The biggest quake in the region in 25 years jolted many residents out of bed when it hit at 3.20am local time (10.20am GMT), centered six miles south of Napa city.
Two people were seriously injured, Barry Martin, public information officer for Napa, said.
There were no reports of any fatalities, Martin said.
Firefighters were still tackling fires, he added.
Most damage appeared centered around Napa.
Unreinforced masonry buildings in downtown Napa including the historic courthouse and the library suffered major damage, city officials said.
"Everything was just shaking, the hanging lamps waving back and forth," said Omar Lopez, 24, night clerk at an inn in St Helena, a small town just north of the city of Napa.