The largest quake, registered at a magnitude 5.5, struck at 1:57 p.m. Sunday and was centered about three miles northwest of Brawley, said Robert Graves, a geophysicist with the U.S. Geological Survey. Another quake about an hour and a half earlier registered at magnitude 5.3.
No injuries were reported.
Several dozen earthquakes with magnitudes of at least 3.5 shook the same area near the southern end of the Salton Sea, Graves said.
By dawn Monday, the U.S. Geological Survey website showed there had been dozens of aftershocks in Imperial County, the largest a magniutude-4.9 at 9:41 p.m. Sunday. There was also a 3.0 at 12:32 a.m. Monday.
"The type of activity that we're seeing could possibly continue for several hours or even days," Graves said.
The quakes pushed 20 mobile homes at a trailer park off their foundations and rendered them inhabitable, said Maria Peinado, a spokeswoman for the Imperial County Emergency Operations Center. A red-tiled roof apparently collapsed and landed on a wooden fence.
Sporadic power outages, at one point affecting 2,500 Imperial Irrigation District customers, also prompted authorities to evacuate 49 patients from one of the county's two hospitals, Peinado said. Police also received numerous calls about gas leaks and water line breaks.
"It's not uncommon for us to have earthquakes out here, but at this frequency and at this magnitude it's fairly unusual," said George Nava, the mayor of Brawley, a town of 25,000.
"And the fact that the aftershocks keep coming are a little alarming," he said.
At the El Sol Market, food packages fell from shelves and littered the aisles.
"It felt like there was quake every 15 minutes. One after another. My kids are small and they're scared and don't want to come back inside," said Mike Patel, who manages Townhouse Inn & Suites.
A TV came crashing down and a few light fixtures broke inside the motel, Patel said.
The first quake, with a magnitude of 3.9, occurred at 10:02 a.m. The USGS said more than 300 aftershocks struck the same approximate epicenter.
Some shaking was felt along the San Diego County coast in Del Mar, some 120 miles from the epicenter, as well as in southwestern Arizona and parts of northern Mexico.
USGS seismologist Lucy Jones said earthquake swarms are characteristic of the region, known as the Brawley Seismic Zone.
"The area sees lots of events at once, with many close to the largest magnitude, rather than one main shock with several much smaller aftershocks," Jones said.
The last major swarm was in 2005, following a magnitude-5.1 quake, she said.
Sunday's quake cluster occurred in what scientists call a transition zone between the Imperial and San Andreas faults, so they weren't assigning the earthquakes to either fault, Graves said.