The US Geological Survey said the quake struck at 0531 (2031 GMT) 84km (52 miles) east of Naha, on the island of Okinawa, Reuters news agency reports.
The quake, only 29km (18 miles) below the seabed, was large enough to trigger at least a tsunami, Reuters said.
Earthquakes often hit Japan. In 1995, a magnitude-7.2 quake in the western port city of Kobe killed 6,400 people.
Note: This is not the actual earthquake footage
image source: greatdreams.com