Japan has sounded an alert for small tsunami waves after a strong earthquake in the southern island chain of Okinawa.

The Meteorological Agency forecast that waves of around 50 centimetres would hit Miyako island in the Okinawan chain after the quake with a magnitude of 6.7 struck at 10:46 am (11:46 AEST) at a depth of 40 kilometres.

It was an aftershock to a quake registering 6.2 on the Richter scale in the same area about an hour earlier.

There were no immediate reports of damage of casualties following the two quakes in Okinawa, a subtropical chain north of Taiwan, officials said.

Late last month, a powerful earthquake with a magnitude of 6.9 hit the Noto peninsula in central Japan, killing one person and injuring more than 200.

Last week another quake hit central Japan, injuring 12 people.

Japan lies at the junction of four tectonic plates and experiences some 20 per cent of the world's most powerful earthquakes.

In 1995, a 7.3-magnitude quake in the western city of Kobe killed more than 6,400 people.