That quake hit just after 9:30 a.m. in the Muscoy area at a depth of just over three miles, according to the U.S. Geological Survey. A few minutes later, a smaller 3.1-magnitude temblor shook the Rialto area nearby.
About an hour before that, the Rialto area saw two smaller quakes - magnitude 3.0 and 2.8. Experts say these were likely foreshocks.
WATCH | Caltech seismologist describes seismic activity after 4.3 IE quake
Caltech seismologist describes seismic activity in Fontana area after 4.3 earthquake
Dr. Gabrielle Tepp, a seismologist at Caltech, says the area where Thursday's quake hit is one that has seen similar sized quakes before.
"This is an active area... This area is also near where the San Jacinto fault, which is one of the major Southern California faults, comes up and meets the San Andreas. So this is an area where we see seismicity, especially the magnitude 4s."
There was last a magnitude 4 earthquake in the same area in January of last year, Tepp said.
People in Fontana react to 4.3 earthquake
People reported feeling shaking in many areas including Jurupa Valley, Eastvale, Ontario and even as far as Redondo Beach. Most callers described it as a jolt or sudden movement.
"I felt it in Garden Grove, in my apartment on the 7th floor," Gregory Christison said on Facebook. "It was a rolling sensation."
Cindy Hecht Sedbrook said she was on the 11th floor of a building in Irvine when she felt the quake.
No damage or injuries were immediately reported.
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Thursday's SoCal quakes were not nearly as powerful as Wednesday's 8.8 magnitude monster earthquake in Russia, which triggered tsunami warnings in Hawaii and here along the West Coast. According to Caltech, Thursday's shakers is not related.
Caltech says more than 100,000 alerts were sent out over the Shake Alert app to cellphones.
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