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SENDAI, Japan -- A big aftershock rocked quake-weary Japan late Thursday, rattling nerves as it knocked out power to the northern part of the country and prompted tsunami warnings that were later canceled.

The Japan Meteorological Agency said the quake was a magnitude 7.4. The U.S. Geological Survey said it was 7.1.

The quake was initially measured at magnitude-7.4, though the U.S. Geological Survey in Golden, Colo., later downgraded it to 7.1. Either way, it was the strongest aftershock since several were recorded on March 11 -- the day of the magnitude-9.0 earthquake and tsunami that killed as many as 25,000 people and touched off a nuclear crisis last month.

There were no immediate reports of serious injuries or major damage, and the operator of the tsunami-ravaged Fukushima Dai-ichi nuclear plant said there was no sign the aftershock had caused new problems there. Workers retreated to a quake-resistant shelter in the complex, with no injuries.

The aftershock around 11:30 p.m. was strong enough to knock items off store shelves and move a large automated teller machine at a FamilyMart convenience store in the major northern industrial center of Sendai. The city is far enough inland that it avoided major tsunami damage, but people there were without gas and electricity for weeks.

Manager Takehiko Akagi said 100 people had showed up within an hour of Thursday's aftershock and cleared the shelves of ice, water and instant noodles -- items that were in short supply after the bigger quake.

Usually at this time of night, there is almost no one, said Akagi, whose store had power even though people in nearby neighborhoods did not. A handful of buildings had broken windows and tiles, and some small electrical fires were reported. In Ichinoseki, which is also inland, buildings shook violently, knocking items from shelves and toppling furniture, but there also appear to be no major damage there. Hotel workers lit candles so guests could find their way around.

Japan's nuclear safety agency said power plants along the northeastern coast were under control after backup generators kicked in at three -- Rokkasho, Higashidori and Onagawa -- that lost power.

The aftershock knocked out two of three power lines at the Onagawa nuclear power plant north of Sendai, which has been shut down since the tsunami. One remaining line was supplying power to the plant and radiation monitoring devices detected no abnormalities. The plant's spent fuel pools briefly lost cooling capacity but an emergency diesel generator quickly kicked in.

It's the way it's supposed to work if power is lost for any reason, said David Lochbaum, director of the nuclear safety project for the U.S.-based Union of Concerned Scientists. Since the tsunami warning was canceled 90 minutes after it was issued, there was no reason to believe the facilities' diesel generators would fail like the ones at the stricken Fukushima plant. The massive wave knocked out cooling systems and triggered a series of mishaps that have left workers struggling to stop radioactivity from spewing nearly a month later.

That was really the blow that the plant didn't recover from, Lochbaum said.

Officials said the aftershock hit 25 miles (40 kilometers) under the water and off the coast of Miyagi prefecture. The USGS later downgraded it to 7.1.

Buildings as far away as Tokyo, which was about 205 miles (330 kilometers) from the epicenter, shook for about a minute.

Prime Minister Naoto Kan met with staff members in his office shortly afterward, according to deputy Cabinet spokesman Noriyuki Shikata.

A separate government emergency response team met shortly after midnight to monitor any reports of damage and urged firefighters, police and other emergency personnel to aid those in need.

USGS geophysicist Paul Caruso said the aftershock struck at about the same location and depth as last month's quake.

The USGS said the aftershock struck off the eastern coast 40 miles (65 kilometers) from Sendai and 70 miles (115 kilometers) from Fukushima.

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