A strong magnitude 6 earthquake has jolted California, the US Geological Survey has said, though there were no immediate reports of deaths or serious damage.

The quake struck at 3:20am local time (10:20 GMT) northwest of American Canyon, California, at a depth of just 10.8km from the surface, USGS said on Sunday.

The power was out in Napa, about nine kilometres from the earthquake's epicentre, as well as in other towns and cities nearby, affecting more than 10,000 households, according to the Pacific Gas and Electric Company.

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.

Residents of the cities of San Francisco, about 64km away, and Davis, just over 112km away, quickly took to Twitter reporting feeling the quake.

Twitter 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. #quake #sf".

Source: AFP