The quake, initially measured at magnitude-5.4, was centered near Borrego Springs and felt from San Diego to Los Angeles. There were no initial reports of major damage.
(Susanne Whatley talks to KPCC's Inland Empire reporter Steven Cuevas.)
The United States Geological Society reported the quake struck at 4:53 P.S.T. No reports yet of major damage to structures in the Los Angeles area.
Borrego Springs is an unincorporated area of northeast San Diego County.
Borrego Springs Resort and Spa marketing director Kim Daniel says the earthquake was the strongest that she's felt in 13 years of living in Southern California.
The earthquake also knocked a few glasses off shelves at Carlee's Place in downtown Borrego Springs. Bartender Tony Wann says there was no apparent damage other than rattled nerves.
KPSP-TV new media producer Renee Schiavone says the earthquake shook her station's Palm Springs newsroom for about 15 seconds, knocking things off shelves and sending workers scurrying under their desks.
USGS seismologist Kate Hutton told reporters at Caltech in Pasadena that the quake likely occurred in the San Jacinto fault zone, and it was followed by about two dozen small aftershocks, the largest a magnitude-3.6.
She said the quake was likely not an aftershock to the magnitude-7.2 temblor centered near the Mexican border that rattled much of Southern California in early April.
"This earthquake, probably, although it's pretty close in time after the April earthquake, it's probably not an aftershock because it's located in a different location,'' Hutton said. "But it probably is what we would call a triggered earthquake, because we're now thinking that the 7.2 earthquake in
April changed the strain slightly in the San Jacinto fault area and the Elsinore fault area, and it increased the number of small earthquakes that were happening there. And this is an example of an earthquake that's like that.''
Hutton said residents should take today's quake as a warning to be prepared for temblors.
The temblor was felt strongly in downtown San Diego, and it rattled buildings in the Coachella Valley in Riverside County. It also rattled buildings in West Los Angeles, West Hills, Hollywood and Palmdale.
There were no reports of any damage or injuries, according to the Los Angeles city and county fire departments.
Disneyland spokeswoman Suzi Brown says rides are being inspected before guests are allowed back on after the magnitude-5.4 quake rolled through the Anaheim theme park.
Brown says some of the rides are coming back in operation in a matter of minutes, while others are taking a little longer.
Airline traffic at Los Angeles International Airport continues to run as scheduled, an LAX spokesperson said.
The shaker was followed by a series of aftershocks in the Borrego Springs area, most measuring around magnitude-3.0.
(Did you feel the earthquake? Tell us.)