There were no immediate reports of injuries or damage, San Diego Sheriff’s Lt. Andrea Arreola said.
“At this point, we are just monitoring” the area, she said.
There were also no immediate reports of damages in Riverside County.
But the earthquake triggered a minor rockslide on Montezuma Valley Road, about 14 miles southeast of Borrego Springs, and California Highway Patrol officers were on scene clearing the area, the CHP reported.
The quake’s epicenter was 13 miles north-northwest of Borrego Springs in San Diego County and 16 miles south-southwest of La Quinta in Riverside County. It was strong enough to be felt in Los Angeles.
The earthquake occurred along the San Jacinto Fault, historically the most active fault in Southern California, according to seismologist Lucy Jones. It was near a magnitude-6 earthquake in 1937 and a magnitude-5.3 earthquake in 1980, Jones reported.
“We have never seen a San Andreas earthquake triggered by a San Jacinto earthquake,” Jones wrote on Twitter, referring to the state’s most famous fault, the one along which the so-called “big one” is expected to hit someday.
“Every earthquake has a 5 percent of triggering an aftershock that is bigger than itself — always within a few miles of location of the first earthquake,” Jones wrote.
Within an hour and 20 minutes, there were seven aftershocks in the same general area. The strongest were magnitude-3.5 shakers at 1:06 a.m. and at 1:33 a.m. with roughly the same epicenter as the main quake but at depths of 6.7 miles and 6.2 miles, respectively, according to the USGS.
Actress McKaley Miller, who was raised in Texas, tweeted in response to the main quake, “I literally thought a ghost was pushing my bed, but thanks to Twitter, I now know it was just an earthquake… I’ve never been so scared.”