OAKLAND, Calif. — A magnitude 4.2 earthquake rattled residents in Northern California on Wednesday, prompting a "ShakeAlert" across the region.

The quake hit near the small community of Isleton in Sacramento County around 9:30 a.m. Wednesday morning, according to the U.S. Geological Survey. There were no immediate reports of injuries or damage.

Isleton city manager Chuck Bergson told KCRA-TV he felt some rumbling at City Hall during the quake and that some levees along the Delta appeared sound.

"There was nothing major with this one," Bergson said.

Wednesday's earthquake comes a day before the annual Great ShakeOut, a global drill where emergency systems will be tested for earthquake preparedness. As a part of this, thousands of MyShake app users will get an earthquake test alert on Thursday.

The quake also occurred one day after the 34th anniversary of the Loma Prieta earthquake that rocked the San Francisco Bay Area in 1989, killing 63 people and injuring nearly 3,800 others. The devastation caused up to $10 billion in damage.

Wednesday's quake was felt in the San Francisco Bay Area suburbs including Antioch, Concord, Fairfield, Martinez, Orinda, Danville, and even Berkeley, the home of the University of California.

As a result, a "USGS ShakeAlert" was sent to potentially millions of residents in Northern California, stretching from as far north as Sacramento to San Francisco and further down south to San Jose and Silicon Valley.

"Earthquake Detected! Drop, Cover, Hold on. Protect Yourself!" the alert said. Any earthquakes above 4.0 will trigger an alert, the USGS said.

The quake also briefly shut down Bay Area Rapid Transit (BART) train service in the area.

Alert made quake 'bigger than it was'

While the quake didn't cause any destruction, the alert surely attracted the attention of millions, said Christine Goulet, director of the USGS' Earthquake Science Center in Los Angeles.

But that's the point, Goulet said as preliminary magnitude estimates ranged from a 5.7 magnitude earthquake on the MyShake app to a 4.6 magnitude quake initially reported on the USGS site.

"There was a seismic shake, and in this case, one extremely close to the quake itself. A longer part of the shake was initially detected, and that triggered a wider area that was alerted," Goulet said. "It made the event appear bigger than it was."

'Tradeoff between accuracy and speed'

Goulet said the wider alert was not a flaw, but part of the alert's design.

"This is the tradeoff sometimes between accuracy and speed," Goulet said. "It is designed to alert as many people as quickly as possible. The more we wait, the less time we have for an alert."

And, unlike major natural disasters such as wildfires, tropical storms, and hurricanes, which could be considered seasonal, "earthquake season is 24 hours, seven days a week," said Goulet. She concluded that Wednesday's alert may have startled more people than intended.

"But it's way better to be safe than sorry," Goulet said. "We know it might be stressful for some, but the alert is meant to save lives, to drop, cover, hold on, and please get in a secure space."

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