Jul 21, 2019 | Updated: 09:46 AM EDT

Why the California Earthquakes Were Back-to-Back

Jul 10, 2019 09:02 AM EDT

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On the morning of July 4, a magnitude 6.4 rocked Southern California, fracturing roads and sending people fleeing to safety. But that wasn't all the Earth had in store: Less than a day and a half later, a powerful magnitude 7.1 temblor shook the region.

While earthquakes are not unexpected, the two most recent temblors are the largest that have struck this area in decades. And they promise to yield fresh clues about its complex geology.

The duo of quakes struck in what's known as the Eastern California shear zone-an area east of the infamous San Andreas fault, where the Pacific Plate grinds against the North American Plate, creeping northwest at roughly two inches each year. The area extends from the southern Mojave Desert, up the eastern side of the Sierra Nevada, and into western Nevada. It's crisscrossed by fractures in the Earth caused by the movement along the nearby tectonic plate boundary.

"The Eastern California shear zone is a really interesting area," says Wendy Bohon, an earthquake geologist at the Incorporated Research Institutions for Seismology, or IRIS. "How is it working? How is it accommodating plate motion? What are going to be the big structures that come out of this, millions of years down the road?"

The recent events are what are known as strike-slip earthquakes, which occur when two blocks of Earth shift side-by-side, grinding past each other. They seemed to have occurred along with the same set of faults, located in an area known as the Little Lake fault zone.

While no deaths or major injuries have yet been reported, the intensity of the ground movement was quite strong-enough to send goods flying off store shelves and buildings swaying. The shaking was also widespread, with reports of light ground movements as far as Chico, California, and Phoenix, Arizona.

Of particular interest in these quakes is that at least the first temblor seemed to have simultaneously broken two sections of faults that cut across each other at nearly a right angle. While such complex quakes are not unheard of, recent research suggests that they may be more common than once believed, explains Zachary Ross, a geophysicist at the California Institute of Technology.

"Historically, the thought has been that earthquakes occurred on individual faults," he says. "And then over time, as the data has gotten better and better, we've started to realize that there's potential for multiple faults to rupture for single events."

This shift in thinking was propelled by the magnitude 7.3 earthquake that shook Landers, California, in 1992. This temblor fractured along at least five fault segments. Subsequent earthquakes have revealed similar complex breakage, including the magnitude 7.2 earthquake in 2010 in Baja California, Ross notes. This latest quake is further evidence that this complexity is common, even for smaller magnitude events.

In most circumstances, big earthquakes strike in a familiar sequence: There's a large earthquake followed by a series of smaller events. That's because the movement that occurs during a large earthquake causes increased strain in the surrounding region. Earthquakes are the Earth's way of relieving this strain.

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