The earthquake was centered 207 miles east of Iwaki, Honshu, and struck at 2:10 a.m. Saturday Tokyo time, according to the USGS. The region includes the crippled Fukushima nuclear site.
Residents in Tokyo -- some 300 miles away -- reported feeling shaking from the earthquake. There were no immediate reports of damage.
The U.S. Pacific Tsunami Warning Center did not post tsunami warnings for the rest of the Pacific.
Japanese broadcaster NHK reported that Tokyo Electric Power Co., the operator of the Fukushima plant, ordered workers near the coast to move to higher ground. Japanese news service Kyodo said there were no signs of trouble at the plant.
The Associated Press contributed to this report.