A strong earthquake shook parts of Northern California early on Tuesday, cutting off power to thousands and causing damage to buildings and roads.

Two injuries were reported after the magnitude 6.4 earthquake occurred at 2.34am near Ferndale, a small community about 210 miles (345 kilometres) northwest of San Francisco and close to the Pacific coast.

The epicentre was just offshore at a depth of about 10 miles (16 kilometres). Numerous aftershocks followed.

The Humboldt County Office of Emergency Services initially tweeted that there were reports of “widespread damages to roads and homes,” but authorities subsequently indicated that damage was less than what might be expected from the size of the quake.

Two injuries were reported but both people were expected to recover, county sheriff’s information specialist Samantha Karges said.

No fatalities were immediately reported, said Brian Ferguson, a spokesperson for the California Governor’s Office of Emergency Services.

There is “some damage” to buildings and infrastructure, and two hospitals in the area lost power and were running on generators, but the scale of the damage appeared to be “minimal”, he said.

Authorities closed a bridge in Ferndale that was showing damage. The state highway department tweeted a photo showing crumpled pavement.

Some gas leaks were also reported and more than 70,000 customers lost power in the area, according to poweroutage.us.

No tsunami was expected, the Humboldt County Sheriff’s Office tweeted.

The county of 136,000 residents is in a region of the state that has a long history of large earthquakes, including a magnitude 7.0 in 1980 and a 6.8 in 2014, according to the California Earthquake Authority.

A city of Eureka resident Dan Dixon, 40, said he and his wife were sleeping when it jolted them awake and shook everything, throwing pictures in their home to the ground.

“It was probably the most violent earthquake we have felt in the 15 years I have lived here,” he said. “It physically moved our bed.”

The quake triggered the West Coast’s warning system that detects the start of a quake and sends alerts to cellphones in the affected region that can give people notice to take safety precautions before strong shaking reaches them.

About 270,000 people received notifications early Tuesday, said Mr Ferguson, the Cal OES spokesperson.

The earthquake came just days after a small magnitude 3.6 earthquake struck the San Francisco Bay Area, causing minor damage.

That earthquake was centered in El Cerrito, about 16 miles (25-kilometres) from San Francisco.