More than 900,000 households remained without electricity on Friday after the strongest aftershock to hit since the March 11 earthquake and tsunami in Japan rocked a wide section of the country’s northeast.
The aftershock on Thursday night prompted a tsunami alert, raised fears of new strains on the crippled Fukushima Daiichi nuclear plant and knocked out external power at three other nuclear facilities.
The public broadcaster, NHK, said two people had died in Miyagi and Yamagata, including a 63-year-old woman whose ventilator stopped working in the blackout. Many more were injured. Of about 3.6 million households initially affected, 950,000 were still without power Friday afternoon, The Associated Press reported, quoting the utility that serves the region.
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No tsunami was detected, the Japan Meteorological Agency said. 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.
But the agency warned of more aftershocks going forward. Many coastal communities were ravaged last month, and some have become even more vulnerable to tsunami waves because sea walls were breached and land levels sank.
Early Friday, injuries were reported in Sendai City and across the region, and blackouts continued in some areas, according to NHK. Five coal-powered power plants also shut down, adding to concerns over energy shortages.
Workers at the Fukushima Daiichi plant were told to take cover until the tsunami warning was lifted, but Japanese officials said at a news conference that water was still able to be pumped into three damaged reactors and a spent-fuel pool at a fourth 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.
Nitrogen also continued to be piped into the No. 1 reactor, the company said, in an effort to prevent a possible explosion.
The Tokyo Electric Power Company, which runs the power station, said Friday that it had found no new damage to the plant, and workers had resumed work to identify the source of leaks, found last week, of radioactive water into pipes and tunnels under the complex. Monitoring posts at the plant were not showing any immediate increase in radiation levels, the company said.
A big aftershock is thought to pose an additional risk to the Fukushima plant because its containment structures, now filled with water that is highly radioactive, may be more vulnerable to rupture, according to an assessment by the United States Nuclear Regulatory Commission in late March.
The temblor has also underscored the sensitivity of other power plants in the region to seismic shocks.
Two other nuclear facilities — a fuel reprocessing plant at Rokkasho and a power plant at Higashidori, both in northern Aomori Prefecture — ran temporarily on emergency diesel generators after their external power was knocked out. Grid power was restored at both plants on Friday morning, according to Japan's nuclear regulator, the Nuclear and Industrial Safety Agency.
The single reactor at Higashidori is shut down for maintenance, and all nuclear fuel had been transferred to the facility's spent fuel pool, which are being cooled by back-up diesel power, according to the operator, Tohoku Electric.
A third site, the Onagawa Nuclear Power Station in Miyagi Prefecture, lost two of its three external power systems, and cooling stopped temporarily at a spent fuel pool there, Tohoku Electric said. All three plants have been shut down since the March 11 quake, but power is needed to cool the nuclear fuel.
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, about 10 miles deeper than the March 11 quake. Hundreds of aftershocks have followed the initial quake, but Thursday's was the strongest, according to the United States Geological Survey.
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.
Early Thursday, before the quake struck, nearly 240 police officers from Tokyo and about 100 from Fukushima Prefecture fanned out wearing protective suits in a search of bodies in the 12-mile evacuation zone around the Daiichi plant, according to Mikio Murakoshi, a police spokesman. Japanese and American soldiers conducted a huge search for the missing last weekend, but avoided the evacuation zone because of the radiation risk. But Mr. Murakoshi said radiation levels had dropped.
Still, concerns about the plant remain high. The Nuclear Regulatory Commission speculated Wednesday that some of the core of the No. 2 reactor had flowed from its steel pressure vessel into the bottom of the containment structure. The theory implies more damage at the unit than previously believed.
While a spokeswoman for Tokyo Electric dismissed the analysis, a spokesman for the Nuclear and Industrial Safety Agency of Japan agreed that it was possible that the core had leaked into the larger containment vessel.
The possibility raised new questions. The Nuclear Regulator Commission said that its speculation about the flow of core material out of the reactor vessel would explain high radiation readings in an area underneath, called the drywell.
But some of the radiation readings at Reactors Nos. 1 and 3 over the last week were nearly as high as or higher than the 3,300 rems per hour that the commission said it was trying to explain, so it would appear that the speculation would apply to them as well. At No. 2, extremely radioactive material continues to ooze out of the reactor pressure vessel, and the leak is likely to widen with time, a western nuclear executive asserted.
“It's a little like pulling a thread out of your tie,” said the executive, who spoke on the condition of anonymity to protect business connections in Japan. “Any breach gets bigger.”
Flashes of extremely intense radioactivity have become a serious problem, he said. Tokyo Electric's difficulties in providing accurate information on radiation are not a result of software problems, as some Japanese officials have suggested, but stem from damage to measurement instruments caused by radiation, the executive said.
Broken pieces of fuel rods have been found outside of Reactor No. 2, and are now being covered with bulldozers, he said. The pieces may be from rods in the spent-fuel pools that were flung out by hydrogen explosions.
Keith Bradsher contributed reporting from Hong Kong, and Matthew L. Wald from Washington.