A powerful earthquake struck near Japan's crippled Fukushima nuclear plant on Tuesday, shaking buildings in Tokyo. No tsunami warning was issued and no damage immediately reported.
The US Geological Survey said the quake had a magnitude of 6.0 and hit less than 11 kilometres (7 miles) below ground, 70 kilometres south of Fukushima city in the prefecture of the same name.
Japanese officials had put the magnitude at 6.3.
Emergency workers grappling to bring stricken reactors under control at the Fukushima Daiichi nuclear plant were temporarily ordered to evacuate, an official from operator Tokyo Electric Power said.
A loss of power to the cooling systems at the plant following the massive tsunami caused by a huge undersea quake on March 11 left reactors heating up dangerously, sparking the world's worst nuclear crisis in 25 years.
A seismologist said he expected aftershocks from the mega-quake to continue for weeks or even months, although their frequency and strength would gradually fade.
"Aftershocks are a completely normal phenomenon," said Jean-Paul Montagner at the Institute of the Physics of the Globe in Paris.
"When there is a very large earthquake, all the tectonic forces in region are disturbed. These aftershocks are simply a way by which the region settles down."
Montagner emphasised, though, the need to be "very humble" when it comes to understanding tectonic dynamics, especially in a place like Japan, which is at the crossroads of four plates.
Each year, Japan accounts for more than a fifth of the most violent earthquakes that occur on the globe, he said.