Earthquakes in Southern California Put Stress on a Major Fault Line That's Been Dormant Since Records Began

A series of earthquakes which shook Southern California earlier this year may have increased stress on parts of a major dormant fault line which has not produced any significant activity since records began, according to a study.

Scientists from the University of Iowa examined the so-called Ridgecrest earthquakes, which began with a 6.4 magnitude foreshock in the Mojave Desert on July 4, followed the next day by a 7.1 magnitude quake—the largest in Southern California for two decades. In addition, more than 100,000 smaller aftershocks were recorded.

By analyzing satellite imagery and data collected by seismic instruments, the researchers were able to work out how this series of earthquakes affected the Garlock Fault—which stretches east to west for 185 miles from the infamous San Andreas Fault to Death Valley.

This fault appears to be lying dormant given that it has not produced a major earthquake for at least a century—when scientists first began measuring seismic activity in the area. Nevertheless, experts say that it still poses an earthquake risk to Southern California.

"The Garlock Fault has been quiet for a long time," lead author of the study Bill Barnhart said in a statement. "But there's geologic evidence that there have been large earthquakes on it. It's a major fault line."

According to the study—which is published in the journal Geophysical Research Letters—the Ridgecrest earthquakes caused a phenomenon known as "aseismic creep" across a specific 12-to-16-mile section of the Garlock Fault.

This term refers to when a fault slips despite not producing an earthquake, and it can indicate an increase in stress.

"The aseismic creep tells us the Garlock Fault is sensitive to stress changes, and that stresses increased across only a limited area of the fault," Barnhart said.

These results were both surprising and unsurprising, according to the lead author.

"This is has been observed several times in California alone," Barnhart told Newsweek. "So to see fault creep on the Garlock fault was not necessarily a surprise."

"What was surprising was how well the region of fault creep corresponded to the region where we estimate that stresses increased on the Garlock fault," he said. "It was almost a one-to-one relationship that very strongly suggests that the Garlock fault is sensitive to and responds to nearby earthquakes."

The lead author argues that this evidence reveals where a future earthquake at the fault could occur.

"So, if—and that's a big if—this area were to slip in a future earthquake, we are showing where that might happen," Barnhart said.

Ridgecrest earthquakes
Ridgecrest residents inspect a recent fault rupture following two large earthquakes in the area on July 7, 2019 near Ridgecrest, California. Mario Tama/Getty Images

Fortunately, the team found that the rest of the Garlock Fault had not been affected by the Ridgecrest quakes in the same way. Most of it actually exhibited a decrease in stress—something Barnhart describes as "good news."

Nevertheless, the study suggests that the a potential rupture of the stressed section identified by the team could result in an earthquake with a magnitude of between 6.7 and 7.0.

"It would be an earthquake of the magnitude of the Ridgecrest sequence," Barnhart said. "That means it would be big. You'd feel some swaying in Los Angeles, but it wouldn't be a magnitude 7.8 that could be more damaging."

"At the same time, earthquakes commonly trigger creep on nearby faults in California without leading to a bigger earthquake, so the creep that we found on the Garlock fault absolutely does not mean that a big earthquake is imminent," he said.

This article was updated to include additional comments from Bill Barnhart.