A 4.2 magnitude earthquake struck in southern Sacramento County on Wednesday, according to the United States Geological Survey, and some people in the Bay Area felt the shaking.
The earthquake hit at 9:29 a.m. and was centered near Isleton in southern Sacramento County, about 12.5 miles northeast of Antioch in eastern Contra Costa County, the USGS said.
No injuries or damage were immediately reported.
"The whole floor was shaking," Devin Petersen of Isleton said. "I felt it underneath my feet. It was weird. I felt waves."
The quake shook docks in Rio Vista, rattled store shelves in Isleton and caused traffic backups on the Rio Vista Bridge, which was shut down post-quake for a quick inspection.
The quake also stopped BART trains briefly as crews checked the tracks for any possible damage. The tracks and service were not affected.
The tremor triggered a warning from the West Coast's ShakeAlert system, which detects the start of a quake and sends alerts to smartphones in areas expected to experience shaking.
In Sacramento, a ShakeAlert arrived before the shaking started. Blinds swayed in The Associated Press’ Sacramento office on the ninth floor of a high-rise building.
One person near the epicenter in Isleton described the shaking as he was working at a local business.
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"We were preparing our stuff, and we just felt the shaking ... everything started moving, it caught us by surprise. Everyone just went outside," a man named Oswaldo said, adding that he received the ShakeAlert on his phone.
In Brentwood, mechanic Tim Walizer was working underneath a car 7 feet in the air when he realized he needed to take cover.
"It's an asymmetrical lift, so the legs move on there. If it scoots around too much, it definitely can fall out of the air," he said. "I’m glad it was nothing bigger."
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USGS geologist Charles Trexler said the quake’s epicenter was on the Midland fault line, which runs along the Delta. He said it's not generally known to cause major earthquakes, but it's still on experts' radar.
"The Midland fault is definitely one of the faults of concern within the Delta," Trexler said. "It's not as active as faults like the San Andreas, but it certainly is capable of producing large earthquakes. This is a good reminder that we should be thinking about earthquakes."
The quake initially was estimated at magnitude 5.7 but was downgraded.
The Associated Press contributed to this report.
