A 6.0-magnitude earthquake rocked California's scenic Napa Valley wine country Sunday, the strongest to hit the region in a quarter of a century, seriously injuring three and jolting thousands from their sleep.

No deaths were reported. But authorities said some 130 people sought minor medical care and that a child was in critical condition after being crushed by a fireplace.

Governor Jerry Brown declared a state of emergency in the wake of the 3:20 am quake, which sparked fires, burst water mains, caused gas leaks and even cracked roads.

The US Geological Survey said the temblor 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 when the ground started shaking.

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, vinegar 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."

Napa city officials said three people suffered serious injuries, including a child who had to be airlifted to a hospital for neurological care.

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

They scrambled to flag 33 buildings, including a senior center, 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 neighborhoods.

- Deluge of damage reports -

As inspectors hustled from building to building to check safety and risk of further collapse, Napa city officials said they had received more than 100 phone calls from people reporting gas leaks.

However, city public works director Jack LaRochelle said "the big thing we're looking at from a public-works and infrastructure standpoint is the water-main system."

The Napa city official said 60 water-main breaks occurred following the quake, which struck near American Canyon some 40 miles (64 kilometers) northeast of San Francisco, according to USGS.

LaRochelle said that 20 of the breaks had been isolated and shut off and that five teams were working in 12-hour shifts to restore water to areas where it didn't exist or had minimal pressure.

He added that Napa's roads were in good shape with only a handful of "buckling streets, but nothing that's really serious enough to cause us to close a road. Our bridges are in pretty good shape."

Dorothy Roberts, Napa city clerk, told AFP multiple reports of structural damage had come in and that brick buildings in particular had been hard-hit.

Power remained knocked out to around 15,000 customers in the area, according to the Pacific Gas and Electric Company.

Some households were also experiencing water problems.

"Water remains safe to drink ... some areas have no water due to main breaks and some areas no or low pressure," a Napa city statement said.

- 'Earthquake country' -

The earthquake was felt as far away as San Francisco, as far east as Sacramento and as far south as Santa Cruz.

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

A handful of small aftershocks had already been recorded by USGS.

"Any time we have an earthquake, whether it's magnitude 6.0 or smaller, they serve as a good reminder that we do live in earthquake country and need to be prepared," USGS spokeswoman Susan Garcia told AFP.

As the dust cleared, merchants covered broken windows with plywood and cleaned glass shards and rubble from sidewalks.

At the Golden Owl Tattoo shop, the shaking took an unexpected toll on a personal collection of paintings, skulls and taxidermy pieces.

"You can't put a price on a 65-year-old African elk skull," owner Donavan Kinyon said.