A 6.0-magnitude earthquake has rocked the northern San Francisco area, injuring dozens of people, damaging historic buildings, setting some homes on fire and causing power outages around the picturesque town of Napa.

The biggest quake in the region in 25 years jolted many residents out of bed when it hit at 3:20am local time on Sunday, centred 10km south of Napa, California.

Three people were seriously injured, including a child who had multiple fractures after a fireplace fell on him, local fire service chief John Callahan told the Reuters news agency.

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

The local Queen of the Valley hospital said it had treated 89 patients.

Most damage appeared centred around Napa, a famous wine-producing region and a major tourist destination in northern California.

The quake knocked out power to about 40,000 homes and businesses in Napa and neighbouring cities of Sonoma, St Helena and Santa Rosa, according to the website for Pacific Gas & Electric.

"They say it went for 50 seconds. It felt like 50 minutes. I was just too terrified to even scream," said antique store owner Patricia Trimble, 50.

The US Geological Survey (USGS) said the epicentre of the quake was eight kilometres northwest of the town of American Canyon, on the northern edge of the San Francisco Bay.

The quake was the largest to hit the Bay Area since the Loma Prieta quake in 1989, which killed 63 people and caused $6bn in property damage.

That quake measured 6.9, while the famous one that levelled San Francisco in 1906 measured 7.8.

Aftershocks can continue for the next several weeks.

Source: Agencies