Seismologists at the US Geological Survey said the shallow 7.0 magnitude tremor had hit around 68 kilometers (42 miles) from the coastal town of Ferndale mid-morning, with people living in the area experiencing strong ground shakes.

Smartphone users received warnings urging them to move to higher ground immediately, AFP reporters in the area said.

"A series of powerful waves and strong currents may impact coasts near you," the warning said. "You are in danger. Get away from coastal waters."

The National Weather Service, which issued the warning, withdrew it around midday, shortly before the first waves had been expected.

The quake appeared to have been felt across the region, including in the San Francisco Bay Area, where some people said they had felt rolling waves underfoot.

Marc Buller, an attorney who lives in Eureka, a port city in northern California close to the offshore epicenter, said he had experienced a lot of quakes, but this one felt unusual.

"It was an intense jolt. When the big jolt stopped, it felt like the house was on rollers," he told AFP.

"It was like when you throw a big stone in water and the ripples go out, it was like the earth was doing that."

The US West Coast is the confluence of several of the Earth's tectonic plates, and tremors are not uncommon.

The area has been struck by a number of major earthquakes, including a 1994 quake that hit Northridge, in the Los Angeles area, killing dozens of people and injuring thousands more, as it wrought billions of dollars of damage to homes and infrastructure.

The San Francisco earthquake of 1906, which also caused a tsunami, is thought to have killed upwards of 3,000 people, some of whom died in fires that erupted after the powerful tremor.