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Chance of big San Andreas earthquake increased by Ridgecrest temblors, study suggests

Rong-Gong Lin II, Los Angeles Times on

Published in News & Features

Ken Hudnut, a U.S. Geological Survey geophysicist, drops, covers and holds on in Ridgecrest as a magnitude 7.1 earthquake ruptures through a nearby fault.

There's a popular conception that earthquakes relieve seismic strain -- they do -- but they also increase seismic strain in other areas.

"An earthquake will relieve stress on the fault that it occurs on. But by relieving that, you're transferring the stress onto something else," Dawson said. "For every action, there's a reaction."

Scientists -- and the public -- have long been fascinated about the prospect of triggered earthquakes. It was a main plot point in the movie "San Andreas," starring Dwayne Johnson in 2015.

It's for a good reason.

Last year's Fourth of July Ridgecrest quake, a magnitude 6.4 temblor, imparted greater stress on a fault that eventually ruptured a day later, causing the more powerful magnitude 7.1 quake on July 5.

The most powerful earthquake in California of the last 68 years, the magnitude 7.3 Landers earthquake that hit the sparsely populated Mojave Desert on June 28, 1992 -- and a magnitude 6.3 aftershock hours later near Big Bear -- was believed to be related to the Joshua Tree earthquake, a magnitude 6.1 event, that occurred two months earlier.

The trio of quakes raised concerns that the San Andreas was next.

 

The theory at the time was that the Joshua Tree-Landers-Big Bear sequence of quakes essentially unclamped a section of the San Andreas fault. That made it plausible the San Andreas fault might be next to rupture, said Ken Hudnut, geophysicist with the USGS.

But the southern San Andreas fault has remained as quiet as it has since the 1850s.

Instead, the next big quakes in Southern California occurred where few scientists were expecting them to hit -- the magnitude 6.7 earthquake that struck Northridge in 1994, and the magnitude 7.1 Hector Mine earthquake in 1999 that was located even deeper in the remote Mojave Desert.

"What has been actually happening in the real world is quite different than what we thought was a plausible scenario back at the time in '92," Hudnut said.

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