SAN DIEGO (KSWB/KUSI) – Nearly 200,000 people in Southern California felt shaking as hundreds of earthquakes rattled the Imperial County desert over the weekend.

The California Governor’s Office of Emergency Services (CAL OES) is working with local agencies after an earthquake swarm struck near Brawley in Imperial County over the weekend, with the strongest earthquake reported as a magnitude 4.7.

As of 12 p.m. local time Sunday, the United States Geological Survey (USGS) had recorded 354 earthquakes that struck in Imperial County, which started at 5:40 p.m. on Friday with several microearthquakes.

Residents near Brawley reported feeling strong shaking from the 4.7 magnitude earthquake that struck at 11:10 p.m. on Saturday, while residents in El Centro, Calexico, El Cajon, Poway, Chula Vista, and more, reported feeling weak to light shaking.

USGS says a series of earthquakes that don’t fit a mainshock-aftershock pattern are typically considered swarms.

The area is part of the Brawley seismic zone, which “has a history of earthquake swarms and geothermal energy exploitation,” the USGS explains. It’s also near the Brawley fault zone, considered “a complex set of faults that is intricately connected to the Imperial fault zone.” This makes it an area “prone to earthquake swarms,” according to the California Institute of Technology.

USGS Shake Alerts were sent to the phones of nearby residents just before midnight Saturday after the 4.7 magnitude earthquake. Those who felt it or other earthquakes can report it here.

The ShakeAlert Earthquake Early Warning system was activated several times during the earthquake swarm.

USGS reports the earthquake swarm caused light to no damage, and that 180,000 people reported feeling light to strong shaking. It’s not clear what was causing the swarm.

If you feel shaking or get an alert, officials advise to “drop, cover, and hold on.”

Nexstar’s Addy Bink contributed to this report.

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