The quake occurred 12.2km south-west of the town of Ferndale, and approximately 400km north of San Francisco, at 2.34am local time.
Several small aftershocks were reported in the area, USGS said.
Widespread damage to roads and homes were reported throughout Humboldt County, the sheriff's office said, adding a tsunami was not expected.
One bridge has been closed while officials investigate it for possible seismic damage, the California Department of Transportation said.
More than 71,000 people in the county reportedly lost power, tracker poweroutage.us showed.
Photos and videos shared online showed items fallen from shelves and goods scattered around the floor of a supermarket.
Earthquakes are a common occurrence in California, though many are too small to feel.
The state government indicates California generally gets between two and three earthquakes per year large enough to cause moderate structural damage, measured as any quake with a magnitude of 5.5 or higher.