This comes just 5 days after a 3.1 magnitude quake struck around that same area.
Measured at a depth of about 3.54 miles, the 3:51 a.m. quake was centered southeast of Ontario, according to the U.S. Geological Survey. The agency initially reported the temblor as a 4.3.
There were no immediate reports of significant damage.
Eyewitnesses reported feeling shaking in Ontario, Pomona, La Verne, Riverside and Anaheim Hills.
A magnitude 2.5 aftershock struck the area at 4:04 a.m., the USGS said.
WATCH | How to make your own earthquake kit
How to make your own earthquake kit