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Hundreds of earthquakes were rattling an area southwest of the Salton Sea Wednesday, including six with a magnitude greater than 4.0, according to the U.S. Geological Survey.

The swarm began around 4 p.m., when a 3.0 magnitude temblor struck. At least 240 earthquakes were recorded by 8 p.m., the largest being a magnitude 4.9 temblor at 5:31 p.m., USGS said in a forecast of seismic activity for the area.

The swarm remained active around 10 p.m.

The events were centered around the small town of Westmorland on the edge of the Salton Sea, about 20 miles north of El Centro.

That’s in the Brawley seismic zone, which is too far from the San Andreas fault to change the probability of a quake occurring there, seismologist Lucy Jones said in a tweet.

Jones also said Wednesday’s is one of the largest swarms to hit the Imperial Valley, an area where swarms are common.

It’s likely that the rate of earthquakes in the area will decrease over the next week, with some additional events with a magnitude between 4.5 and 5.4. It’s less likely, but possible, that a larger quake with a magnitude up to 6.9 will strike, according to USGS.

Probability for an earthquake of magnitude 7.0 or larger occurring is very low, seismologists say.

“The chance of large earthquakes will remain elevated as long as the swarm continues,” USGS said in its forecast.

Past swarms in the area have stayed active for about a week on average. Those include a 1981 swarm around Westmorland that included a 5.8 magnitude quake, and a 2012 swarm near Brawley that included a magnitude 5.4 temblor.

Wednesday’s swarm is about 25 miles south of another one that occurred last month on the eastern shore of the Salton Sea, near Bombay Beach.

USGS said the latest swarm is “rapidly evolving,” and seismologists will update their quake forecast as more data is collected.