A Pacific Ocean earthquake registering a 6.9 magnitude struck late Sunday off the coast of Northern California.
The powerful quake struck 50 miles west of Eureka in Humboldt County, and was felt over a wide area from Oregon down to the Bay Area.
Police and fire department officials in Humboldt County said there were no reports of injuries and no tsunami warning was in effect, according to the National Tsunami Warning Center.
A further nine minor aftershocks hit following the first tremblor, registering between 2.9-4.6 magnitude, the National Earthquake Information Center said on its website.
It also said there was a 90% chance of a "strong and possibly damaging aftershock" reaching magnitude 5.0 or more over the next seven days.
One report described it as a "long, slow earthquake," and residents have reported significant shaking of buildings in Oakland, San Francisco, and Redding.
Eureka police department officials said shaking lasted up to 30 seconds.
The region is a hotbed of seismic activity, with similar magnitude earthquakes causing fatalities in recent years.
A 6.7-magnitude earthquake in Los Angeles killed 60 people in 1994 and a quake in San Francisco in 1989 claimed the lives of 67.
