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SANOMA, Calif. - Officials say an earthquake with a preliminary magnitude of 6.0 has been reported in California's northern San Francisco Bay area.

Leslie Gordon of the U.S. Geological Survey says the tremor struck at just before 3:30 a.m. Sunday about 10 miles northwest of American Canyon, which is about six miles southwest of Napa. The USGS says it's the largest tremor to shake the Bay Area since the 1989 6.9 magnitude Loma Prieta quake.

The tremor set off car alarms and had residents of neighboring Sonoma County running out of their houses in the middle of night. Power was knocked out in some areas. According to Pacific Gas & Electric Company's Electric Outage map, found here, at least 15,000 people in Northern California are without power.

The USGS says the depth of the earthquake was just less than seven miles, and numerous small aftershocks have occurred in the Napa wine country.

A member of Napa County dispatch tells The Associated Press that there has been one report of structural damage, but additional details were not available.

There are no immediate reports of damage or injury.

Copyright 2014 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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