The first 3.5 magnitude earthquake struck around 10:05 a.m. and was centered about three miles southeast of Ontario, USGS said.
It was followed by another 3.9 magnitude quake centered east-southeast of Ontario at 10:34 a.m. almost 30 minutes after the first.
At 11:45 a.m., a 2.6 magnitude aftershock struck about two miles southeast of Ontario.
The USGS also recorded dozens of smaller quakes, between magnitudes 1 and 2 throughout the day.
"All of a sudden everything shook," said Lucy Campa, a store clerk in Ontario. "I felt like the truck was coming through the store. Especially with all the merchandise, it felt a lot stronger."
"That second one hit and it was just, it was a bigger one, it was more noticeable and everyone froze for a second," said Gaby Galdamez of Ontario.
There were no immediate reports of significant damage.
Eyewitnesses reported feeling shaking in Ontario, Fontana, San Bernardino, Riverside, Pomona and Chino.
"Today we are having a small swarm in the Fontana Seismicity Lineation, a northeast-striking line of many small quakes," seismologist Dr. Lucy Jones said in a post on X.
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