It comes after dozens of earthquakes shook the area over the weekend, including a 5.3 magnitude on Saturday that triggered shaking across the San Diego area and into Mexico.
Monday’s largest quake hit at about 2:08 p.m. at the south tip of the sea, about 8 miles northwest of Calipatria and 25 miles north of El Centro, USGS said.
People reported feeling strong shaking at the epicenter and weak movement as far away as San Diego, Cathedral City and El Centro.
The other eight temblors that had struck as of late Monday afternoon were between magnitudes 2.5 and 3.6. Overall, the seismic activity seems to be diminishing since it kicked off over the weekend.
Swarms are common in the Imperial Valley, and 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.
The main fault involved appears to be the Westmoreland fault, which is not long enough for a quake much bigger than magnitude 5.8, famed seismologist Lucy Jones said in a series of tweets over the weekend.
The area is in the Brawley seismic zone, which Jones says is too far from the San Andreas fault to change the probability of a quake occurring there.