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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 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 U.S. 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 of up to 3 feet and possibly higher in some areas, was lifted after about an hour and a half, and the Japan Meteorological Agency said no tsunami had been detected.

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 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 Co.

Monitoring posts around the plant were not showing any increase 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 by the U.S. Nuclear Regulatory Commission in late March.

Thursday’s aftershock was the strongest since March 11, according to the Geological Survey. Hundreds of aftershocks have followed the initial quake.

In an attempt to account for missing people, the police on Thursday searched the evacuation zone around the Fukushima Daiichi plant.

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. An estimated 160,000 people remain housed in temporary shelters.