Japan's Meteorological Agency says it considers Saturday's earthquake off the east coast to be an aftershock of the deadly 9.0-magnitude earthquake that struck the same area almost 10 years ago.
The March 11, 2011, earthquake caused the country's worst nuclear disaster when three reactors at the Fukushima Daiichi nuclear plant melted down, releasing radioactive materials into the air and forcing more than 100,000 people to be evacuated.
More than 20,000 people died or went missing in the quake and tsunami, while hundreds of thousands more lost their homes.