The strongest of the temblors were magnitudes 5.5 and 5.3, said Robert Graves of the USGS. The quakes started around 8:30 a.m. and by 6 p.m. about 300 had been recorded, many in the four range and more than 30 with a magnitude of 3.5 and larger.
The earthquakes were expected to continue Sunday and there was no estimate for when the shaking might end.
"It's possible that this will continue on for a number of hours or even several days. This has happened in the past," Graves said. "We don't necessarily expect larger events, but that certainly is a possibility at this time."
There were no immediate reports of injuries or significant damage. Images from the around town showed shattered storefront windows and at least one trailer home fell off its jacks.
The largest temblor struck four miles north of the small farming town of Brawley around 1:57 p.m. More than a dozen quakes followed, including one that measured in at 4.3, according to the USGS.
Earlier in the day, a 5.3-magnitude quake hit the same place around 12:31 p.m. The Imperial Valley area was shaken by several noticeable temblors, ranging from magnitude 2.0 to 4.5, in the morning, starting around 10:00 a.m.
It was unclear what fault the quake cluster was on, but it was in the Brawley Seismic Zone near the San Andreas and Imperial faults.
Eyewitness News viewers said that they felt the quakes all across Southern California, including in Brea, San Diego, Tustin, Indio and Temecula. The shaking was felt east to Arizona.
"The message here, if nothing else, make sure you're prepared," Graves said.