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.
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.
"Now, you can think of the Ridgecrest earthquake as being so far from Greater Los Angeles ... that it is nearly harmless," said Stein, an earthquake scientist emeritus of the U.S. Geological Survey and adjunct professor of geophysics at Stanford University.
"But the problem is that ... the Ridgecrest earthquake brought the Garlock fault closer to rupture. If that fault ruptures — and it gets within about 25 miles of the San Andreas — then there's a high likelihood, maybe a 50/50 shot, that it would immediately rupture on the San Andreas," Stein said. Stein's coauthor on the study is Shinji Toda, of Tohoku University in Japan.
If the Garlock fault did rupture close to the San Andreas fault — but the San Andreas did not immediately rupture — Los Angeles would face the prospect of having a metaphorical sword of Damocles hanging over its neck, Stein said, with the prospect of L.A. facing a larger risk of a San Andreas quake within a matter of months, or perhaps decades.
"In a way, if the fault ruptures all at once, life is simpler. It's done," Stein said. "But if it doesn't — if it hangs, and plenty of faults do hang — that would put the city in a really difficult ... position."
A hypothetical magnitude 7.8 quake on the San Andreas could cause more than 1,800 deaths, injure 5,000, displace some 500,000 to 1 million people from their homes and hobble the region economically for a generation. A quake of that magnitude produces 45 times more energy than the 1994 magnitude 6.7 Northridge quake.
Out of the many faults in California, the San Andreas is singularly poised to be the one that unleashes a megaquake in our lifetime because it is the main tectonic plate boundary between the Pacific and North American plates, and because of how fast the fault accumulates seismic strain.
Another troubling scenario Jones has mentioned before was a hypothetical magnitude 6 earthquake at the Cajon Pass north of San Bernardino. It's a particularly vexing scenario because such a quake could trigger large quakes on three major faults: the San Andreas, the San Jacinto and the Cucamonga.
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.
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