The USGS says the 1:30 p.m. tremor had a preliminary magnitude of 3.5 and the epicenter was 4 miles north of Fontana, about 50 miles east of downtown Los Angeles.
A 2.6-magnitude quake reportedly struck two minutes later in the same area.
There were no immediate reports of injuries or damage.
Eyewitness News viewers in Riverside, Pomona, Rialto, Rancho Cucamonga, Lake Elsinore reported feeling shaking from the quake.
The Associated Press contributed to this report.
