A powerful earthquake with a magnitude of 7.3 jolted Japan's northeast coast on Wednesday, shaking buildings as far away as Tokyo where it left hundreds of thousands briefly without power.
The tremor hit off the coast of Fukushima prefecture, some 275 kilometres (170 miles) northeast of Tokyo and at a depth of 60 kilometres, the Japan Meteorological Agency (JMA) said.
It revived memories of a devastating quake 11 years earlier in the same region.
There were some reports of fires, but there otherwise were no immediate reports or signs of major damage. A number of people sustained injuries across northeastern Japan, but none of them appeared serious, with most sustained in falls or from being struck by falling objects.
Large parts of the capital, Tokyo, were plunged into darkness for an hour or more. Separately, a Shinkansen bullet train derailed with some 100 people on board, although there were no reports of injuries.
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There were no abnormalities at the country's nuclear power plants, Prime Minister Fumio Kishida told reporters, adding that he expected power to be largely back within an hour.
Authorities earlier said a fire alarm had been triggered at a turbine at the crippled Fukushima Daiichi nuclear power plant. That plant was devastated by a magnitude 9 earthquake and following tsunami in March 2011, causing a radiation leak and the worst nuclear crisis in a quarter of a century.
Tokyo Electric Power Company said that around 2 million households lost power on Wednesday, including 700,000 in the capital.
Authorities issued a tsunami warning for the region of as high as 1 metre (3.3 ft), with public broadcaster NHK reporting waves of 20 centimetres (8 inches) in some places.
Authorities warned residents in Fukushima, Miyagi and Yamagata prefectures to expect aftershocks.
Sitting on the boundary of several tectonic plates, Japan experiences around a fifth of the world's earthquakes of magnitude 6 or greater.