A magnitude 4.0 earthquake that struck in the Santa Ana Mountains on Thursday morning was felt throughout much of Southern California, from northern L.A. County to San Diego.
The earthquake struck at 2:09 a.m., centered along the boundary of Orange and Riverside counties.
The temblor was located near the Elsinore fault but not on it, seismologist Lucy Jones tweeted. "Many small quakes are on … small faults, too small to be mapped," she said.
The Elsinore fault zone is one of the largest in Southern California, according to Caltech. The last major earthquake on that fault was in 1910, a magnitude 6 event that was close to Thursday's epicenter, the university says on its website; the Elsinore fault's northern end splits into the Chino and Whittier faults.
Residents reported light shaking in the San Fernando Valley, Long Beach, Mission Viejo, Riverside, San Bernardino and San Diego.
On Twitter, residents throughout the Southland reported shaking.
8:35 a.m.: This article was updated with more information on the epicenter, nearby faults and a quote from Lucy Jones.
4:00 a.m.: This article was updated with the magnitude downgraded to 4.0.