A strong 6.0-magnitude earthquake jolted California early Sunday morning, the US Geological Survey (USGS) said, though there were no immediate reports of deaths or serious damage.
The quake struck at 3.20 am (1020 GMT) near American Canyon, 40 miles northeast of San Francisco, USGS said.
There was a "low likelihood for casualties," it said, but issued an "orange alert" for possible damage, a rating which means "significant damage is likely and the disaster is potentially widespread".
An aftershock of 2.6-magnitude hit about 30 minutes afterwards, it said.
USGS expert Jessica Turner told KCBS radio that aftershocks of up to 5.0 are likely in the next week.
Early reports suggested some damage closest to the earthquake's epicenter, but no reports of anything major.
However, power outages, affecting more than 28,000 households were reported in Napa, about six miles from the epicenter, as well as in other towns and cities in the region, according to the Pacific Gas and Electric Company.
It said it had sent crews out to assess the damage and restore power, and estimated electricity would be back up within several hours.
The California Highway Patrol in the San Francisco Bay Area tweeted that it was "checking over crossings and bridges for obvious signs of structural integrity", and asked residents to report any signs of problems.
They closed a bridge near Vallejo on Highway 37 while inspecting for possible damage, KCBS reported.
Residents in the quake zone, including as far away as San Francisco and Davis, just over 40 miles northeast, quickly took to Twitter.
One user Tyson Winter wrote: "Shook violently here in Napa. Power's out."
Ann Marie Christy wrote that her mother, also in Napa, said the shaking "was very violent with a lot of broken glass".
And in San Francisco, Om Malik posted "Damn, woke me up. Was super long."
Further north, Tim Kerbavaz tweeted: "Felt it in Davis too. Quite long."