A 6.5 magnitude earthquake struck off the coast of Northern California on Saturday afternoon, shaking buildings south of the Oregon border and knocking out power in several coastal communities.

The U.S. Geological Survey said the quake hit at about 4:27 p.m. local time about 43 kilometres from Eureka, a city of about 26,000. No injuries have been reported.

Pacific Gas&Electric Co. spokesman J.D. Guidi said power outages were widespread across most of Humboldt County, affecting about 25,000 customers.

Several traffic lights have fallen and numerous residents have reported water, gas and sewer leaks, Humboldt County Office of Emergency Services spokeswoman Jo Wattle said.

"People have chimneys down, and we're hearing about minor property damage and lots of glassware broken," Wattle said. "People are really shaken up. It was shaking pretty good, then it had a big jolt to it at the end."

Police in Ferndale, the community closest to the quake's epicentre, said the jolt caused stucco to fall off City Hall and broke shop windows, strewing the historic downtown streets with glass shards.

Televisions tumbled and objects were knocked off walls in Arcata, a small town that's home to Humboldt State University.

Arcata and Eureka are located about 435 kilometres north of San Francisco in a coastal area known for periodic earthquakes.

The quake was felt as far south as Capitola in central California, and as far north as central Oregon, USGS geophysicist Richard Buckmaster said.

The National Oceanic and Atmospheric Administration said there was no threat of the quake generating a tsunami.