The quake hit at 2:10 a.m. Saturday Tokyo time (1710 GMT), the USGS said.
The tremor was felt in Tokyo, some 300 miles (480 kilometers) away.
All but two of Japan's 50 reactors have been offline since the March 2011 magnitude-9.0 earthquake and ensuing tsunami triggered multiple meltdowns and massive radiation leaks at the Fukushima Dai-ichi nuclear power plant, about 250 kilometers (160 miles) northeast of Tokyo.
About 19,000 people were killed.
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