Local officials had to set up a triage tent at the Queen of the Valley Medical Center in Napa to handle the influx of the people injured in the earthquake, AP reported.
According to hospital's CEO Walt Mickens, most of the injured had cuts, bumps and bruises. Three people were also admitted with broken bones, and two for heart attacks.
The 6.0 quake struck at 3:20 am (10:20 GMT), the US Geological Survey said. Its epicenter was 6 kilometers from American Canyon and 9 kilometers from the city of Napa, at a depth of 10.8 kilometers from the surface.
Residents of the cities of San Francisco, some 40 miles away, and Davis, just over 70 miles away, quickly took to Twitter reporting feeling the quake.
"Everything was just shaking, the hanging lamps waving back and forth," said Omar Lopez, 24, night clerk at a small inn in St. Helena, 15 minutes outside of Napa. "Guests came into the front desk after the quake and they said the swimming pool looked like a bunch of people had jumped in at the same time."
Three people - two adults and one child - sustained critical wounds. At least 87 people were treated for minor injuries at Napa hospital after the earthquake, the spokesperson told AP. Most patients have cuts, bumps, bruises, said Vanessa DeGier, spokeswoman for the hospital in Napa.
The quake caused six significant fires including a blaze at the Napa Valley mobile home park, where 4 trailers were completely destroyed, according to local firefighters. Up to 16 buildings are now considered uninhabitable; access to many others is restricted.
The quake has been described as the most powerful in the area since the deadly 1989 Loma Prieta 6.9 magnitude earthquake, in which 63 people were killed and more than 3,000 injured.