Geophysicists from the California Institute of Technology and NASA’s Jet Propulsion Laboratory found the Garlock Fault is beginning a process called fault creep and has slipped 0.8 inches since July, The Associated Press reported. The fault runs east to west between Death Valley and the San Andreas Fault.
On July 4, the largest earthquake in recent memory, a 7.1 magnitude tremor, struck the area in the Mojave Desert. Hours before that, a 6.4 magnitude earthquake hit.
“It’s going to force people to think hard about how we quantify seismic hazard and whether our approach to defining faults needs to change,” Zachary Ross, assistant professor of geophysics at Caltech and lead author of the paper, told the Los Angeles Times. “We can’t just assume that the largest faults dominate the seismic hazard if many smaller faults can link up to create these major quakes.”
He added that the findings were a surprise.
“We’ve never seen the Garlock fault do anything. Here, all of a sudden, it changed its behavior,” he told the newspaper. “We don’t know what it means.”
Ross said the 6.4 quake broke faults at right angles to each other, AP reported.
“This was a real test of our modern seismic monitoring system,” he said in a press release. “It ended up being one of the best-documented earthquake sequences in history and sheds light on how these types of events occur.”
The study was published on the same day that California released an earthquake early warning app and on the 30th anniversary of the 6.9 Loma Prieta earthquake along the San Andreas Fault, according to reports.
Officials encouraged Californians to download the app as a precaution.
“There is earthquake risk throughout the state,” the FAQ page of the app’s website says. “If earthquakes are large enough, the energy spreads far wider than most imagine.”
The two earthquakes in July underscored how little scientists actually understand.
“Over the last century, the largest earthquakes in California have probably looked more like Ridgecrest than the 1906 San Francisco earthquake, which was along a single fault. It becomes an almost intractable problem to construct every possible scenario of these faults failing together–especially when you consider that the faults that ruptured during the Ridgecrest Sequence were unmapped in the first place,” Ross said in the release.