The quake struck at 6:50 a.m. and had a depth of 6.2 miles.
It hit about 110 miles off the coast of Eureka, which is 250 miles north of San Francisco.
USGS initially registered the earthquake as magnitude-6.9, but downgraded it about 15 minutes later.
The Pacific Tsunami Warning Center said there was no tsunami threat. There were also no immediate reports of damage or injuries.
A hotel front desk worker at the Gingerbread Mansion Inn in Ferndale, a city in Humboldt County, said the temblor was "definitely felt" for about 15-20 seconds, but there was no damage.
Two smaller-scale quakes followed just hours after the first. A 4.7-magnitude quake hit near the same epicenter about two hours later, and a 2.9 temblor struck southwest of Rio Dell at about 10 a.m.