A massive clean-up operation is underway after a powerful earthquake rocked California's scenic Napa Valley wine country, leaving more than 130 injured.

The 6.0-magnitude quake struck early on Sunday morning - the strongest tremor to hit the region in a quarter of a century.

Three people were seriously injured, including a child who is fighting for life after being crushed by a falling fireplace.

No deaths were reported but authorities said more than 130 people sought minor medical care.

Governor Jerry Brown declared a state of emergency in the wake of the quake, which sparked fires, burst water mains, caused gas leaks and cracked roads.

The US Geological Survey (USGS) said the earthquake was the most powerful to hit the San Francisco Bay area since the 1989 6.9-magnitude Loma Prieta earthquake.

Many residents of Napa and the surrounding area, home to some of California's most celebrated wineries, were roused from sleep in a state of panic.

Among them was David Gadlin, manager of Lucero Olive Oil shop in downtown Napa, who raced to the store to find the floor coated with olive oil and shattered glass.

"It could have been a lot worse if it happened during the day when customers and workers were inside," he said. "We will get through this."

The Queen of the Valley Medical Centre treated 125 patients for minor injuries, including cuts and bruises. Another eight received care at St Helena Hospital, according to officials.

They scrambled to flag 33 buildings as too damaged for occupancy, as portions of the city's downtown were cordoned off with yellow tape.

Fire destroyed four mobile homes and damaged two others at a trailer park in the area, while crews extinguished blazes in two other residential neighbourhoods.

60 water-main breaks occurred following the quake, which struck near American Canyon some 64kms northeast of San Francisco.

This man did not realise what was happening.