California earthquake increases five times the risk of tremors

The earthquakes that struck Ridgecrest (California) in 2019 could create a domino effect and cause tremors across the San Andreas Fault, which in turn increased the likelihood of a major earthquake in 2021. Writes about it National Geographic.

Photo: Shutterstock

In Southern California, there is a fault shaped like a giant Z. The top consists of a sinuous series of cracks that were responsible for the earthquakes that hit Ridgecrest in 2019. The diagonal cross-section is an ancient fault called Garlock, which runs west. and the lower part of the Z is the mighty San Andreas.

Earthquakes along this long fault, which extends over 800 miles (1 km) through California, are an ongoing problem, and a new study suggests that a strong earthquake in the Los Angeles area could occur 287-2021 times in 3 more likely than previously thought.

A study published in the Bulletin of the Seismological Society of America showed that the Ridgecrest earthquakes in 2019 created all the conditions for a likely future major earthquake along the Garlock Fault. If a strong enough earthquake hits Garlock, it can also cause an earthquake in San Andreas. Such a series of events, according to researchers, has a probability of 1 in 87 next year.

However, the overall likelihood of such an event remains low. The research team estimates there is a 7,7 percent chance of a magnitude 2,3 earthquake hitting Garlock next year, and a 1,15 percent chance of a similar quake hitting San Andreas.

“In our opinion, the likelihood of a major earthquake in 2021 is significantly higher due to the Ridgecrest earthquake,” says study co-author Ross Stein, CEO of Temblor, Inc., a company that assesses the risks associated with earthquake hazards .

On the subject: The probability of a powerful earthquake in San Francisco exceeded all expectations

Assessing the likelihood of earthquakes is difficult. Scientists are increasingly realizing that the deep faults that give rise to them are complex networks of cracks and chasms. “They are fractals. They are rough. They have bends and breaks,” Stein says.

Faults can also interact: Movement along one can add stress to another, causing a series of earthquakes, “like a domino effect,” says Alessandro Verdecchia, a geologist at McGill University. The new model is the latest attempt to estimate the likelihood of this potentially deadly scenario.

The domino effect

The San Andreas Fault marks the boundary where the North American tectonic plate and the Pacific plate cross each other. As the Pacific Plate slowly moves along the northwestern route, stress builds up until the soil breaks, causing the surface to move.

There were a lot of earthquakes in California in the last century, but the last time a great one happened on San Andreas itself in 1906, when an earthquake of magnitude 7,9 stretched 300 miles (482 km) from the fault, destroying buildings in San Francisco and killing more than 3000 people. It was the deadliest earthquake in US history.

A new study suggests that the Ridgecrest earthquakes increased the likelihood of another 5 times more likely, this time in southern California.

The 2019 earthquake was double, in one day there were two tremors with a magnitude of 6,4, and then 7,1. The movement from these earthquakes distorted the surrounding landscape, transferring stress to nearby faults such as Garlock.

To assess this change, Stein and co-author of the Shinji Toda study at Tohoku University in Japan created a model based on movement along faults during the Ridgecrest earthquake. According to Stein, they also included data on many earthquakes to visualize the fault.

According to model estimates, during the year after the Ridgecrest earthquake, there was an 8 percent chance of an earthquake of magnitude 7,7 along Garlock. Although this did not happen, researchers suggest that there is an even greater risk than previously recognized. Next year, the likelihood of such an earthquake will remain at 2,3 percent, which is about 100 times more than previous models.

A strong enough earthquake along Garlock—magnitude 7,5 or greater, according to researchers—could trigger a quake along the San Andreas that would travel south toward Los Angeles.

On the subject: 10 strongest earthquake in California history

“The fact that the probability of an earthquake is higher is interesting and perhaps encourages us to look at the whole situation more carefully,” says John Vidale, a geophysicist at the University of Southern California who was not involved in the study, citing the perceived likelihood of a large earthquake. But many uncertainties still remain, he says, and the time period with the greatest risk of a Garlock earthquake has passed, so the new model "doesn't necessarily mean we should be any more scared."

Regardless, the new work is a good reminder that everyone living in a state prone to earthquakes should be prepared, Stein says. If a major earthquake hits Garlock, it could be weeks, months, or even longer before San Andreas also wakes up—if it does at all. But earthquakes in this region are inevitable at some point in the future.

The study does not take into account many factors

“All models, including the latest ones, make assumptions about our amazingly complex planet. For example, the new model does not take into account the complexities of fluid interactions that can change fault stresses over long periods of time, says Pablo Gonzalez, a geophysicist at the University of Liverpool in England.

The model also assumes that the base is uniform in composition. But moving along the Garlock Fault over the course of millions of years displaced the earth by about 40 miles (64 km), which means that the rock in the north is different from the rock in the south, says Gonzalez.

According to Chris Goldfinger, an earthquake geologist at the University of Oregon, one of the specific problems with all earthquake predictions is that researchers do not know how many additional shocks are required to cause an earthquake.

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