A strong earthquake measuring 6.6 on the Richter scale has shaken northern Japan, triggering a minor tsunami.
There were no immediate reports of casualties or damage.
The tremor struck on Saturday with its epicentre located in the Pacific off Fukushima prefecture at a depth of 10km.
A 20cm-high tsunami was observed in Miyagi and other areas north of Tokyo.
An aftershock measuring 5.3 hit northern Japan shortly after the first jolt, the Japanese meteorological agency says.
The Government set up a liaison office at the crisis management centre at the Prime Minister's office in Tokyo.
A powerful quake hit the northern prefecture of Miyagi on June 13, killing 13 people with another 10 missing.
Japan experiences 20 per cent of the world's major earthquakes.