The focus of the tremor, which hit at about 1:00 am (1600 GMT Tuesday), was about 30 kilometers (20 miles) below sea level in the Sea of Japan off Niigata prefecture on the main island of Honshu, the agency said.

Last year, 51 people were killed and hundreds injured in a 6.8-magnitude quake in the Niigata region, 200 kilometers northwest of Tokyo.

Japan endures 20 percent of the world's major earthquakes.

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