Japan's emergency agencies declared a tsunami warning for the region that includes the crippled Fukushima nuclear site.
Japan's Meteorological Agency raised a 1m tsunami warning for a long stretch of Japan's northeastern coast, including Fukushima.
It put the magnitude of the quake at 7.1.
The US Pacific Tsunami Warning Centre did not post warnings for the rest of the Pacific.
There were no immediate reports of damage on land.
The quake hit at 02:10 Saturday Tokyo time (1710 GMT) about 290km off Fukushima.
Japanese broadcaster NHK reported that Tokyo Electric Power, the operator of the Fukushima plant, ordered workers near the coast to move to higher ground.
The tremor was felt in Tokyo, some 480km away.
All but two of Japan's 50 reactors have been offline since the March 2011 magnitude-9.0 earthquake and ensuing tsunami triggered multiple meltdowns and massive radiation leaks at the Fukushima Dai-ichi nuclear power plant, about 250km northeast of Tokyo.
About 19 000 people were killed.