Earthquake Centered in Mojave Desert Area Known for ‘Robust’ Aftershock Sequences

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The magnitude 6.4 earthquake that rocked Southern California on Fourth of July was centered in an area of the Mojave Desert known for "robust" aftershock sequences but remote enough to prevent damage in more heavily populated areas, famed former USGS seismologist Lucy Jones said Thursday.

The quake struck east of the southernmost part of the mighty San Andreas Fault, likely on a small fault about 10 miles long, Jones said at a news conference at Caltech. She later said new evidence suggested the quake involved two faults.

The epicenter was about 7 miles southwest of Searles Valley, in northwest San Bernardino County, at about 10:33 a.m. The epicenter was on the sprawling Naval Air Warfare Center Weapons Division at China Lake, about 11 miles east-northeast of Kern County's Ridgecrest, where about 29,000 people live.

"This was not unexpected in this area," Rob Graves of the U.S. Geological Survey said, calling the temblor "totally appropriate" for the region.

Ridgecrest and the small town of Trona may have been particularly hard hit because of the nature of the quake, Jones said. Ridgecrest Regional Hospital was evacuated.

No damage was documented in Los Angeles County or more populated areas of San Bernardino County farther from the epicenter.

"It's a sparsely inhabited area so the number of people who would have received damage is much lower," she said.

USGS geologists were headed to the region to examine reports of "ground cracks," Graves said.

About three dozen aftershocks were felt within an hour of the quake, she added. They generally were not being felt in the L.A. Basin, but probably in the Owens Valley area.

Within five hours, more than 220 aftershocks had occurred, according to the Southern California Seismic Network.

The quake area has likewise in past seen "very robust sequences," Jones said.

A magnitude 5.1 quake struck close by in 1982, and six to eight magnitude 4 quakes were associated with that temblor, along with many in the magnitude 3 range, she said.

"It went on for six months. So we should be expecting lots of aftershocks," Jones said.

Within the next few days, there's a 9% chance of a quake larger than magnitude 6.4 within the next week, Graves and Jones said. The most likely time for larger quakes is on Thursday, and probabilities will shrink by more than half Friday, they said.

It's about a 100% chance of another quake of magnitude 4.

"It's certain that this area is going to be shaken a lot today," Jones said.

Aftershocks were occurring on "two conjugate fault structures in the Airport Lake Fault Zone," USGS said on Twitter.

Generally, over the last 20 years, it's been an "extremely quiet time" for quakes in Southern California, Jones added.

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