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Chance of big San Andreas earthquake increased by Ridgecrest temblors, study suggests

Rong-Gong Lin II, Los Angeles Times on

Published in News & Features

The last time scientists in California warned about an increased risk of a big earthquake on the San Andreas fault, however, the initial statewide response was flatfooted.

In 2016, state officials didn't issue a statement of the increased threat of a big quake on the San Andreas fault until about 39 hours after the first concerning quake hit in the Salton Sea.

Even when state officials finally put out a statement, they inserted an error that inaccurately downplayed the increased seismic risk. It was corrected after an inquiry from the Los Angeles Times.

A hypothetical magnitude 7 earthquake along the San Francisco Bay Area's Hayward fault would cause severe, violent or extreme shaking along large swaths of the East Bay, North Bay and Silicon Valley, according to the U.S. Geological Survey. By contrast, the magnitude 6.9 Loma Prieta earthquake of 1989 caused only such shaking in the Santa Cruz Mountains, Watsonville and Gilroy.

The study published Monday is not the first time scientists have suggested the Ridgecrest earthquakes could be the first domino to fall that eventually leads Southern California's section of the San Andreas fault to rupture in a significant way for the first time since 1857, when a magnitude 7.8 earthquake ruptured 225 miles of fault between Monterey County and the Cajon Pass in San Bernardino County.

A year ago, the U.S. Geological Survey -- the nation's primary earthquake science agency -- calculated that there was an extremely remote chance the San Andreas could be triggered by the Ridgecrest quakes.

 

And a USC professor of earth sciences, James Dolan, articulated the same Ridgecrest-to-Garlock-to-San Andreas scenario in an interview with The Times last year.

The Garlock fault ruptures on average every 1,300 years, said Tim Dawson, senior engineering geologist with the California Geological Survey, but earthquakes can occur as often as every few hundred years or have a drought between large quakes of as long as 3,000 years. The last big earthquake on the Garlock fault happened about 500 years ago, Dawson said.

Big quakes on the southern San Andreas fault along the Grapevine section of Interstate 5 can happen on average every 100 years, although there's wide variation in how often they can happen; there has been a time when it was just 20 years between major quakes, and another when there was a gap of 200 years between huge quakes.

Though it's far from a sure bet the Garlock fault will rupture in our lifetime, the southern San Andreas is a likely candidate for such a big quake in our lifetime. "It's really the fastest moving fault in California," Dawson said of the San Andreas, meaning it accumulates strain far faster than other faults. "It's always going to play the most significant role in earthquake hazard in California."

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