A spokesman for Pacific Gas & Electric Co. says power outages are widespread across most of Humboldt County, affecting about 25,000 customers.
The U.S. Geological Survey says the powerful quake hit late this afternoon, about 22 miles from Ferndale.
Authorities in the nearby city of Eureka and other area communities say no major injuries have been reported.
Humboldt County Office of Emergency Services spokeswoman Jo Wattle says several traffic lights are down and numerous residents are reporting water, gas and sewer leaks. Wattle says there are also reports of chimneys down and minor property damage. As she puts it, "people are really shaken up."