More than 200 people, many of them children on summer vacation, were in an indoor pool in coastal Sendai city when the quake on Tuesday showered tiles and metal onto them, injuring 17, officials said.
Tumbling rocks and roof tiles hurt others elsewhere. TV news footage showed a collapsed house outside Tokyo and landslides in the quake-hit area, 300km north of the capital. An area police spokesman said an 80-year-old woman was trapped and later rescued.
"The shaking felt like it lasted forever. It was scary thinking when it was going to end and what was going to happen," said Sendai city official Yoji Kimura.
National broadcaster NHK said about 60 people were reported injured. Officials were still tallying figures, but police in Miyagi prefecture, which bore the worst of the quake, could only confirm 26 injuries. Neighbouring Fukushima prefecture reported four.
A 72-year-old man suffered a broken leg from a falling stone lantern, and two women were hurt when hot oil spilled on them in a supermarket, Fukushima police spokesman Ichiro Oizumi said.
A seven-year-old child was injured by falling rocks in the town of Zao, according to local official Mitsuharu Shishido.
The quake was followed by
at least four aftershocks
The temblor knocked out power to about 17,000 households, suspended bullet train services in northern Japan and temporarily grounded flights at Tokyo's Haneda airport, Kyodo said.
The quake hit at 11.46am (0246 GMT) and was centred 42km below the ocean floor about 80km off the coast of Miyagi prefecture in northeastern Japan, the Meteorological Agency said.
Two 10cm tsunamis hit the nearby coast shortly after noon, and officials expected little damage from the waves.
The quake was followed by at least four aftershocks and additional quakes of up to magnitude 6 could follow, the agency said.
Authorities in Miyagi prefecture were still assessing damage in the area, local government official Masami Oshima said, adding that it was premature to give estimates.
"The horizontal shaking was very strong, so much so that I almost couldn't remain standing"
"The horizontal shaking was very strong, so much so that I almost couldn't remain standing," Oshima said.
Sendai, the capital of Miyagi, is about 300km northeast of Tokyo. A nuclear power plant in the neighbouring prefecture of Fukushima was not affected by the earthquake, Kyodo reported, citing the plant's operator.
Another plant in Onagawa in Miyagi prefecture shut down automatically and was being checked for damage, news reports said.
The US Geological Survey registered the quake with a 7.2 magnitude, after initially logging it at 7.
Japan is at the juncture of four tectonic plates, or moving slabs of the earth's outer crust, and is one of the world's most quake-prone regions.
A quake with a magnitude of six on the Richter shook the Tokyo area on 23 July, injuring more than two dozen people and suspending flights and trains for hours.
Earlier on Tuesday, a 4.9 magnitude earthquake shook Japan's northern island of Hokkaido. There was no danger of a tsunami in that one.