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Swarm Of Earthquakes Hits Just South Of San Andreas Fault

A cluster of earthquakes occurred beneath the Salton Sea in the Brawley Seismic Zone, just south of the San Andreas Fault on August 10, 2020. (Courtesy of the U.S. Geological Survey)
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Starting this morning, a swarm of dozens of small earthquakes hit just south of the San Andreas Fault, beneath the Salton Sea, all within a few hours of each other.

The largest was a magnitude 4.6, and was followed by numerous magnitude threes.

They occurred in an area known as the Brawley Seismic Zone, a transition point between the San Andreas Fault and the Imperial Fault, which extends down into Mexico.

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The quakes occurred about seven miles south of the San Andreas so, naturally, people had concerns about a larger quake being triggered on the fabled fault.

"It's a really active area right there in the Sea. Lots of small earthquakes and they tend to be swarmy," said Morgan Page, research geophysicist with the U.S. Geological Survey. "In the past these swarms have gone on for about a week or so."

Page said of the most recent quakes: "Hopefully it'll just die out and nothing bigger will happen."

Over the past 20 years there've been three similar clusters of earthquakes in the Brawley Seismic Zone, most recently in 2016.

"There's quite a long history of these fairly short lived swarm episodes that are reasonably close to the San Andreas and none of those have led to anything," said Zachary Ross, a seismologist at Caltech.

After the 2016 cluster, Ross co-authored a paper that dug into earthquake clusters in the area. It's particularly seismically active because that's where the Pacific and North American tectonic plates meet.

"We have faults that are building up strain all the time," Ross said.

The USGS is currently running models to determine just how much this series of quakes might've increased the likelihood of a bigger quake in the region.

On any given week there's a one in 10,000 chance that a magnitude seven, or greater, earthquake will hit on the southern San Andreas, according to Page. There's a 20% chance one will hit sometime in the next 30 years.

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When it does hit, a major San Andreas quake could have disastrous consequences for much of Southern California.

It's always a good time to get your earthquake supplies and plan ready.


We don't want to scare you, but the Big One is coming. We don't know when, but we know it'll be at least 44 times stronger than Northridge and 11 times stronger than the Ridgcrest quakes last year. To help you get prepared, we've compiled a handy reading list

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