The undersea quake struck at 8:30 pm (1130 GMT) off the east coast of Japan's main island of Honshu, at a depth of 33 kilometres (20 miles).
The epicentre was located about 100 kilometres east of the city of Iwaki.
Two hours earlier, a quake with a magnitude of 5.0 hit the same area, the USGS said. The Japan Meteorological Agency put the magnitude of the earlier quake at 5.2.
The earlier quake did not trigger a tsunami warning and there were no reports of casualties or damage.
Japan experiences 20 percent of the world's major earthquakes. A powerful inland quake in the north of the country on June 13 triggered landslides and killed 12 people.
© 2008 AFP
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