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.
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