The most powerful earthquake in 25 years hit the Francisco Bay Area Sunday morning at about 3:30 a.m. damaging homes and causing power outages.

Initial reports said there were only a few casualties.

The U.S. Geological Survey (USGS) said the earthquake that hit northern California measured a magnitude 6.0 on the Richter Scale and lasted some 30 seconds.

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It noted that the tremor hit about 10 miles northwest of American Canyon and some 6 miles southwest of Napa. USGS said the quake was the strongest to strike the Bay Area since the 6.9 magnitude Loma Prieta earthquake in 1989 that killed 63 persons.

USGS said the tremor's epicenter was less than 7 miles deep. It reported some 36 aftershocks, the strongest being a magnitude 3.6, following the earthquake.

USGS reported some 2.3 million people from the Santa Cruz area to Wine Country were affected by the quake.

Government officials reported widespread power outages in Sonoma County. The quake triggered car alarms and forced residents of Sonoma County to flee their homes in the middle of night and head into the darkened streets.

The Napa City government said the city library and the historic Chinese laundry building suffered significant damage. Bricks and glass were scattered across downtown sidewalks along Second and Third Streets.

Several people in Napa were injured by broken glass.

The earthquake was categorized as "severe" by the USGS that said an earthquake this powerful is not uncommon in the area historically.

It said that after 1969, earthquakes with a magnitude of 6 occur every few years. It noted that a pair of 5.7 and 5.8 quakes shook Santa Rosa in 1969; a 5.8 and 5.9 hit in Livermore in 1980 and a 6.2 hit Morgan Hill in 1984.