Big California quake just got ‘a little likelier’

An analysis of recent changes along earthquake faults in Southern California suggests there is an increased possibility of a major quake on the San Andreas Fault, researchers said Monday.

The changes in fault stresses, resulting from a pair of strong earthquakes in July 2019, increase the likelihood of a quake on a stretch of the San Andreas in the next 12 months to about 1%, or three to five times the probability of earlier forecasts, the researchers said.

A major quake on that section of the fault, called the Mojave, could devastate Los Angeles and its surrounding communities, which are home to 18 million people.

“We are still saying this is unlikely,” said one of the researchers, Ross S. Stein, a former U.S. Geological Survey geophysicist who now runs a consulting company. “It’s just a little likelier.”

The findings were published in the Bulletin of the Seismological Society of America.

But other researchers took issue with the analysis, saying it overstated the probability. Morgan T. Page, a research geophysicist who works on earthquake forecasting at the geological survey, said that while she agreed with the basic premise of the study, “their numbers are too high.”

Earthquake forecasts describe the likelihood of a quake occurring over a given time period; they are not predictions of a specific event at a specific time. Currently the geological survey forecasts a 31% probability of a 7.5 magnitude quake occurring in the Los Angeles area in the next 30 years.

Stein and a longtime collaborator, Shinji Toda of Tohoku University in Japan, modeled changes in stresses in the complex structure of surrounding faults that resulted from the two 2019 quakes, which occurred in sequence near Ridgecrest, California, about 120 miles north of Los Angeles. The quakes, of magnitudes 6.4 and 7.1, resulted in one death, about two dozen injuries and at least $1 billion in damage.

In an earthquake, stresses that have built up along a fault reach a breaking point, releasing huge amounts of energy. That release can alter stresses on other parts of the fault or on nearby faults, increasing or in some cases reducing the potential for more quakes.

At Ridgecrest, the changes from the first earthquake triggered the second 34 hours later. But in general these kinds of stress changes are temporary; the likelihood of another quake decreases over time. That is why the forecast by Stein and Toda is only for the next 12 months.

In their analysis, they showed that the Ridgecrest quakes changed stresses along a nearby fault, the Garlock, and increased the possibility of a major quake on a 75-mile length of it. They said there was a 2.3% chance of a Garlock quake over the next year.

But the Garlock runs through a relatively unpopulated area. Of far greater concern is the San Andreas, the major fault that runs from northern to Southern California. The Garlock is perpendicular to it, and the researchers found that a major quake on the Garlock had the potential to set off one on the San Andreas, if the Garlock rupture came near the bigger fault.

“The Garlock is the link in the chain,” Stein said. “If it comes within about 25 miles of the San Andreas, it increases the chances of an earthquake there by about 150 times.”

Susan Hough, a research seismologist with the geological survey, said she thought such a chain of events was unlikely.

“It’s kind of a Rube Goldberg scenario,” she said. “It’s possible, but in terms of something to worry about, it’s a low probability.”

Kenneth Hudnut, another geological survey geophysicist who studied an earthquake sequence that occurred in the Imperial Valley of California in 1987, said the chain of events suggested in the new study was just one of many plausible scenarios for what could happen in Southern California.

“I wouldn’t call it farfetched,” he said. “I wouldn’t challenge the notion that the Garlock could have a larger event.

“But just because we can create a plausible scenario does not mean it’s going to happen,” Hudnut said.

In studying earthquakes, he added, “‘What’s next’ is a really tough problem for us. But it’s what everybody wants to know.

“I think this paper does a good job representing the uncertainties.”

Stein said that another way of looking at his and Toda’s findings is that they are still forecasting a 99% chance that a major San Andreas quake will not happen this year.

“The sky is not falling,” he said. “Nobody should panic.

“But at the same time, the inference that the San Andreas likelihood of rupture has increased should be a reminder that anybody in Los Angeles should ask themselves, ‘Am I ready?’ “