Geologists have warned of "strong and possibly damaging aftershocks" on the coast of California after a magnitude 6.9 quake struck late on Sunday night (5.18am UK time) beneath the Pacific seabed.
The US Geological Survey (USGS) warned there is a 90% probability of a strong aftershock of magnitude 5 or more in the next seven days. The USGS also warned that there was a 5-10% chance of a quake equal to or greater than Sunday's tremor.
The epicentre of last night's quake was 48 miles west-northwest of the town of Ferndale at a depth of 4.3 miles. The quake was felt as far away as 250 miles away in San Francisco.
The National Tsunami Warning Center said there was no tsunami danger for the region.
More than 3,000 people, including some in Oregon, reported on the USGS website that they felt the quake. Some reported a long, rolling shake that woke children or knocked items off shelves.
"This lasted longer than any earthquake I've ever felt," Raquel Maytorena, 52, who lives about a mile from the coast in Ferndale near Eureka, told The Los Angeles Times. "It just kept going and going, very slowly and softly. It was not violent. It almost felt like you were in a boat that was rocking."
Maytorena said she felt a little rattling in her nearly 100-year-old home, but power remained on without any interruptions. The quake felt like it lasted about 20 seconds, she said.
"The animals, they felt it," she said. "My two horses were running around out by the barn, and my dogs, six dogs, were ready to get out of the house."
Jana Pursley, a geophysicist with the National Earthquake Information Center, said that based on the area's tectonics and past temblors, damages or casualties were unlikely.
Earthquakes are very common in Eureka, a city of about 27,000 people about 270 miles (435 kilometers) northwest of San Francisco and 100 miles (160 kilometers) south of the Oregon state line. Nearby Arcata is home to about 17,000 people and Humboldt State University.
Additional reporting by Associated Press.