California's governor declared a state of emergency Sunday following a strong 6.0-magnitude earthquake that seriously injured three people including a child and ignited fires in the scenic Napa valley wine region.

The US Geological Service said that the quake was the most powerful to hit the San Francisco Bay area since the 1989 6.9-magnitude Loma Prieta earthquake.

The quake sent at least a top corner of a brick building tumbling into the street in Napa. Elsewhere it ignited fires, burst water mains and cracked roads in the area.

No deaths were reported but Napa city officials said three people suffered serious injuries after the quake struck at 3:20 am.

Among them was a child who was critically injured by a fallen fireplace and had been flown to a hospital for neurological care.

Queen of the Valley Medical Center in Napa told AFP it had treated 89 patients for minor injuries such as cuts and bruises and that all were "non-life-threatening."

Meanwhile fire destroyed four mobile homes and damaged two others at a trailer park in the area, and officials said a fire broke out at another mobile home park as well.

The city of Napa told AFP that a family home had also caught fire.

Issuing a state of emergency, Governor Edmund Brown said that "the circumstances of the earthquake, by reason of their magnitude, are or are likely to be beyond the control of the services, personnel, equipment and facilities of any single local government."

- Damage reports -

Napa city officials said they had had 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 infrastructure standpoint is the water main system."

The Napa city official said there had been around 30 reports of water main leaks or breaks following the quake, which struck near American Canyon some 40 miles (64 kilometers) northeast of San Francisco, according to USGS.

It roused people from sleep as far away as San Francisco, and was felt as far east as Sacramento and as far south as Santa Cruz.

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.

USGS issued an "orange alert" for possible damage, a rating which means "significant damage is likely and the disaster is potentially widespread."

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.

"Cracks in road may cause flat tires," it said on its Twitter account.

Meanwhile sections of streets were closed in Napa due to glass on the road.

And power was still knocked out to around 20,000 customers in the area, home to California's most celebrated wine producing regions, according to the Pacific Gas and Electric Company. Brown said 69,000 people lost power in all.

The power company said it had sent crews out to assess the damage and restore power, and estimated electricity would be back up within several hours.

- 'Earthquake country' -

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.

"Anytime 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.

Ceebee Thompson and Jack Fairchild were shaken from slumber to see chandeliers swinging and cabinets flung open in their house in the Wine County town of Calistoga about 30 miles from the quake's epicenter.

"That was pretty scary," Thompson said. "I've been in an earthquake in Oakland and a nasty one on Guam, and this was right up there with the one in Guam."

She described the quake as a long rolling shudder of the earth's crust, marked by jolts.

Napa's historic downtown featuring vintage brick buildings was hard-hit, with facades breaking and windows shattering.

Residents in the quake zone, including as far away as San Francisco and Davis, just more than 40 miles northeast, quickly took to Twitter.

One user Tyson Winter wrote: "Shook violently here in Napa. Power's out."

And in San Francisco, Om Malik posted "Damn, woke me up. Was super long."

Further north, Tim Kerbavaz tweeted: "Felt it in Davis too. Quite long."