Water leaks out of spent fuel pools at the Onagawa Nuclear Power Plant in Miyagi prefecture in northeast Japan after a strong aftershock rocked the region on Thursday, but there was no change in the radiation levels outside the plant, operator Tohoku Electric Power Co said. [Photo/Agencies]

SENDAI, Japan - Shoppers emptied store shelves, traffic snarled with stoplights knocked out and drivers waited in long lines to buy gasoline in a new wave of anxiety on Friday after a magnitude-7.1 aftershock struck disaster-weary northeastern Japan.

Nearly a half-million homes were without power after the latest tremor, which dealt another setback for those struggling to recover from the earthquake-spawned tsunami that wiped out hundreds of miles of the northeastern coast last month and killed as many as 25,000 people.

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Electricity was cut across a huge area of northern Japan, forcing cooling systems at three nuclear plants to switch to emergency power. At least one backup 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 stabilize reactors at tsunami-hit Fukushima.

Officials and reports said that four people had been killed by the tremor, which struck off the coast near Sendai - one of the most powerful to hit Japan since the country's worst post-war disaster four weeks ago.

Thursday's quake swayed buildings in Tokyo, more than 300 kilometers away from the epicenter, and generated unease even well away from the coast.

In the town of Kitakami, northwest of the 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."

Workers battling to control the stricken Fukushima Daiichi nuclear plant on the northeast coast were ordered to evacuate but have since returned, plant operator Tokyo Electric Power Co (TEPCO) 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 that a buildup of hydrogen might react with oxygen to cause an explosion.

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

The loss of external power sources at Fukushima in the March 11 tsunami left reactor cores heating up uncontrollably, resulting in the world's worst nuclear emergency since Chernobyl 25 years ago.

The crippled plant has leaked radiation which has made its way into tap water and farm produce, sparking food export bans.

Some highly radioactive water has leaked into the Pacific Ocean and TEPCO began dumping 11,500 tons of low-level radioactive water from the plant into the sea this week to free up urgently needed storage space.

The move has angered the Japanese fishing industry and raised concerns in neighboring countries including China, which on Friday urged Tokyo to ensure that the marine environment was protected.

The nuclear agency said that Thursday's quake had knocked out some external power sources used for cooling at plants in Onagawa in Miyagi prefecture and at Rokkasho and Higashidori in Aomori prefecture. But at least one emergency power source remained operational at each, and there was no indication that the loss of regular power was causing a problem.