A powerful aftershock that rocked an area of Japan already reeling from last month's earthquake and tsunami disaster killed two people and injured more than 100, broadcaster NHK said.
The quake was initially measured at magnitude-7.4, though the US Geological Survey later downgraded it to 7.1.
Either way, it was the strongest aftershock since several were recorded on March 11 - the day of the magnitude-9.0 earthquake and tsunami that killed as many as 25,000 people and touched off a nuclear crisis in March.
The aftershock, which struck at 11.32pm local time, prompted the operator of the stricken Fukushima Daiichi nuclear plant to evacuate workers while they were cooling radioactive fuel.
No unusual conditions were observed at the Fukushima plant, according to statements from Tokyo Electric Power Co and Japan's Nuclear and Industrial Safety Agency.
"Indications of new leakage or a change in radiation levels will be the only way they'll tell if there's further damage," Murray Jennix, a nuclear engineer who specialised in radioactive containment leaks and teaches at San Diego State University, said.
"You've got cracks that could have been made bigger."
Two people died and 93 were injured from yesterday's quake, the Fire and Disaster Management Agency said, as more than 3.6 million households lost power.
Tokyo Electric, the operator of the Fukushima Daiichi plant 220 kilometres north of Tokyo, evacuated 15 workers who were pumping nitrogen into the No.1 reactor to prevent hydrogen explosions of the type that damaged radiation containment buildings last month.
Work at Fukushima wasn't affected by the quake, Tokyo Electric spokesman Takashi Kurita said today.
There have been no signs of changes in radiation levels or damage at the plant, he said.
Agencies
