In Southern California, the landscape is fractured in the shape of an enormous letter Z. The top arm is made up of a winding series of cracks that were responsible for quakes that rattled the city of Ridgecrest last year. The diagonal section is an ancient fault called Garlock that runs to the west. And along the bottom sits the mighty San Andreas.
Earthquakes along this lengthy fault, which runs more than 800 miles through California, are an ever-looming concern—and a new study suggests that in the next year, a large quake near the bustling city of Los Angeles could be three to five times more likely than previously thought. The research, published in the Bulletin of the Seismological Society of America, found that the 2019 Ridgecrest earthquakes made a future quake along the nearby Garlock fault more likely. If a big enough quake hits Garlock, it could trigger the San Andreas fault as well—a series of events that the researchers estimate has about a 1 in 87 probability of occurring within the next year.
However, the overall probability of such an event remains low. The research team estimates that there is a 2.3 percent chance of a magnitude 7.7 earthquake occurring on the Garlock fault in the next year, and a 1.15 percent chance of a similar quake hitting San Andreas.
“So, the sky is not falling,” says study co-author Ross Stein, CEO of Temblor, Inc., a company that assesses risks from hazards such as earthquakes. “But it is significantly higher, in our judgement, than what it would have been had the Ridgecrest earthquake not occurred.”
Estimating the probability of earthquakes is notoriously tricky. The deep faults that generate them, scientists have increasingly realized, are complex networks of cracks and chasms. “They’re fractal. They’re grungy. They have bends and breaks,” Stein says.
Faults can also interact: Movement along one might increase stresses on another, sparking a sequence of quakes, “like a domino effect,” says Alessandro Verdecchia, a geologist at McGill University who was not part of the study. The new model is the latest attempt to assess the likelihood of this potentially deadly scenario.
How the dominos fall
The San Andreas fault marks the boundary where the North American tectonic plate and the Pacific plate grind past each other. As the Pacific plate inches along a northwesterly route, stresses build until the ground breaks, which sends the surface rolling in an earthquake.
There have been many quakes in California over the past century, but the last time a big temblor occurred along the San Andreas itself was in 1906, when a magnitude 7.9 earthquake unzipped some 300 miles of the fault, leveling buildings across San Francisco and killing more than 3,000 people. It was the deadliest quake in U.S. history.
The new study suggests that the Ridgecrest quakes have increased the chances of another big one occurring, this time in southern California.
The 2019 event was a double whammy, with a magnitude 6.4 and then 7.1 quake striking one day apart. The movement from these quakes distorted the surrounding landscape, shifting the stresses on nearby faults such as the Garlock.
To estimate this change in stress, Stein and study co-author Shinji Toda of Tohoku University in Japan created a model based on the motion along faults during the Ridgecrest quakes. They also incorporated data from a host of earlier quakes to visualize the fault as a spidery zone of fractures, Stein says.
The model estimates that in the year after Ridgecrest, there was an eight percent chance of a magnitude 7.7 event along the Garlock. While that did not come to pass, the work suggests a greater risk still remains than previously recognized. In the upcoming year, the chance of such a quake remains at 2.3 percent, about 100 times as large as previous models found.
A big enough quake along the Garlock—magnitude 7.5 or bigger, by the researchers’ calculations—could spark a quake along the San Andreas that travels southward toward Los Angeles.
“The fact that it’s higher is interesting and maybe motivates us to look at it more closely,” says John Vidale, a geophysicist at the University of Southern California who was not involved in the study, referring to the estimated probability of a major quake. But many uncertainties still remain, he says, and the time period with the greatest risk of a Garlock rupture has already passed, so the new model “doesn’t necessarily mean we need to be more scared than we otherwise would be.”
Even so, the new work is a good reminder that all residents living in earthquake country need to be prepared, Stein says. If a big quake hits the Garlock fault, it could be weeks, months, or more before the San Andreas slips as well—if it does at all. But quakes in this region at some point in the future are inevitable. (Learn more about earthquake safety and how to prepare.)
“Creaking limb of assumptions”
All models, including the latest, make simplifying assumptions about our astoundingly complex planet. For example, the new model doesn’t account for the complexities of fluid interactions, which can change the fault stresses over long periods of time, says Pablo Gonzalez, a geophysicist with the University of Liverpool in England and part of the Spanish National Research Council who was not part of the study.
The model also assumes that the ground is uniform in composition. But movement along the Garlock fault over millions of years has offset the land by some 40 miles, meaning the rocks to the north differ from those to the south, Gonzalez says.
One particular challenge with all earthquake forecasts is that researchers don’t know how much additional stress is required to cause a fault to break, says Chris Goldfinger, an earthquake geologist at Oregon State University who was not part of the new study.
“When you get over to the San Andreas, you’re kind of on a long creaking limb of assumptions,” Goldfinger says. “I would still sleep well in L.A. tonight—or as well as you would otherwise.”