San Andreas a fault with mystique

27 June 2005 | 16:08 Code : 5279 Geoscience events
The Golden Gate Bridge. The Hollywood sign. The San Andreas Fault. Maybe it's not the geologic feature a state would embrace as a cultural icon ...
The Golden Gate Bridge. The Hollywood sign. The San Andreas Fault.

Maybe it's not the geologic feature a state would embrace as a cultural icon, but the 800-mile- long boundary where the Pacific tectonic plate and the North American plate grind together is easily the most famous earthquake fault in the world.

"It's the longest. It's the fastest moving. It's the plate boundary. It created the biggest (California) earthquake,' said Lucy Jones, a geologist with Caltech in Pasadena. "All of that plays into its mystique.'

In the relatively short American history of California, it has produced two of California's three great earthquakes.

In 1857, near Fort Tejon in the Tehachapi Mountains, not far from Bakersfield, the fault tore off an estimated magnitude 8- plus temblor.

And in the most famous or infamous earthquake in the United States, the San Andreas ruptured in San Francisco in 1906, triggering a conflagration that torched most of the buildings that were still standing after the 7.8 quake.

The 4.9-magnitude quake centered in Yucaipa on June 16 was along the fractured fault zone of the San Andreas.

In Southern California, the San Andreas may not be the one that generates the most death and destruction, even though it is still the only fault capable of causing a magnitude 8.

The San Andreas is a strike-slip fault, meaning the two plates grind horizontally past each other.

The faults that can generate earthquakes greater than 9, such as those in Alaska and Chile, and California's North Coast, are in subduction zones where one plate is riding over another.

Smaller thrust faults also can be dangerous.

The Southern California Earthquake Center in May released a report estimating that the Puente Hills fault beneath Los Angeles discovered in 1999 could kill 3,000 to 18,000 people and cause $250 billion in damage if an earthquake of more than magnitude 7 hit there.

That's because the fault is under a heavily populated, industrialized area. Damage would certainly spread to Orange and San Bernardino counties as well, researchers said.

In the Inland Empire, San Bernardino sits amid its own scary triangle of death. The San Andreas runs out of the Cajon Pass and along the foothills a few hundred yards above Cal State San Bernardino as it heads east through rapidly growing Highland and the San Gorgonio Pass.

Indeed, the San Andreas is why the San Bernardino Mountains are there. In the city itself, the San Andreas' biggest tributary, the San Jacinto fault, traces through the city, through the middle of San Bernardino Valley College and under the Guthrie Interchange where Interstates 10 and 215 intersect.

Using disaster-preparedness software that could give Stephen King shudders, Miles Wagner, a staff analyst for the county Office of Emergency Services, ran a credible scenario.

After the computer clicked and whirred for many minutes, it spat out the possible consequences of a magnitude 7.0 earthquake on the San Jacinto fault near the Guthrie Interchange.

The program estimated up to 1,530 buildings could be destroyed and 38,332 would suffer at least moderate damage. Nearly 10,000 would be without water for a stretch of days.

And if it happened at 2 p.m. on a weekday, it predicts 594 people could be killed, with more than 8,000 injured.

"It gets your heart rate up,' said Denise Benson, manager of the Office of Emergency Services after she looked at the estimates.

And if the San Andreas rumbled to a magnitude 8, that would mean the ground would shake 10 times worse than a magnitude 7.

Research is nonstop on the fault.

On Friday morning, geology professor Sally McGill, at Cal State San Bernardino, sent 45 geology students and others to set up global positioning system receivers at predetermined points to measure how much the two continent-size plates on either side of the fault are moving.

Standing next to a yard a few feet from a parked freight train on Seventh Street in San Bernardino, science teacher Patricia Pitts and geology student Stephanie Montgomery carefully lined up the antenna made of concentric brass rings above a 3-inch-wide brass disk implanted by the U.S. Geological Survey.

"We'll take data to see how much strain is building up on the plate,' said Pitts, who teaches at East Valley High School in Redlands.

Efforts are under way to plant permanent GPS monitors along fault boundaries to measure the movement full time.

And yes, though geologists like to avoid the term "overdue,' the last big quake along the southern section of the fault is thought to have been in 1812, centered near Wrightwood.

That's a little past the estimated cycle of big quakes, McGill said, meaning the Big One could happen any time.

But most folks take the risk in stride, even if there is good reason to be prepared, and no small reason to be afraid.

Last week, as he walked along the greenbelt concealing the San Jacinto Fault in a south San Bernardino neighborhood, 25-year-old Anthony Hughes said the Yucaipa quake was only the second one he has experienced since arriving in California from the Midwest a year-and-a-half ago.

tags: QAZVIN

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