A powerful 7.1-magnitude earthquake late on Thursday hit the same area of Japan that was ravaged by disaster a month ago, seismologists said, prompting a local tsunami alert that was later cancelled.
Power was cut to parts of the northeast of the country, much of which is still struggling with the effects of the monster tsunami that roared ashore four weeks ago.
The new quake caused a handful of injuries, national broadcaster NHK said, but there were no reported deaths.
Workers battling to control the stricken Fukushima Daiichi nuclear plant on the northeast coast were temporarily ordered to evacuate, plant operator Tokyo Electric Power Co. (TEPCO) said.
The evacuation order came less than 24 hours after they began pumping nitrogen into reactor No. 1, where engineers were concerned a build-up of hydrogen react with oxygen to cause an explosion.
A company spokesman later said that work at the plant was nonetheless continuing with no problem.
A TEPCO spokesman told a press conference that there was 'no information immediately indicating any abnormality at Fukushima Daiichi plant'.
The tremor hit at 11:32 pm local time (1432 GMT) with an offshore epicentre 66 kilometres (40 miles) east of Sendai, a city severely impacted by the March 11 quake and tsunami, according to the US Geological Survey.
Japan's weather bureau promptly issued a tsunami alert for the Pacific coast, saying that waves of up to two metres (six feet) could hit the shoreline, but the alert was cancelled 83 minutes after the quake.
In a statement on its website the Hawaii-based Pacific Tsunami Warning Center had said earlier it did not expect an ocean-wide tsunami.
Footage from broadcaster NHK showed that power was off in parts of Sendai, a regional commercial hub shaken by the March 11 quake.
The broadcaster said gas and water leaks were being reported in some areas of the city.
Jiji Press news agency said that shortly after midnight there were five fires and 13 gas leaks in Sendai city, according to the Miyagi prefectural office.
In Iwate prefecture, local authorities ordered some 500 households to evacuate, NHK said. The broadcaster also reported three fires in Iwate and Miyage prefectures.
An AFP photographer in Kitakami city in Iwate reported that power had gone off following Thursday's quake.
The quake had a depth of 49 kilometres, the USGS said. Although the epicentre was 330 kilometres (205 miles) from Tokyo, it shook buildings in the Japanese capital.
A Meteorological Agency official told a televised news conference: "We have determined this was an aftershock of the (March 11) earthquake."
The quake came as workers at the Fukushima plant were battling round the clock to prevent a possible explosion, but the tremor appeared not to have impacted the work.
Early Thursday morning workers at the Fukushima plant began pumping in nitrogen, an inert gas, which they hope will dilute the concentration of both hydrogen and oxygen inside the reactor casing.
The process to inject 6,000 cubic metres (210,000 cubic feet) was expected to take around six days, TEPCO said.
"We haven't noticed any problem in parameters with regard to the injection of nitrogen," a nuclear safety official said after the late-night earthquake.
Earlier, chief government spokesman Yukio Edano indicated Tokyo was considering widening the 20-kilometre (12-mile) evacuation zone around the stricken plant, a week after a UN nuclear watchdog said it should be increased.
"The existing safety standards for local residents are that an evacuation order is issued if there is a possibility that they might receive radiation 50 millisieverts or above," he said.
"The standard assumed that a high level of radiation is emitted temporarily. We are discussing how best to issue evacuation orders based on data and standards for accumulative radiation," Edano said.
Around 3,400 people are unaccounted for along the 40-kilometre stretch of coast covered by the exclusion zone and on Thursday, around 300 police began searching for bodies in the the outer 10-kilometre band of the zone.
Television pictures showed officers in full body suits entering the area, while a police spokesman said all officers were armed with radiation meters.
The Bank of Japan on Thursday warned of pressures as a result of the triple disaster and bolstered funding for quake-hit areas, unveiling a 1.0 trillion yen ($11.7 billion) scheme to keep banks in affected areas liquid.
The BoJ also downgraded its view of the economy due to last month's disasters.
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