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California’s Earthquake Risk Extends Far Beyond San Andreas Fault

Jul 9, 2019

When people think about California earthquakes, what likely comes to mind is the San Andreas Fault. But most of the state is not near the San Andreas, and yet there are still plenty of opportunities for seismicity.

The two damaging earthquakes that shook the Ridgecrest-Trona area over Independence Day weekend may have taken locals by surprise, but the same may not be said for geologists.

The Ridgecrest area is located in what’s called the Eastern California Shear Zone, where small tremors occur regularly when faults slip alongside one another. “It’s an area where we see a lot of strike-slip movement, a lot like what we do see on the San Andreas,” says CSU Bakersfield geologist William C. Krugh. It’s “the same type of fault motion, but in a slightly different area.”

Those faults are likely also responsible for nearby hot springs and other geothermal activity.

One of the faults ruptured last week, however, was relatively unexplored. The ensuing earthquake swarm has provided an opportunity to learn more about it. “This is a good example of a fault actually growing in a short period of time,” Krugh says. “We can see that sequences of earthquakes along a fault causing that fault to grow.”

Faults occur on the other side of the Sierra, too. Although the middle of the San Joaquin Valley isn’t tectonically active, the edges can be.

In 1857, the magnitude-7.9 Fort Tejon earthquake—tied for California’s largest in recorded history—struck the Monterey County City of Parkfield, but caused the most damage and killed two people in Kern County north of the Grapevine.

The 1952 Kern County quake killed 12 and caused tens of millions of dollars’ worth of damage. The epicenter of the magnitude-7.3 quake was located near the community of Wheeler Ridge, though damage was concentrated in the City of Tehachapi.

The Fresno County City of Coalinga was also near the epicenter of a magnitude-6.2 earthquake in 1983, which led to no fatalities and caused roughly $10 million in damage.