A powerful 6.9-magnitude earthquake struck late Sunday off the coast of northern California, the US Geological Survey said, but there were no immediate reports of damage or a tsunami threat.
The quake struck at 0518 GMT with an epicenter located 77 kilometers (48 miles) west-northwest of the town of Ferndale and at a depth of seven kilometers (4.3 miles), said the USGS, which monitors earthquakes worldwide.
There were no immediate reports of casualties or damage, but early reports indicate the quake was felt as far away as San Francisco, around 400 kilometers south of Ferndale.
Authorities in Humboldt County, the part of sparsely-populated northern California some 250 miles up the coast from San Francisco, said they had no calls about damage of injuries, local media reported.
The Pacific Tsunami Warning Center issued a bulletin announcing the quake, but downplayed any danger.
"A widespread destructive tsunami threat does not exist based on historical earthquake and tsunami data," the Center said.
The USGS said there is a 90 percent probability of a "strong and possibly damaging aftershock" of magnitude 5 or more in the next seven days, and a 5-10 percent chance of a quake equal to or great than Sunday's temblor.
Up to 300 small aftershocks can be expected over the next week, it added. A series of mostly 3-3.5 magnitude shocks, and one of 4.6, were recorded in the hour after the main quake, according to the USGS.
California has long braced for the Big One. It is on the so-called Ring of Fire, which circles the Pacific and has produced a number of devastating quakes including Japan's March 2011 quake-tsunami, which killed thousands of people.
Geologists say a quake capable of causing widespread destruction is 99 percent certain of hitting California in the next 30 years. A magnitude 7.8 quake could kill 1,800 people, injure 50,000 more and damage 300,000 buildings.
A 6.7-magnitude earthquake in Los Angeles left at least 60 people dead and did an estimated $10 billion damage in 1994, while a 6.9 quake in San Francisco in 1989 claimed the lives of 67 people.