Vanessa DeGier, spokeswoman for Queen of the Valley hospital in Napa, says the facility has treated more than 70 people, most for cuts, bumps and bruises.
She says the facility has treated a hip fracture and heart attack, but it's unclear if it was related to the 6.0-magnitude earthquake.
The hospital has set up a triage tent and many people are still coming in, DeGier said.
The quake caused six significant fires, including at four mobile homes, Napa Division Fire Chief Darren Drake said. The damage from the fires is not yet clear but it appears significant, he said.
Several other smaller fires have been reported and firefighting efforts have been complicated by broken water mains.
The earthquake struck just before 3:30 am local time about 6 miles (10 kilometers) southwest of Napa, in California wine country, Leslie Gordon of the US Geological Survey said. It's the largest earthquake to shake the Bay Area since the 6.9-magnitude Loma Prieta quake in 1989, the USGS said.
"There's collapses, fires," said Napa Fire Capt. Doug Bridewell, standing in front of large pieces of masonry that broke loose from a turn of the century office building where a fire had just been extinguished. "That's the worst shaking I've ever been in.
