document: The USGS said it struck at 03:20 local time (10:20 GMT) four miles (6km) north-west of the town of American Canyon, at a depth of 6.7 miles.
  At least 87 people have been taken to hospital in the Napa area, a renowned wine-producing region and tourist area.
  California Governor Jerry Brown has declared a state of emergency in order to deal with the effects of the quake.
  Officials in Napa said in a statement that the quake - believed to be the biggest in the region in 25 years - had destroyed four mobile homes and made 16 buildings "uninhabitable", as well as causing "approximately 50" gas main breaks and around 30 leaks from water mains.
  The clear-up operation will begin in earnest on Monday.
  At least three of those injured in Napa are in a serious condition.
  Three historic buildings in the town had been hit and two commercial buildings "severely damaged", the statement went on.
  By Sunday evening, between 11,000 and 15,000 people were still reported without power, down from 70,000 people in the immediate aftermath of the earthquake.
  At least 33 buildings in the city of 77,000 have been "red-tagged", which means that they have been designated as unsafe to enter, Community Development Director Rick Tooker was quoted by the Reuters news agency as saying.
  A Red Cross evacuation centre has been set up in a church in the city.
  The California Highway Patrol in the San Francisco Bay area tweeted that it was "checking over crossings and bridges for obvious signs of structural integrity", and asked residents to report any signs of problems.
  "They say [the quake] went for 50 seconds. It felt like 50 minutes. I was just too terrified to even scream," antique shop owner Patricia Trimble told Reuters.
  The front window of her shop was destroyed, with cabinets on their sides and merchandise strewn across the floor.
  The site of the earthquake was 51 miles from Sacramento and about 30 miles north-east of San Francisco.
  Numerous small aftershocks were also reported on Sunday.
  California lies on the San Andreas Fault, which forms the tectonic boundary between the Pacific Plate and the North American Plate, two of the large moving plates that form the Earth's crust.
  The San Francisco earthquake of 1906 and subsequent fires devastated the city.
  In 1989, a powerful earthquake struck San Francisco, killing more than 60 people and injuring hundreds.
summary: An earthquake with a magnitude of 6.0 has shaken northern California, causing injuries and damage to buildings.