A 5.1 magnitude earthquake struck off the coast of Japan's Fukushima prefecture early on Saturday morning, the United States Geological Survey said.
The moderate quake hit 34 kilometres (21 miles) east of coastal Namie city in Fukushima prefecture, where a devastating quake-tsunami in March 2011 sparked a nuclear disaster and caused massive damage.
Saturday's earthquake occurred at 2:18 am local time (1718 GMT) off the east coast of Japan's main Honshu island, at a depth of 63 kilometres, US geologists said in a preliminary report.
No tsunami warning was issued after the tremor, the Japan Meteorological Agency said, according to Kyodo news agency.