More than 600 earthquakes strike small California town in swarm around Salton Sea

A swarm of hundreds of small earthquakes struck in and around the town of Westmoreland in Imperial County over the past couple days

The Southern California Seismic Network recorded nearly 600 earthquakes, the smallest a magnitude 1.1, as of 6 a.m. Thursday, according to a report from the project by Caltech and the U.S. Geological Survey.

The biggest of the temblors measured a magnitude 4.9 and struck about 5:30 p.m. Wednesday near the town of Westmorland, the USGS said.

The other, smaller quakes included at least seven that measured magnitude 4.0 or greater and 59 that measured 3.0 or greater, according to preliminary figures from the USGS.

At least 45 of those earthquakes took place within two and a half hours Wednesday, seismologist Lucy Jones wrote on Twitter. She described the event as “one of the largest swarms we have had in the Imperial Valley,” which itself is one of the most active regions for earthquake swarms in Southern California.

The swarm was continuing Thursday at a lower frequency, with four magnitude 3 earthquakes recorded in the first seven hours of the day versus 40 recorded in the last seven hours of Wednesday, the USGS said. Past earthquake swarms have lasted between one and 20 days and average about a week, according to the USGS.

The quakes were largely felt in relatively remote regions of Imperial and Riverside counties, the USGS said.

The earthquakes were taking place in the Brawley Seismic Zone, a seismically active network of small faults that connects the much larger San Andreas fault to the north and the Imperial fault to the south, Jones tweeted. The seismic zone extends about 30 miles northwest from the city of Brawley, across the Salton Sea’s southern half, and ends near Bombay Beach.

The zone is known for producing earthquake swarms, with a 1981 swarm in Westmorland that included a magnitude 5.8 earthquake and one in Brawley in 2012 that included a magnitude 5.4 quake that damaged a handful of buildings, the USGS said.

Another swarm took place near Bombay Beach in August. USGS researchers said at the time that the event increased the chance of a big earthquake on the San Andreas fault, which is considered one of the state’s most dangerous, during the week that followed.

This week’s earthquake swarm took place took place about 25 miles south — too far away from the San Andreas to increase the probability of a quake there, Jones tweeted.

The swarm could still trigger a larger earthquake over the next week, with the probability of a magnitude 7 quake or greater taking place in the vicinity of the swarm increasing to about 1 in 300 from about 1 in 3,000 during a typical week, the USGS said. The USGS was also forecasting a roughly 10% chance of a nearby quake with a magnitude of 5.5 to 6.9, similar to the one seen in the 1981 swarm.

But the probability is only heightened in the area within a couple miles of the earthquake swarm, Jones tweeted.

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