The strongest earthquake to hit northern California in a quarter of a century has caused widespread damage and injured at least 100 people. 

A state of emergency is in force and rescue crews are assessing how to make safe damaged buildings. 

The 6.0 magnitude earthquake caused much damage to the town of Napa in California's wine country, north of San Francisco.

The temblor damaged historic buildings, set some homes on fire and caused power and water outages.

It was centred 10km south of the city of Napa.          

There were no known fatalities, but three people were seriously injured, including a child who suffered multiple fractures after a fireplace fell on him, local fire battalion chief John Callahan said.

Six fires broke out, including one that consumed six mobile homes, he said.

At least 33 buildings in the city of Napa, a city of 77,000, had been "red-tagged," meaning they were unsafe to enter, said Napa Community Development Director Rick Tooker.

Inspectors had accessed about a third of all structures and planned to complete the work today.

Napa's Queen of the Valley Medical Centre said it had treated 172 patients injured in the quake.             

California Governor Jerry Brown declared a state of emergency, putting all state resources at the disposal of his Office of Emergency Services.

The quake was felt throughout the San Francisco Bay area.

The state, which sits along a series of seismic faults, is forecast to experience a much more powerful earthquake at some point, but scientists do not know when it might come or how strong it would measure, a geophysicist with the US Geological Survey (USGS), Don Blakeman, said.

"Usually when people talk about 'The Big One,' they're talking about something in the order of a magnitude 9, which of course is tremendously more powerful" than this quake, he said.