The quake had a magnitude of 4.9 and was centered near Yucaipa in San Bernardino County, east of Los Angeles, according to the U.S. Geological Survey. Witnesses said the quake wasn't strong at the outset but quickly grew in strength.
"We just had a few items falling, that's all," said Andrea Cabrera, an employee at the Walgreens drug store in Yucaipa. "The customers were just stunned, and they just stood there."
The Los Angeles Fire Department received no immediate reports of major damage, spokesman Brian Humphrey said.
It was the third significant quake to hit California this week: A magnitude-5.2 quake shook Riverside County on Sunday, and a magnitude-7.0 quake struck Tuesday under the ocean 90 miles off Northern California. U.S. Geological Survey seismologist Susan Hough in Pasadena said she believed the latest quake was likely triggered by Sunday's temblor.
"The shaking from the Anza earthquake sort of juggled the fault in this other area that probably caused this earthquake to happen," Hough said.
Channon Kelly, 31, was eating her lunch in downtown Los Angeles when Thursday's quake hit.
"I almost jumped out of my seat," Kelly said. "I'm starting to get freaked out. We've had so many in the last week, the one Sunday and then in Northern California. I could hear the windows rattling and feel it all at the same time."