Swarm of hundreds of small earthquakes continues in Imperial Valley, but no elevated risk for L.A., Lucy Jones says

California
A USGS map shows the location of a swam of earthquakes in the Imperial Valley on Sept. 30 and Oct. 1, 2020. The locations marked are for quakes of magnitude 2.5 or larger.

A USGS map shows the location of a swam of earthquakes in the Imperial Valley on Sept. 30 and Oct. 1, 2020. The locations marked are for quakes of magnitude 2.5 or larger.

An earthquake swarm is continuing on Thursday to roil a rural area of southeastern California with a history of such seismic activity, but a renowned seismologist said there is no increased risk for a temblor in other areas.

The hundreds of earthquakes in the Imperial Valley since Wednesday include a magnitude 4.9 tremor. The swarm is centered beneath agricultural fields south of the Salton Sea, about 2 1/2 miles northeast of the small town of Westmorland in Imperial County.

Nearly 600 quakes have been recorded in the swarm, the Southern California Seismic Network reported. The smallest event had a magnitude of 1.0.

In one span of 2 1/2 hours Wednesday, the region had 45 earthquakes with a magnitudes of at least 3.0, according to seismologist Lucy Jones.

“One of the largest swarms we have had in the Imperial Valley – and it is historically the most active swarms in SoCal,” she tweeted.

Past swarms have remained active for up to 20 days, but the average is about a week, according to the U.S. Geological Survey.

The earthquake activity is taking place in the Brawley Seismic Zone, which is comprised of a a series of “small faults that connect the San Andreas and Imperial faults,” according to Jones.

But the zone doesn’t have a history of producing major earthquakes.

A magnitude 5.8 temblor is the largest ever recorded in the Brawley zone. Wednesday’s quakes struck near the epicenter of that 1981 temblor, she said.

Notable past seismic activity in the region also included a 2012 swarm that produced a magnitude 5.4.

Jones explained that, as ever, there is an elevated earthquake risk for the immediate area where temblors occur. That means more magnitude 3’s are expected in the location and there is even the possibility of a magnitude 5 quake.

But, she emphasized, that would have no impact elsewhere, including Los Angeles, San Diego and San Francisco.

“In other words, there is no scientific reason to predict a big quake in another location today,” Jones said.

But the Geological Survey does say that there is a “significantly greater” probability of a larger earthquake in the region during the swarm: from an approximately 1 in 3000 chance of a temblor of magnitude 7.0 or higher, to a 1 in 300 chance of such an occurrence.

USGS outlined three possible scenarios during the swarm.

The first, and — at 90% by far — most likely, is that quakes will continue, and may include tremors up to magnitude 5.4.

The second scenario is that a larger earthquake, measuring between a 5.5 and 6.9, would strike the area within the next week. There’s about a 10% chance of that happening.

And the least likely scenario, with 1 in 300 odds, is that a 7.0 earthquake hits the region in the next seven days.

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