2019 Jolts May Have Boosted Risk of ‘Big One’ on San Andreas Fault

Several fault lines in Southern California form a “Z” pattern, where seismicity could trigger earthquakes in other locations. (Graphic courtesy of Ross Stein, Ph.D. / Temblor)

(CN) — The 2019 earthquakes in the small desert town of Ridgecrest, California, may have been the first domino to fall ahead of a devastating earthquake on the San Andreas Fault near Los Angeles and other metropolitan hubs in Southern California, according to a study released Monday.

Published in the Bulletin of the Seismological Society of America, the study finds the likelihood of an earthquake on the 800-mile San Andreas Fault within the next year tripled from 0.35% to 1.15%.

The likelihood of an earthquake of magnitude 7.5 or higher in the next year on the 160-mile Garlock fault, which runs along the Mojave Desert where the 7.1 and 6.4 temblors jolted Ridgecrest in July 2019, has also increased 100 fold from 0.023% to 2.3%.

If the Garlock Fault ruptures within 30 miles of its juncture with the San Andreas Fault, it has a 50/50 chance of triggering a rupture in the Mojave Desert section of the larger fault — up to five times higher than the normal probability, according to the study.

While the numbers may still seem paltry, they represent a substantial jump in the likelihood of a devasting earthquake impacting California, triggered by previous seismic activity.

Researchers Ross Stein and Shinji Toda, the co-founder and developer of catastrophe modeling company Temblor, found stress caused by four magnitude 7.0+ earthquakes within 90 miles of Ridgecrest in the 150 years before the 2019 quakes didn’t directly cause the jolts. But they may have promoted it.

The four large earthquakes may not have only “influenced” the Ridgecrest quakes, but the pattern of seismicity in Southern California.

Stein, an adjunct professor of geophysics at Stanford University, said in an interview that earthquakes “communicate” with each other by transferring stress. The four large earthquakes may have caused enough stress to bring the Garlock fault closer to failure but not enough to actually cause the Ridgecrest quakes last year.

Three major faults form a “Z” pattern, Stein said, with Ridgecrest at the top of the “Z”, causing stress to the Garlock Fault at the diagonal of the “Z” and the San Andreas Fault at the bottom of the “Z.”

As for the 50/50 potential of a “big one” on the San Andreas triggered by a quake on the Garlock Fault, Stein said it would be “an unusual situation” if there wasn’t an immediate domino effect causing multiple earthquakes and Southern California residents were forced to “wait and see” when the earthquake would hit.

“Do people evacuate, is there time to retrofit, insurance would immediately stop selling policies,” Stein said.

“We would really be out there naked, waiting for that shoe to drop, we hope we wouldn’t be in that situation, but if Covid has taught us anything, it’s that chain reactions can occur,” he added.

And if a San Andreas Fault earthquake was triggered by one on the Garlock Fault, Stein said it would be the ideal scenario since the earthquake would start far enough away from Los Angeles that early warning systems could give residents the chance to duck and cover.

“The best scenario is the earthquake starts way off in the distance and comes your way slowly, giving you the maximum amount of time to prepare,” Stein said.

“Earthquake early warning, if successful, has the chance to save lives, that’s why this is a uniquely helpful situation to envision,” he added.

The researchers developed a new method to forecast future seismicity based on past seismicity, stress imparted by recent large and moderate earthquakes and the equations which govern how fault friction varies in time and space to estimate the future likelihood of earthquakes of different sizes.

The forecast for earthquakes in Southern California over the next year finds the areas with the highest earthquake potential are close to the Ridgecrest rupture.

The two largest shocks to have struck since last July — a 5.8 magnitude earthquake June 24 and 5.5 magnitude earthquake June 4 — fall within the most stressed areas predicted to experience quaking following Ridgecrest, according to the study.

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