The Fire and Disaster Management Agency says the magnitude-6.5 earthquake struck at 3:54 a.m. Sunday (1854 GMT Saturday) off the coast of Fukushima, a region struck by the March 11 massive quake and tsunami. The epicenter was 35 miles (57 kilometers) below the sea surface.
Plant operator Tokyo Electric Power Co. says the temblor caused no further damage to its Fukushima Dai-ichi nuclear power complex, which was crippled by the March disasters.
Seven people in nearby cities were injured mildly.
The March disasters left about 23,000 people dead or missing across Japan's northeast coast and forced 80,000 others to evacuate their homes due to the radiation threat.