A 5.7-magnitude earthquake struck eastern Japan on Thursday near the crippled Fukushima nuclear plant, but did not cause any further problems at the power station, officials said.
The quake hit in the Pacific, 22 kilometres east of Iwaki in southern Fukushima, at 12.20pm (0320 GMT) at a depth of nine kilometres, the US Geological Survey said.
A tsunami was not expected, said the Japan Meteorological Agency, and there were no immediate reports of damage.
Nuclear plant operator Tokyo Electric Power Co. (TEPCO) said the stricken Fukushima Daiichi nuclear plant remained stable.
A 9.0-magnitude earthquake on March 11 triggered a monster tsunami that killed more than 19,000 people and crippled the cooling systems at the Fukushima plant, causing meltdowns in some of its reactors.
Radiation leaked into the air, soil and sea around the plant, forcing tens of thousands of people to evacuate their homes.
A number of powerful aftershocks have shaken the region since the March quake.