including 1994's magnitude 6.7 Northridge earthquake in Los Angeles—may be a sign that tectonic stress in the region is reaching a breaking point, according to Zoback.

And if the Big One hits, what are the consequences?

Unlike the northern section of the San Andreas Fault, which slices through the middle of the heavily populated San Francisco Bay Area, the southern section of the fault passes through mostly uninhabited desert, Zoback says.

However, computer simulations show that a southern California earthquake that ruptures toward the north could be devastating. The energy from an earthquake is focused in the direction of the rupture, Fialko explains.

For an earthquake of magnitude 7.5 to 8 that ruptures east of San Diego toward San Bernardino, the "shaking in Los Angeles would be extremely strong and extensive," he said.

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