Workers at the damaged Fukushima Daiichi nuclear plant were evacuated after a fresh earthquake with a magnitude of 7.1, but officials said it had no detectable effect at the plant.
A tsunami alert for north-east Japan was issued but lifted after 90 minutes.
The earthquake - at a depth of 49km (32 miles) - struck close to the epicenter of the 11 March quake, which triggered a devastating tsunami.
Japanese authorities ordered a new evacuation from the warning zone.
All seven of the workers at Fukushima Daiichi were safe, a spokesman for plant operator Tepco told a news conference in Tokyo."They have not been injured and they have all taken shelter in our seismic resistant building. We are continuing to inject water, or we are continuing the injection operation at reactors 1, 2 and 3," said the spokesman, whose name was not given.
The workers are trying to keep the damaged reactors cool to stop further releases of radioactive material.
Thursday's quake struck at 2332 local time (1432 GMT) on Thursday, 118km (78 miles) north of Fukushima, 40km offshore.
First reports said it had a magnitude of 7.4 but that was later revised downwards to 7.1, according to the US Geological Survey (USGS).
Last month's earthquake had a magnitude of 9.0 and struck at 32km deep.
USGS geophysicist Paul Caruso said Thursday's quake struck at about the same location as the 11 March quake, the AP news agency reported.
The quake was strong enough to shake buildings in Tokyo, 265km to the south.