Tokyo:
The strongest aftershock to hit since the day of the March 11 earthquake and tsunami in Japan rocked a wide section of the country's northeast on Thursday night, prompting a tsunami alert, raising fears of further damage to the already crippled Fukushima Daiichi nuclear plant and knocking out external power at three other nuclear facilities.
The public broadcaster, NHK, said there were local reports of injuries, fires and blackouts. The aftershock had a magnitude of 7.1, according to the United States Geological Survey; last month's quake, which devastated much of the northeastern coast, was measured at 9.0.
The tsunami alert, which warned of waves up to three feet and possibly higher in some areas, was lifted after about an hour and a half and Japan's Meteorological Agency said no tsunami had been detected.
But it warned that slight changes in sea level were still possible and it was unclear whether there was any damage along the coast. Many coastal communities were ravaged last month, and some are vulnerable because sea walls were breached and land levels have sunk.
Workers at the Fukushima plant were told to take cover until the tsunami warning was lifted, but Japanese officials said at a news conference that water was being automatically pumped into three damaged reactors in the crucial effort to keep their nuclear fuel cool. The plant's cooling systems were knocked out by last month's quake and tsunami, and there was no immediate word of whether there was new damage to the plant, according to its operator, the Tokyo Electric Power Company.
Nitrogen also continued to be piped into the No. 1 reactor, the company said, in an effort to prevent a possible explosion. Tokyo Electric said it was unsure of the status of the damaged No. 4 reactor because it has not been able to station workers there.
Monitoring posts around the plant were not showing any rise in radiation levels, the company said.
Experts have said that a big aftershock poses an additional risk to the Fukushima plant because its containment structures are filled with water that was used in the cooling efforts and is now highly radioactive. The strain from holding that water could make the structures more vulnerable to rupture in the event of an earthquake, according to an assessment made by the United States Nuclear Regulatory Commission in late March.
Two other nuclear facilities -- a fuel reprocessing plant at Rokkasho and a nuclear power plant at Higashidori, both in northern Aomori Prefecture -- were running on emergency diesel generators after their external power supplies were knocked out by the aftershock.
A third site, the Onagawa nuclear power station in Miyagi Prefecture, lost two of its three external power systems. All three facilities have been shut down since the March 11 quake, but power is needed to keep the nuclear fuel cool.
The aftershock hit at 11:32 p.m. local time and was centered 41 miles east of Sendai, 72 miles from Fukushima and 205 miles from Tokyo, officials said. It was about 30 miles below the ocean floor, considerably deeper than March 11's magnitude 9.0 quake, which hit about 20 miles below the sea floor.
Thursday's aftershock was the strongest since the day of the March 11 quake, according to the United States Geological Survey. There have been hundreds of aftershocks since the initial quake.
Also on Thursday, the police searched for people missing in an evacuation zone around the Fukushima Daiichi plant.
Nearly 240 police officers from Tokyo and about 100 from Fukushima Prefecture fanned out wearing protective suits in a search for bodies in the 12-mile evacuation zone around the plant, according to Mikio Murakoshi, a spokesman for the Fukushima Prefecture police.
Japanese and American soldiers last weekend conducted a huge search for the missing but avoided the evacuation zone because of the radiation risk. But Mr. Murakoshi said radiation levels had dropped, making a search in the area possible.
The police say about 12,600 people have died as a result of the March 11 earthquake and tsunami. More than 14,700 are listed as missing, including about 4,200 in the evacuation zone around the Fukushima plant.
The magnitude 9.0 quake and tsunami flattened communities, has kept an estimated 160,000 still housed in temporary shelters and knocked out power at the Fukushima Daiichi plant, where workers have since battled to stabilize the reactors.
Workers at the facility's No. 1 reactor began injecting nitrogen early on Thursday to reduce the risk of an explosion from hydrogen gas that might be building. Officials said the step was being taken as a precaution, not because an explosion was deemed imminent.
This is the first injection of nitrogen into any of the reactors. The same approach might be tried later for the No. 2 and No. 3 reactors.
Hydrogen explosions occurred in some of the reactors in the days after March 11, damaging the outer buildings around the reactors. It was thought that the hydrogen was produced when zirconium from fuel rods reacted with steam.
The injection of nitrogen was one of the steps recommended by the United States Nuclear Regulatory Commission in a confidential assessment dated March 26.
On Wednesday, the commission said that some of the core of the No. 2 reactor had probably leaked from its steel pressure vessel into the bottom of the containment structure. The assessment implied that the damage at the No. 2 unit was worse than previously believed.
The agency emphasized its interpretation was speculative and based on high radiation readings that Tokyo Electric had found in the lower part of unit No. 2's primary containment structure, called the drywell. The statement said that the commission "does not believe that the reactor vessel has given way, and we do believe practically all of the core remains in the vessel."