SENDAI, Japan (AFP) ― A powerful aftershock rocked Japan’s tsunami disaster zone, killing at least four and triggering new concerns over nuclear power plants in a region still grappling with an atomic emergency.

Electricity was cut across a huge area of northern Japan, forcing cooling systems at three nuclear plants to switch to emergency power and plunging more than 3.3 million households into darkness late Thursday night.

At least one back-up supply remained online at all three plants, but the aftershock highlighted the potential risks of nuclear generation in an earthquake zone amid a battle to stabilise reactors at tsunami-hit Fukushima.

Officials and reports said four people had been killed by the 7.1 magnitude tremor ― one of the most powerful to hit Japan since the country’s worst post-war disaster nearly four weeks ago.

The Japanese Meteorological Agency immediately issued a tsunami alert, warning 2-meter high waves could hit an area where much of the coastline still lies in ruins from the March 11 catastrophe.

The alert, which had sent people fleeing to higher ground, was withdrawn 83 minutes after the 11:32 p.m. quake, but set already frayed nerves on edge.

In the town of Kitakami, northwest of Thursday night’s epicenter, an AFP reporter witnessed queues forming at convenience stores as people tried to stock up anew on food, water and batteries.

“It was so scary,” said Kazuyuki Shiroiwa, who had been to four shops in central Kitakami in a vain effort to find batteries.

“The midnight quake reminded me of the fear I felt a month ago,” he said. “I’m fed up with earthquakes. No more quakes, please.”

The Fire and Disaster Management Agency said a 63-year-old woman in Yamagata prefecture died after her home respirator failed when the power was knocked out.

Broadcaster NHK said two men aged 79 and 85 were killed in Miyagi prefecture ― the area worst hit by the 9.0 magnitude quake of March 11 and the towering tsunami it spawned.

The Jiji news agency said an 83-year-old woman in the same prefecture was taken to hospital immediately after the earthquake and later confirmed dead.

The disaster management agency said at least 93 people were confirmed injured as of Friday morning, while Jiji Press said about 140 were hurt.

Electricity was restored to some areas but 2.62 million households were still without power as of mid-Friday, Tohoku-Electric Power said.

The U.S. Geological Survey said the 7.1 aftershock’s epicenter was under the sea 66 kilometers east of Sendai, a city severely affected by the March 11 disaster.

It swayed buildings in the capital Tokyo, more than 300 kilometres away.

Workers battling to control the stricken Fukushima Daiichi nuclear plant on the northeast coast were ordered to evacuate temporarily, plant operator Tokyo Electric Power Co. said.

The evacuation order came less than 24 hours after the workers began pumping nitrogen, an inert gas, into reactor No. 1, where engineers were concerned a build-up of hydrogen might react with oxygen to cause an explosion.

Work at the plant was remotely controlled and had continued uninterrupted, the company said.

A nuclear safety agency official told reporters: “There are no abnormal readings at the Fukushima Daiichi’s monitoring posts.” He added: “We have not seen any problem... with regard to the injection of nitrogen.”

The nuclear agency said some external power sources used for cooling had been lost at plants in Onagawa in Miyagi prefecture and at Rokkasho and Higashidori in Aomori prefecture, but at least one emergency source remained operational at each.