A 4.8 earthquake lightly shook most of San Diego County. ‘Is it a foreshock to something bigger? No one knows.’

A magnitude 4.8 earthquake occurred near El Centro in Imperial County early Monday, producing shaking across much of San Diego County, including in coastal cities, according to the U.S. Geological Survey.

The agency said the mainshock at 12:36 a.m. was followed six minutes later by a magnitude 4.5 aftershock and a series of smaller aftershocks, all of which originated west of the Salton Sea.

Nearly a dozen aftershocks in the 3.0-to-3.9 range had occurred through 3:30 p.m. Monday.

The epicenter of the mainshock was 11.7 miles deep, at a point roughly 113 miles east of San Diego. The temblor occurred on the Weinert fault, a branch of the San Jacinto fault system, one of the most active in Southern California.

“I don’t recall a swarm of aftershocks like this ever occurring on the Weinert,” said Tom Rockwell, a San Diego State University geologist who has studied that region for decades. “Is it a foreshock to something bigger? No one knows.”

The Weinert fault, located beneath agricultural land in the El Centro area, contributed to the 6.6 Superstition Hills earthquake on Nov. 23, 1987. That temblor was mostly caused by the larger San Jacinto system.

The fault system runs through parts of Imperial, San Diego, Riverside and San Bernardino counties. On Dec. 25, 1899, the San Jacinto produced a magnitude 6.7 quake that was felt throughout much of Southern California.