California: July earthquake caused fault to move for first time on record

Sam Levin in Los Angeles and agency
<span>Photograph: Mario Tama/Getty Images</span>
Photograph: Mario Tama/Getty Images

The large earthquake that hit southern California over the summer has increased strain on a major nearby fault, causing it to move for the first time on record, researchers said on Thursday.

Ruptures in the Ridgecrest earthquake sequence in July ended a few miles from the Garlock fault, which runs east-west for 185 miles (300km) from the San Andreas fault to Death valley. That fault has been relatively quiet for 500 years, but now has begun slowly creeping, according to a new study released in the journal Science.

Related: Thirty years after devastating quake, is San Francisco ready for the next?

“It’s surprising,” Zachary Ross, assistant professor of geophysics at Caltech and lead author of the paper, said. “In California, we’ve been monitoring earthquakes for a long time. We think we have a pretty good sense of what typical behavior is over long timescales … and then when something happens that’s anomalous, that obviously stands out.

“We just don’t know what this could mean,” he added.

The Ridgecrest earthquake sequence was the largest in two decades in southern California and began 4 July in the Mojave desert about 120 miles (190km) north of Los Angeles. After a magnitude 6.4 foreshock, there was a magnitude 7.1 mainshock the next day, followed by more than 100,000 aftershocks.

The Garlock fault is capable of producing a magnitude 8 quake and has slipped 0.8in (2cm) since July, the study by geophysicists from the California Institute of Technology and Nasa’s Jet Propulsion Laboratory found. It moved over a large enough length of the fault that radar satellite in space detected it, Ross said.

“We’ve never seen this fault do that before,” Ross said, emphasizing that it was impossible to speculate what the movement could mean for future hazard.

The study also found that the Ridgecrest sequence involved ruptures on a web of interconnected faults. The researchers noted that while major earthquakes are commonly thought to be caused by the rupture of a single long fault, seismologists have since been rethinking that model.

The Ridgecrest sequence involved about 20 previously undiscovered faults crisscrossing, providing another example of how large earthquakes can be generated by a weblike network of smaller “faults that, when they rupture, trigger one another like falling dominoes”, the authors said.

This means, seismologists may have to rethink how they quantify potential hazards and define faults moving forward, said Ross: “It injects a lot more uncertainty … It’s hard to come up with all the different possible scenarios where tiny faults activate together to produce some event … There’s an infinite number of combinations.”

The study was published on the 30th anniversary of the deadly magnitude 6.9 Loma Prieta earthquake that badly damaged the San Francisco Bay Area and the same day that California implemented a statewide earthquake early warning system for the public.

Alerts previously were made available to schools, government agencies, industries and industries but not the general public, except in Los Angeles county, where an app-based system has been in use since January.