A moderate 5.6-magnitude quake rattled Japan's southern Ryukyu islands on Tuesday, US seismologists said, but there was no immediate tsunami warning nor reports of damage.
The quake struck at 4.31 am (1931 GMT Monday) 83 kilometres (52 miles) southwest of the city of Naha, the United States Geological Survey said.
It struck at a relatively shallow depth of 18 kilometres.
Japan, located at the junction of four tectonic plates, experiences 20 percent of the strongest quakes recorded on Earth each year.
A 9.0-magnitude earthquake in March 2011 triggered a monster tsunami that killed more than 19,000 people and crippled the cooling systems at Japan's Fukushima nuclear plant, causing meltdowns in some of its reactors.