An earthquake with a magnitude of 6.0 has hit Northern California, according to reports.
The US Geological Survey said the earthquake struck at 3.20am local time (10.20am BST) four miles north-west of American Canyon, Napa County, at a depth of 6.7 miles.
The earthquake struck 51 miles from California's capital city Sacramento, and could be felt in San Francisco, where several residents said it woke them up.
NPR's southern US bureau chief Russell Lewis said on Twitter that there were 23 aftershocks after the quake.
Thousands of people were reportedly left without electricity in the cities of Napa, Sonoma and Santa Rosa. A map on the Pacific Gas and Electric Company's website showed that at least 15,000 customers were left without power across Northern California.
There were no immediate reports of injuries or serious damage to property. Police in Santa Rosa said they are responding to downed power poles.
Several local residents took to Twitter to share their experiences of the earthquake. Sarah McLellan tweeted: "On 6th floor entire building swaying, shaking", while Matt Dyar posted: "Major shaker. Stuff off shelves. Broken glass. No house damage that is obvious."
Last December, Dr Lucy Jones the US Geological Survey's adviser for risk reduction, said Southern California, particularly Los Angeles, is not prepared for a major earthquake.
Speaking at the American Geophysical Union in San Francisco, Jones said that when the "Big One" hits the southern part of the state, the damage could be much worse than in previous incidents, and could devastate as many as 10 million people.
In the Northridge Earthquake of 1994, which occurred in the north-central San Fernando Valley region of Los Angeles, 57 people died and more than 8,700 were injured. Damage to property was estimated to reach $20bn (£12bn, €15.1bn), making it one of the most costly earthquakes in recent history.