San Diego County sheriff's Lt. Anthony Ray said there were no immediate reports of damage or injury.
Busy phone circuits made it difficult to get information from Mexicali or authorities on the U.S. side of the border in Imperial County immediately after the quake. People in Calexico, on the U.S. side of the border from Mexicali, reported buildings shaking violently.
Two smaller quakes preceded the main shock, which was initially reported at magnitude-5.9 before being downgraded slightly.
Julie Dutton, a geophysicist with the USGS, said the area is seismically active.
"It's definitely nothing that's unusual," she said.
The quake occurred 4.3 miles deep and is considered shallow. Shallower quakes have the potential to cause more damage than deeper ones, Dutton said.
Residents of Mexicali and Calexico occasionally endure short periods of time when small, frequent tremors rattle nerves but cause little damage.
