Officials indicated there were no immediate reports of damage.
The powerful earthquake shook the northeastern coast, setting off a small tsunami and causing buildings to shake in Tokyo, 300 miles away, Voice of America reported.
Authorities said 3-foot waves rolled ashore in Miyagi prefecture -- in the same region where a massive earthquake and tsunami killed thousands of people last year.
The U.S Geological Survey said the quake struck 22 miles under the Pacific Ocean, about 152 miles southeast of Kamaishi.
Nuclear power-plant operators said they hadn't received any reports of danger at their facilities, Japan's national broadcaster HNK reported.
Tohoku Electric Power Co. officials said all three reactors at the Onagawa plant in Miyagi prefecture were off-line before the quake and that another nuclear power plant in Aomori prefecture didn't show signs of stress.
Tepco officials said the quake did not affect the company's Fukushima Daiichi and Fukushima Daini nuclear plants.
A 9-magnitude earthquake followed by a gigantic tsunami in March 2011 killed more than 15,000 people and crippled the Fukushima-Daiichi plant.
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