The tremor struck far down at a depth of 332 kilometres, USGS added. There were no initial reports of damage.
Japan lies on the so-called "Ring of Fire", a series of seismic fault lines encircling the Pacific Ocean which create frequent seismic and volcanic activity.
More than 18,000 people died in March 2011 when a 9.0-magnitude sub-sea earthquake sent a towering tsunami barrelling into Japan's northeast coast in the country's worst post-World War II disaster.
Cooling systems at the Fukushima nuclear power plant were knocked out, sending reactors into meltdown and forcing tens of thousands of people to flee.
A 5.5-magnitude earthquake hit eastern Japan last week, rocking buildings in Tokyo. That tremor struck at a much shallower depth of 59 kilometres.


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