Damage to the Napa County Superior Court building is seen after an earthquake in Napa, California August 24, 2014. The 6.0 earthquake rocked wine county north of San Francisco early Sunday, injuring dozens of people, damaging historical buildings, setting some homes on fire and causing power outages around the picturesque town of Napa. [Photo/Agencies]

'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, and on its website the city reported 30 water line breaks and many broken windows and buildings with interior damage.

The quake 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.

"They say it went for 50 seconds. It felt like 50 minutes. I was just too terrified to even scream," said antique store owner Patricia Trimble, 50. 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.

The U.S. Geological Survey (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.

'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.

Sunday's quake was strong enough to be felt throughout the Bay Area.

"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, south of the area where the quake was felt more strongly.

"Nothing tipped over, thank God. Rolling back and forth. Just woke us all up," she said.

Aaron Moreno, 18, rushed to Lola's Market, the Mexican grocery store where he works in Napa, right after the earthquake to prevent potential looting and to begin cleaning up the mess. He said there were broken wine bottles and glass everywhere and the store would probably be closed for two days.

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.

President Barack Obama, who is on his last day of vacation at Martha's Vineyard, Massachusetts, was briefed about the quake and would continue to receive updates, the White House said.