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

A new study suggests that last year’s Ridgecrest earthquakes increased the chance of a large earthquake on California’s San Andreas fault.

The study, published in the Bulletin of the Seismological Society of America on Monday, says there is now a 2.3% chance of an earthquake of magnitude 7.5 or greater in the next 12 months on a section of the 160-mile-long Garlock fault, which runs along the northern edge of the Mojave Desert.

That increased likelihood, in turn, would cause there to be a 1.15% chance of a large earthquake on the San Andreas fault in the next year.

Those odds may seem small. But they’re a substantial jump from what the chances were before last year’s Ridgecrest, Calif., earthquakes, whose epicenters were about 125 miles northeast of downtown L.A.

The new odds mean a large quake on the Garlock fault is now calculated to be 100 times more likely — rising from 0.023% in the next year to 2.3%.

And the chance of a large quake on the San Andreas has roughly tripled, from 0.35% in the next year to 1.15%, said Ross Stein, a coauthor of the study and the CEO of Temblor, a catastrophe modeling company in the Bay Area that has built a free earthquake hazards app for smartphones.

One plausible scenario involves the Ridgecrest, Calif., quakes triggering a large temblor on the Garlock fault, which then triggers a seismic event on the San Andreas. The chances of such an event happening are small. Another plausible scenario, not mapped, involves a rupture of faults southeast of the Ridgecrest quakes.

Seismologist Lucy Jones, who did not play a role in the report released Monday, called the study “elegant science” but added that its conclusions are not confirmed.

“It’s really interesting science, and I like the way they’ve been able to increase the complexity of how they do their modeling. That’s a real advance. But it’s not yet proven,” Jones said.

That said, Jones said that government officials in California should be prepared for a scenario in which an earthquake occurs that immediately raises the risk of a large quake on the San Andreas fault.

“If the Garlock happens, yes, we will be saying the San Andreas is at increased risk,” Jones said. “What do you do when there’s an earthquake that could be a foreshock to the San Andreas? What do you say? What do you do?”

The study is the latest suggestion of a plausible scenario in which last summer’s earthquakes in a remote part of California might have started a chain of events that could result in a devastating earthquake on the San Andreas fault that has not been seen in Southern California in 163 years.

At its closest, the San Andreas fault comes within 35 miles of downtown Los Angeles.

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