The two Ridgecrest earthquakes and subsequent aftershocks that jolted the Southland in July occurred in a domino-like sequence of ruptures on a web of connecting faults, sparking the first significant movement in 500 years along a major Mojave Desert fault, according to a report released Thursday by Caltech and NASA’s Jet Propulsion Laboratory.
While there is no definitive way to know if the sudden movement on the 185-mile-long Garlock Fault increases the likelihood of a major quake, geophysicists concluded in their report that the Ridgecrest temblors have shown that such large shakers occur in a fashion far more complex than previously believed.
“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,” said Zachary Ross, assistant professor of geophysics at Caltech and lead author of the study that appears in the journal Science. “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.”
The Ridgecrest quakes began July 4 with a 6.4 magnitude shaker, followed the next day with a quake that measured 7.1. More than 100,000 aftershocks have occurred since.
According to the Caltech/JPL study, the Ridgecrest ruptures ended within a few miles of the Garlock Fault, which runs east-and-west from the San Andreas Fault to Death Valley. Researchers said the fault has been quiet for at least 500 years, but since Ridgecrest, it has moved 2 centimeters, or 0.8 inches.
“This is surprising, because we’ve never seen the Garlock Fault do anything,” Ross told the Los Angeles Times. “Here, all of a sudden, it changed its behavior. We don’t know what it means.”
Ross told the paper it’s unclear what the sudden movement might mean, but it highlights the potential risk the fault poses to the state. The Times reported the fault could create an earthquake with a magnitude of up to 8.0, causing strong shaking in the San Fernando Valley, Santa Clarita, Lancaster, Palmdale, Ventura, Oxnard, Bakersfield and Kern County.
The Southern California Earthquake Data Center at Caltech estimates possible magnitudes from the Garlock Fault at 6.8 to 7.6.
According to the study, major quakes were commonly believed to be caused by a single long fault. Although the 1992 Landers quake — with a magnitude of 7.3 — prompted some re-evaluation of that theory, the Ridgecrest quakes showed that large quakes can be triggered by a web of smaller, inter-connected faults that set off a series of ruptures, like falling dominoes.
“We actually see that the magnitude-6.4 quake simultaneously broke faults at right angles to each other, which is surprising because standard models of rock friction view this as unlikely,” Ross said. “It is remarkable that we now can resolve this level of detail.”
Ross called the Ridgecrest quakes “one of the best-documented earthquake sequences in history.” Researches used computer analysis of the seismic data from the series and combined it with satellite data to map the fault ruptures.
“I was surprised to see how much complexity there was and the number of faults that ruptured,” report co-author Eric Fielding of JPL said.
Ross said the Ridgecrest series showed how little researchers still know about earthquakes.
“Over the last century, the largest earthquakes in California history have probably looked more like Ridgecrest than the 1906 San Francisco earthquake, which was along a single fault,” he said. “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.”
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