An earthquake with a preliminary reading of magnitude 6.4 has jolted northeast Japan.
It occurred at 3.54am on Sunday at a depth of 44km. The tremor was also felt in Tokyo.
The focus of the quake was off the coast of Fukushima prefecture.
NHK said there were no immediate reports of injuries or damage.
Tokyo Electric Power Company says there are no reports of abnormalities at the Fukushima Daiichi nuclear plants.
Earthquakes are common in Japan. A magnitude 9.0 quake and tsunami on 11 March caused a radiation crisis when reactor cooling systems at the Fukushima plant were knocked out.
Up to 23,000 people are dead or missing in the disaster.
The Japan Meteorological Agency said the tremor was presumed to be an aftershock of the earthquake on 11 March.