LOS ANGELES, California - Northern California experienced its strongest earthquake in 25 years Sunday, with more than 120 people injured and hundreds of historic buildings in downtown Napa damaged by the 6.1-magnitude quake.

The earthquake struck just six miles southwest of Napa, but remarkably all except six of the injured had been treated and released within a few hours of the quake. Most of the injured only suffered bruises and lacerations, according to Queen of the Valley Hospital president, Walt Mickens.

The six remaining patients, including one child, had reportedly sustained critical injuries.

The damage to the city is "fairly significant", CEO of the California Earthquake Authority Glenn Pomeroy told reporters Sunday, with California Governor Jerry Brown declaring a state of emergency to free up resources to tackle the aftermath of the quake.

The city has had to put out six major fires, deal with more than 100 gas leaks, and clear downed power lines from dozens of roads, which have left 15,000 people without power. According to Napa public works director Jack Rochelle, there have also been at least 60 minor water main breaks and leaks.

All schools remained close in Napa Monday to give authorities time to clear debris and check school buildings to make sure they were structurally safe.

According to the United States Geological Service, an estimated 15,000 people felt severe shaking, 106,000 people felt very strong shaking, while 738,000 felt it moderately.

The quake struck at a depth of 7 miles and was considered "strong" by the USGS, but not 'major'.

Major quakes start at a 7-magnitude, according to the USGS scale. The centre added that the quake would likely cost the city around USD $1-billion.

More than 60 aftershocks have been felt in the hours after the quake, ranging from 0.6-magnitude to 3.6-magnitude, which was the strongest quake to hit the area since 1989.

The 6.9-magnitude Loma Prieta earthquake caused 63 deaths, 3,757 injuries and an estimated USD $6-billion in property damage.