The earthquake was at a depth of 16.1km, USGS said, adding that it struck 12km west-southwest of Ferndale, California.
No tsunami warning was issued after the earthquake under the US Tsunami Warning System, Metro reported.
The tremor led to thousands of people losing power, with at least 15,623 people briefly without electricity in Humboldt County, according to the provider PG&E.
MNA/PR