Tsunami waves were expected to start hitting the California coast Thursday after a powerful earthquake struck offshore, US seismologists said.
The 7.0-magnitude quake was recorded at a depth of just 10 kilometers (six miles), almost 100 kilometers west-southwest of Ferndale, according to the United States Geological Survey.
Smartphone users received warnings urging them to move to higher ground immediately, the San Francisco Chronicle reported.
"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 paper said tsunami warning sirens were activated along a stretch of the coast, including in Bonny Doon in Santa Cruz County.
The first waves were expected to hit the San Francisco area around 12.10 pm (2010 GMT).
"Based on preliminary earthquake parameters... hazardous tsunami are possible for coasts located within 300 KM of the earthquake epicenter," said a warning issued by the National Weather Service's Tsunami Warning Center in Honolulu.
The USGS, which monitors seismic activity, said the quake struck at 10.44 am local time (1844 GMT).
Seismologist Maureen Long told CNN the time lag between the quake and the arrival of the tsunami waves was because of the difference in the way that the energy of the two phenomena behaved.
"Tsunami waves... in the open ocean tend to travel about as fast as a jet plane flies, whereas earthquake waves travel much faster," she said.
"It's easier for us to to give tsunami warnings because there's a little bit of a time delay there."
The quake appeared to have been felt across the region, including in the San Francisco Bay Area, where some people said they had felt strong shaking and 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 a number 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.
