Dec 20 (Reuters) - A strong 6.4-magnitude earthquake struck off the coast of northern California on Tuesday, the U.S. Geological Survey said, as local authorities and media reported thousands of power outages and bridge damage.
The earthquake was about 10 miles (16.1km) deep, USGS said, and struck about 7.4 (12km) west-southwest of Ferndale, California, about a four-hour drive north of San Francisco.
There was no risk of a tsunami after the quake, the U.S. tsunami warning system said.
In Ferndale and surrounding Humboldt County, more than 55,000 homes and businesses were without power early Tuesday, the electric grid tracking website Poweroutageus.com said.
The California Independent System Operator, which oversees much of the state's electrical grid, issued a transmission emergency notice for the area following the earthquake.