Science

3 questions seismologists are asking after the California earthquakes

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A week after two large earthquakes rattled southern California, scientists are scrambling to understand the sequence of events that led to the temblors and what it might tell us about future quakes.

A magnitude 6.4 quake struck July 4 near Ridgecrest — about 194 kilometers northeast of Los Angeles — followed by a magnitude 7.1 quake in the same region on July 5. Both quakes occurred not along the famous San Andreas Fault but in a region of crisscrossing faults in the states high desert area, known as the Eastern California Shear Zone.

The San Andreas Fault system, which stretches nearly 1,300 kilometers, generally takes center stage when it comes to Californias earthquake activity. Thats where, as the Pacific tectonic plate and the North American tectonic plate slowly grind past each other, sections of ground can lock together for a time, slowly building up strain until they suddenly release, producing powerful quakes.

For the last few tens of millions of years, the San Andreas has been the primary origin of massive earthquakes in the region. Now overdue for a massive earthquake, based on historical precedent, many people fear its only a matter of time before the “Big One” strikes.

But as the July 4 and July 5 quakes — and their many aftershocks — show, the San Andreas Fault system isnt the only source of concern. The state is riddled with faults, says geophysicist Susan Hough of the U.S. Geological Survey in Pasadena, Calif. Thats because almost all of California is part of the general boundary between the plates. The Eastern California Shear Zone alone has been the source of several large quakes in the last few decades, including the magnitude 7.1 Hector Mine quake in 1999, the magnitude 6.7 Northridge quake in 1994 and the magnitude 7.3 Landers quake in 1992 (SN Online: 8/29/18).

Here are three questions scientists are trying to answer in the wake of the most recent quakes.

Which faults ruptured, and how?

The quakes appear to have occurred along previously unmapped faults within a part of the Eastern California Shear Zone known as the Little Lake Fault Zone, a broad bunch of cracks difficult to map, Hough says. “Its not like the San Andreas, where you can go out and put your hand on a single fault,” she says. And, she adds, the zone also lies within a U.S. Navy base that isnt generally accessible to geologists for mapping.

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But preliminary data do offer some clues. The data suggest that the first rupture may actually have been a twofer: Instead of one fault rupturing, two connected faults, called conjugate faults, may have ruptured nearly simultaneously, producing the initial magnitude 6.4 quake.

Its possible that the first quake didnt fully release the strain on that fault, but the second, larger quake did. “My guess is that they will turn out to be complementary,” Hough says.

The jury is still out, though, says Wendy Bohon, a geologist at the Incorporated Research Institutions for Seismology in Washington, D.C. “What parts of the fault broke, and whether a part of the fault broke twice … Im waiting to see what the scientific consensus is on that.”

And whether a simultaneous rupture of a conjugate fault is surprising, or may actually be common, isnt yet clear, she says. “In nature, we see a lot of conjugate fault pairs. I dont think they normally rupture at the same time — or maybe they do, and we havent had enough data to see that.”

Is the center of tectonic action moving away from the San Andreas Fault?

GPS data have revealed exactly how the ground is shifting in California as the giant tectonic plates slide past one another. The San Andreas Fault system bears the brunt of the strain, about 70 percent, those data show. But the Eastern California Shear Zone bears the other 30 percent. And the large quakes witnessed in that region over the last few decades raise a tantalizing possibility, Hough says: We may be witnessing the birth pangs of a new boundary.

“The plate boundary system has been evolving for a long time already,” Hough says. For the last 30 million years or so, the San Andreas Fault system has been the primary locus of action. But just north of Santa Barbara lies the “big bend,” a kink that separates the northern from the southern portion of the fault system. Where the fault bends, the Pacific and North American plates arent sliding sideways past one another but colliding.

“The plates are trying to move, but the San Andreas is actually not well aligned with that motion,” she says. But the Eastern California Shear Zone is. And, Hough says, theres some speculation that its a new plate boundary in the making. “But it would happen over millions of years,” she adds. “Its not going to be in anyones lifetime.”

Will these quakes trigger the Big One on the San Andreas?

Such large quakes inevitably raise these fears. Historically, the San Andreas Fault system has produced a massive quake about every 150 years ago. But “for whatever reason, it has been pretty quiet in the San Andreas since 1906,” when an estimated magnitude 7.9 quake along the northern portion of the fault devastated San Francisco, Hough says. And the southern portion of San AndreasRead More – Source