A 4.8-magnitude earthquake struck El Centro in Imperial County just after midnight and was followed by a swarm of aftershocks, many over magnitude 3, the U.S. Geological Survey reported
The first quake struck at 36 minutes after midnight about two miles north of downtown El Centro. Many of the aftershocks were also centered within the city limits.
There were no immediate reports of injury or damage, but the quakes were widely felt in San Diego County.
Shaking continued into the morning, with a 2.7-magnitude quake just after 7 a.m.
The swarm of quakes were likely caused by the San Jacinto fault system, one of the most active faults in Southern California.