San Andreas Fault earthquake more likely after Ridgecrest quakes, scientists warn

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Aftershocks rattle Southern California following massive 6.4 earthquake near Ridgecrest

Seismologist warns the event isn't over and it's possible there could be even a stronger earthquake on the way; Jeff Paul reports.

Scientists have warned that last year’s Ridgecrest quakes in Southern California have increased the risk of a major San Andreas Fault earthquake.

The quakes, warn catastrophe modeling company Temblor, have placed stress on the Garlock Fault that links the Ridgecrest Faults with the San Andreas Fault. In a blog post, Temblor CEO Ross Stein and Shinji Toda of Tohoku University explained that if the Garlock Fault ruptured in a large shock, “it would likely trigger a San Andreas earthquake north of Los Angeles.”

A paper on the research has been published in the Bulletin of the Seismological Society of America.

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“In our study, we developed a new method to forecast future seismicity,” Stein and Toda wrote in the post. “These forecasts are not earthquake predictions, which have so far proven impossible. Rather, they are probabilistic, framed in the language of chance. We use past seismicity, the stress imparted by recent large and moderate earthquakes and the equations governing how fault friction varies in time and space (‘rate-state friction’) to estimate the likelihood of future earthquakes of different sizes.”

The 9.3-mile deep San Andreas Fault in California is one of the longest and most active in the world. This computer-generated image is enhanced to show its features - illustration. (SRTM, JPL, NASA)

The experts used machine learning technology in their analysis, which estimates a 2.3 percent chance of a magnitude-7.7 Garlock Fault rupture in the next year. “This is 100 times higher than its annual chances in the ‘UCERF3’ benchmark model for California, which is jointly issued by the [U.S. Geological Survey], the Southern California Earthquake Center, and the California Geological Survey,” Stein and Toda wrote.

The scientists calculate that if the Garlock Fault ruptured to within about 30 miles of its junction with the San Andreas Fault, “it would raise the probability of a San Andreas rupture extending to the southeast, on the so-called ‘Mojave section,’ by a factor of about 150.”

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“That translates into a 50/50 chance of a San Andreas Mojave section rupture (with a range, 25 percent-67 percent), either immediately following a Garlock quake, or after some delay,” they added.

Part of the visible section of the San Andreas Fault. (AP)

Set against this backdrop, they estimate that the net chance of a large San Andreas earthquake in the next 12 months is 1.15 percent. “While small, this probability is 3.5-5.0 times higher than the annual chance of a large San Andreas earthquake in the ‘UCERF3’ benchmark model for California,” they added.

Given the small likelihood of a large San Andreas earthquake in the next year, the scientists say no one should panic.

“But, as we have seen during the COVID-19 pandemic, low probability chain-reaction events do occur in nature, and when they do, their consequences can be great,” they explained. “It is the responsibility of scientists to assess the likelihood of such events, and then to bring them to the attention of the public and decision-makers.”

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Seismologist Lucy Jones, who was not involved in the study, described the analysis as “elegant” in a tweet, but added that it “assumes a reason for quake triggering that is not consensus.”

“The possibility of a San Andreas quake AT ANY TIME should already be part of your planning or you shouldn't be in California,” she wrote, in another tweet.

Follow James Rogers on Twitter @jamesjrogers