The temblor, centered in Napa, struck at 3:20 a.m. at a depth of 6.7 miles, the U.S. Geological Survey said.
It was the largest earthquake to strike the region since the 6.9-magnitude Loma Prieta temblor in 1989, which killed about 60 people.
The Valley Medical Center in Napa treated some 70 people, mostly for cuts, bumps and bruises early Sunday morning. Some have been admitted, but the severity of their injuries was unknown.
More than 42,000 people lost power in American Canyon, about 4 miles from the earthquake's epicenter, Napa, Saint Helena, Santa Rosa and Sonoma.
There was also damage to roadways. The California Highway Patrol warned drivers large cracks in routes 121 and 29 could cause flat tires.
Some homeowners in the area reported significant structural damage to their homes, while others woke up to the contents of their refrigerators and cabinets spilled on the ground.
A church in Vallejo lost its bell tower.