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San Andreas fault capable of magnitude 8.1 earthquake over 340-mile swath of California, researchers say [Updated]

October 8, 2010 |  6:51 am

San andreas The "Big One" on the San Andreas fault just got a little bigger.

New research showing a section of the fault is long overdue for a major earthquake has some scientists saying that the fault is capable of a magnitude 8.1 earthquake that could run 340 miles from Monterey County to the Salton Sea. 

Whether such a quake would happen in our lifetime had been a subject of hot debate among scientists. That's because experts had believed that a major section of the southern San Andreas, which runs through the Carrizo Plain 100 miles northwest of Los Angeles, would remain dormant for at least another century.

But that rosy hypothesis seemed to be shattered by a recent report in the journal Geology, which said that even that section of the San Andreas is far overdue for the "Big One." [Updated, Oct. 9: The report, published in August, was written by Sinan Akciz and Lisa Grant Ludwig of UC Irvine, and J. Ramon Arrowsmith and Olaf Zielke of Arizona State University.]

Now, according to U.S. Geological Survey seismologist Lucy Jones, it is entirely possible that all 340 miles of the southern San Andreas could be ready to erupt at any time. Such a scenario would trigger a magnitude 8.1 earthquake, said Thomas Jordan, director of the Southern California Earthquake Center, a calculation with which Jones agreed.

"All of it has plenty enough stress for it to be ready to go," Jones said. "The biggest implication of [the report] is that it increases the likelihood that when we do have a big earthquake, it will grow into the 'wall-to-wall' rupture."

[Updated, Oct. 9: Such a temblor could cause much more damage because with a longer stretch of the fault rupturing, a larger area is exposed to the quake, and the shaking would last longer.]

The walls Jones is referring to are the boundaries of the southern San Andreas, which begins in the Salton Sea and ends in the town of Parkfield in Monterey County. Scientists consider the southern San Andreas fault as one segment generally because it behaves the same -- it rarely rumbles, but when awakened, the shaking can be devastating.

In contrast, the section of the San Andreas north of Parkfield up to Hollister in San Benito County behaves differently. That section constantly moves at a creep -- meaning stress is relieved regularly, so large quakes don't occur there.

Large quakes haven't occurred anywhere on the southern San Andreas for more than a century, making it a sleeping giant that has been building stress for so long it could snap at any moment.

"My concern is that we will get a series of large earthquakes along the San Andreas fault," Jordan said. The last "Big One" to rip through Southern California occurred in 1857, when an estimated magnitude-7.9-quake, ruptured 200 miles of fault between Monterey and San Bernardino counties. It wasn't a wall-to-wall quake: It stopped near the Cajon Pass, near the present-day 15 Freeway, probably because the fault south of it shook just a few decades earlier, in 1812, Jones said. Because the 1812 quake had relieved tectonic tension in that area, it effectively put a brake to the 1857 quake from moving further south.

But with the San Bernardino County section of the fault now having accumulated two centuries' worth of strain, there may not be any brakes now. "Can I imagine the 1857 earthquake happening again and stopping at the Cajon Pass? Probably not," Jones said. "Once you have a big slip, you're more likely to move along down the fault," Jones said. "If the rupture has been made ... that’s a lot of momentum that will keep the rupture moving down the fault."

The San Andreas has long been considered one of the most dangerous faults in Southern California because of its length. Not only do longer faults produce bigger quakes, they emit a type of shaking energy that can travel longer distances.

"So a much larger area is affected by a really large earthquake," Jones said.

In 2008, seismologists developed a scenario for a large earthquake on the San Andreas -- a magnitude 7.8 shaker that begins at the Salton Sea and barrels northwest along the fault toward San Bernardino and Los Angeles counties.

-- Rong-Gong Lin II

Photo: Quake researchers study a portion of the San Andreas fault. Ricardo DeAratanha / Los Angeles Times; Map: U.S. Geological Survey