(Adds comments from city officials, updates power outages; schools closed; adds link to map)
By Jim Christie
NAPA, Calif., Aug 24 (Reuters) - A 6.0 magnitude earthquake rocked wine country north of San Francisco early on Sunday, injuring dozens of people, damaging historic buildings, setting some homes on fire and causing power outages around the picturesque town of Napa.
The quake, the biggest in the region in 25 years, jolted many residents out of bed when it hit at 3:20 a.m. (1020 GMT). It was centered 6 miles (10 km) south of the city of Napa.
There were no known fatalities, but three people were seriously injured, including a child who suffered multiple fractures after a fireplace fell on him, local fire battalion chief John Callahan said. Six fires broke out, including one that consumed six mobile homes, he said.
Queen of the Valley Medical Center in Napa said it had treated 89 patients.
Although the quake was only moderately strong, it shook up residents, said Barry Martin, community outreach coordinator for Napa, a city of 77,000 located about 50 miles northeast of San Francisco.
The quake was strong enough to be felt throughout the Bay Area.
"This was a pretty big jolt in Napa, but it certainly is not 'The Big One,'" Martin said on local television, referring to fears Californians have of a catastrophic quake. (Graphic on the Napa quake: http://link.reuters.com/tud72w)
California, which sits along a series of seismic faults, is forecast to experience a much more powerful earthquake at some point, but scientists do not know when it might come or how strong it would measure, a geophysicist with the U.S. Geological Survey (USGS), Don Blakeman, said.
"Usually when people talk about 'The Big One,' they're talking about something on the order of a magnitude 9, which of course is tremendously more powerful" than Sunday's quake, he said.
California Governor Jerry Brown declared a state of emergency, putting all state resources at the disposal of his Office of Emergency Services.
The biggest impact appeared centered around Napa, a famous wine-producing region and a major tourist destination in northern California, where about 15 buildings were damaged.
Napa City Manager Mike Parness said it could take a full week before the city was fully restored.
"We're seeing people coming together and helping people and getting buildings back on line as soon as possible," Parness told a midday news conference.
One building housing winery tasting rooms had to be closed to tourists, and the floors of many wine stores were stained red from the contents of broken wine bottles.
Tyler Paradise, general manager of Cult 24 wine bar in Napa, estimated his business lost $50,000 worth of bottles. They were shaken out of their holding cabinets and littered the floor.
USGS said the epicenter of the quake was 5 miles (8 km) northwest of the town of American Canyon, on the northern edge of the San Francisco Bay.
"TOO TERRIFIED TO EVEN SCREAM"
Brick facades gave way in the historic section of downtown Napa, and bricks fell off a second floor corner of the courthouse, which showed cracks. On the main street, masonry collapsed onto a car.
Callahan said officials had responded to more than 100 calls regarding problems with gas lines and power lines. The city, on its website, reported 30 water line breaks and many broken windows and buildings with interior damage.
The quake initially knocked out power to about 40,000 homes and businesses in Napa and neighboring cities of Sonoma, St. Helena and Santa Rosa, according to the website for Pacific Gas & Electric. By midday only about 20,000 customers, mostly in Napa, were still without power.
"They say it went for 50 seconds. It felt like 50 minutes. I was just too terrified to even scream," said Patricia Trimble, 50, the owner of an antique store. She rushed to her store in central Napa and found the front window blown out, cabinets on their sides and merchandise littering the floor.
As dawn broke, merchants were on the streets sweeping up debris and boarding up windows.
"WOKE US ALL UP"
The quake was the largest to hit the Bay Area since the Loma Prieta quake in 1989, which killed 63 people and caused $6 billion in property damage. That quake measured 6.9, while the famous one that leveled San Francisco in 1906 measured 7.8.
"It was long. I think it was the biggest one since I felt the 1989 quake," said Stephanie Martin, 47, a nursing assistant in Oakland, which sits just across the bay from San Francisco.
"Nothing tipped over, thank God. Rolling back and forth. Just woke us all up," she said.
City officials said all 30 Napa Valley Unified School District buildings will be closed to students on Monday while structural engineers assess damage to school buildings. Officials said damage included broken windows and computers that fell but did not offer details on larger structural damage.
The quake was followed by small aftershocks of about magnitude 2, USGS geophysicist Randy Baldwin said.
USGS's Blakeman said there was about a 50 percent chance there would be an aftershock of at least magnitude 5 within the next week.
"That's the thing people need to worry about if their house is already damaged, because magnitude 5 can cause more problems," he said.
(Additional reporting by Fiona Ortiz and Robin Respaut; Writing by Peter Henderson, Dina Kyriakidou and Eric Beech; Editing by Leslie Adler)