Police and fire officials said no injuries were reported and the temblor caused no damage at the Fukushima No. 1 and No. 2 nuclear reactors, Kyodo News said. No tsunami warning was issued.
The Japanese Meteorological Agency said the quake was 28.5 miles below the ocean floor off the coast of Fukushima, and officials said it was an aftershock of the 2011 quake that generated a tsunami that devastated the region and nearly caused a full scale nuclear meltdown at two of the reactors in the Fukushima nuclear plant.
"Aftershock activities have become less frequent but we urge the public to be careful, since there are also unusually strong aftershocks like this one," an agency official said.
Air traffic at Sendai Airport was briefly halted so runways could be checked for cracks. Train service in the region was halted for about 6 minutes.