Bigger temblor unlikely after swarm of quakes hits near Anza, but SoCal should be prepared: SDSU geologist

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The location of a magnitude 4.9 earthquake that struck near Anza on April 3, 2020. (USGS)

The location of a magnitude 4.9 earthquake that struck near Anza on April 3, 2020. (USGS)

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Small aftershocks continued overnight after a series of earthquakes centered near Anza in Riverside County rattled residents as far as Los Angeles on Friday, but a bigger temblor is unlikely.

Southern Californians, however, should still be prepared, said Dr. Thomas Rockwell, a paleoseismologist and structural geologist at San Diego State University.

The U.S. Geological Survey reported at least seven quakes on Friday evening, with the first and largest recorded at a magnitude 4.9 at 6:53 p.m. about 10 miles southwest of Anza and 16 miles southwest of La Quinta. Six more temblors immediately followed and ranged in magnitude from 3.6 to 2.6.

Residents as far as San Diego, Hollywood, Santa Clarita, Hesperia and Corona said they felt some shaking.

There have been more than 200 aftershocks since then, most of them registering in the 1 and 2 magnitude range, Rockwell said. Thirty-one aftershocks were between magnitude 2 and 3, and six were between magnitude 3 and 4.

On Friday night, seismologist Dr. Lucy Jones said the public shouldn’t worry about the San Andreas fault, but that the chance of a quake on the San Jacinto fault, which is capable of magnitude 7 earthquakes, has increased.

“This was a fairly typical earthquake for that section of the San Jacinto fault,” Rockwell told KTLA. “There have been several of similar magnitude during the past 25 years, so not too surprising.”

It’s been 220 years since the San Jacinto fault produced an earthquake in the magnitude 7.5 range, Rockwell said. “The average recurrence interval for such events is about 260-270 years, but with quite a bit of variability so we are in the window for a large event.”

There’s only a 5% likelihood that Friday’s swarm was a foreshock to a bigger quake, but “it is best that we are prepared,” Rockwell said.

“Would be a bad time for a major earthquake considering the current coronavirus outbreak,” he added.

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