A pair of earthquakes that struck the remote California desert 1 year ago have raised the risk of “the big one” hitting Southern California, according to a new study. The research finds that the 2019 Ridgecrest, California, quakes shifted underground stresses, making the San Andreas fault—the state’s longest and most dangerous fault—three times more likely to rupture.
“You would think an earthquake … out in the desert would have no impact on Los Angeles,” says Ross Stein, a seismologist and one of the authors of the new study. “But that is because we do not appreciate the way the network of fault lines connect across the state.”
In July 2019, two faults near the town of Ridgecrest ruptured in quick succession: a magnitude 6.4 on 4 July, followed by a substantial magnitude 7.1 a day and a half later. The temblors damaged buildings in the area, but residents in Los Angeles, nearly 200 kilometers away, felt little more than light shaking.
Yet the faraway earthquakes have raised the hazard for Los Angelenos, says Stein, CEO of Temblor, a company that specializes in catastrophe modeling. The reason, he explains, is that two quakes put new stresses on the Garlock fault, a relatively dormant fault that runs through the desert toward the San Andreas. The Garlock fault has not ruptured in 600 years, and given its location in a sparsely populated region, it is not regarded as a great threat. Yet, based on modeling from Stein and Shinji Toda, a seismologist at Tohoku University, stresses resulting from the Ridgecrest quakes have made the Garlock fault 100 times more likely to rupture—which would in turn boost the chances of the San Andreas rupturing.
U.S. Geological Survey (USGS) estimates for the annual probability of an earthquake on this part of the San Andreas are about one-third of a percent—equivalent to expecting a magnitude 7.8 every 300 years, on average. The new modeling triples that hazard to 1% per year—or a big one every century, the team reports today in the Bulletin of the Seismological Society of America. And if the Garlock actually does rupture, then the hazard really rises on the San Andreas, by a factor of 150: The probability of a big one rises to 50% in the following year. “What does LA do then?” Stein asks.
In principle, a Garlock earthquake could lead to rupture on the San Andreas in a matter of hours or days, much as the two Ridgecrest events came within a day or two. USGS regional scenarios anticipate 1800 deaths and 50,000 injuries in the event of a major San Andreas earthquake. More than 3 million homes could be damaged, at a reconstruction cost of $289 billion.
Stress-transfer models of the kind used in the new study have a long history in seismology, but their value remains uncertain. Following the 1999 Izmit earthquake in Turkey, Stein and colleagues made a similar forecast of heightened risk in Istanbul, predicting a 60% chance of a major earthquake there within 3 decades. So far it hasn’t happened.
On the other hand, immediately after the disastrous 2004 Sumatran earthquake, another team published calculations showing it was likely to trigger a subsequent nearby rupture, a forecast published days before a massive event did hit the island of Simeulue, just as foreseen. The different stories illustrate the promise and limitations of these models, Stein says.
Seth Stein, a Northwestern University seismologist who has no relation to Ross Stein, finds the approach worthwhile, but takes a dimmer view of the result. “The trouble is the uncertainties on all the numbers in this sequence are huge,” he says. “And you’re cascading those all together.”
Morgan Page, a seismologist at the California Institute of Technology who models quake risk for the state, has her doubts, too. She says the recorded behavior of aftershocks shows a large earthquake on the Garlock would be unlikely to trigger an equally large earthquake on the San Andreas. “Doublets are not that common,” she says.
Amid all this anxiety, there is some good news. California has deployed a system that can provide digital warnings that outpace the speed of earthquake waves. If a Garlock earthquake triggered a San Andreas earthquake, it would be likely to begin far north of Los Angeles—in a perfect position for the warning system.
“In most early-warning sequences, the warning would be a matter of seconds,” Stein says. “But here for people in the densely built-up areas, it could be tens of seconds.” That’s enough time for trains to stop, the gas network to be shut down, and for a dentist to remove the drill from a patient’s mouth.