A strong 6.0-magnitude earthquake jolted northern California early this morning, knocking out power and causing some damage to roads in Napa and Sonoma counties.

There were no immediate reports of deaths, injuries or major damage from the quake, which struck at 3.20 am (11.20am Irish time), rousing people from sleep as far away as San Francisco.

The epicenter was near American Canyon, 64km northeast of San Francisco, the US Geological Service 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 2.6-magnitude aftershock hit about 30 minutes afterwards.

USGS expert Jessica Turner told KCBS radio that aftershocks of up to 5.0 are likely in the next week. 

The California Highway Patrol closed several off-ramps to highways and at least two roads in Napa Valley because of "significant roadway damage."

Drivers were cautioned to be careful at an intersection of two state roads in Napa Valley.

Power was knocked out to nearly 50,000 customers in Napa and Sonoma, which anchor two of California's most celebrated wine producing regions, 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.