The quake struck just before 4 a.m. off the coast of Fukushima.
The epicenter was 35 miles below the sea surface, but no tsunami warning has been issued.
Fukushima was hit by a massive quake and tsunami back in March. The combination of the two left about 23,000 people dead or missing.
It also forced about 80,000 people to evacuate their homes due to a radiation threat from a damaged nuclear reactor.
The Associated Press contributed to this report.