Photo: The Canadian Press. All rights reserved.

A cracked section of roadway is shown in the Carneros district following an earthquake Sunday, Aug. 24, 2014, in Napa, Calif.  (AP Photo/Eric Risberg)

A powerful earthquake that struck the heart of California's wine country caught many people sound asleep, sending dressers, mirrors and pictures crashing down around them.

The magnitude-6.0 quake struck at 3:20 a.m. Sunday near the city of Napa, an oasis of Victorian-era buildings nestled in the vineyard-studded hills of Northern California.

Scores were injured as the temblor knocked out power to thousands, caused gas and water lines to rupture and sparked fires.

By midday Sunday, the fires were out and power was starting to be restored, said Mark Ghilarducci, director of the California Governor's Office of Emergency Services.

"While it was bad, it wasn't as bad as it could be and it was very manageable from a regional perspective," Ghilarducci said.

The quake struck about six miles (10 kilometres) south of Napa and lasted 10 to 20 seconds depending on proximity to the epicenter, according to the U.S. Geological Survey. It was the largest to shake the San Francisco Bay Area since the magnitude-6.9 Loma Prieta quake struck in 1989, collapsing part of the Bay Bridge roadway and killing more than 60 people, most when an Oakland freeway fell.

It was felt widely throughout the region, with people reporting feeling it more than 200 miles (320 kilometres) south of Napa and as far east as the Nevada border.

The quake's timing was also bad for Napa Valley's famed vineyards, where winemakers were just getting ready to harvest the 2014 crop. The quake broke thousands of bottles of wine and toppled barrels.

Vintner Richard Ward of Saintsbury Winery south of Napa watched Sunday afternoon as workers righted toppled barrels and rescued a 500 pound (225 kilogram) grape de-stemmer that the quake had thrown to the ground.

"That's what happens when you're a mile from the epicenter," said Ward, who lost 300 to 400 bottles in the winery's basement.

The grape harvest was supposed to start overnight Monday, but it would now be pushed back a few days, he said. Had the harvest started a day earlier, the quake could have caught the workers among the heavy barrels when it struck, Ward said.

For many, the quake struck at the worst time possible, rousing them in the middle of the night and sending them fumbling in darkness to take cover and find loved ones. A number of the injuries were caused by people stepping on broken glass, falling down or being hit by furniture.

Omar Rodriguez, 23, of Napa, was treated for a gash on his forehead in one of the triage tents outside a hospital that handled the victims.

"We woke up to the earthquake and I thought I was dreaming because I fell off my bed, you know, it was all dark, and I just got back up. She noticed it was all bleeding," he said, referring to his girlfriend.

Officials were still assessing the damage late Sunday in hopes of getting a cost estimate they could submit for possible federal assistance. But the initial assessment found that dozens of homes and buildings in the region were unsafe to occupy, including an historic Napa County courthouse, where a 10-foot (3-meter) wide hole opened a view of the offices inside.

Queen of the Valley Medical Center in Napa reported treating 172 people in the emergency room, although hospital officials could not say how many of them were there for injuries suffered in the quake and how many for more routine injuries and illnesses, hospital CEO Walt Mickens said.

Twelve people were admitted for broken bones and other medical problems directly related to the earthquake, including an adult who remained in critical condition on Sunday night and a 13-year-old boy.

The teen was hit by flying debris from a collapsed fireplace and had to be airlifted to the children's hospital at the University of California Davis Medical Center for a neurological evaluation. He condition was listed as serious, hospital spokeswoman Phyllis Brown said.