A 6.6-magnitude undersea earthquake has hit off the eastern coast of Japan's main Honshu island, not far from the area devastated by a huge quake and tsunami in March, US geologists said.
However there were no immediate reports of damage or casualties from the early-morning quake and no widespread tsunami warning.
The United States Geological Survey reported the 36.2km deep quake hit 108km from the coastal town of Hachinohe at 4:26am local time (20:26pm Irish time).
The Pacific Tsunami Warning Center said that "no destructive widespread tsunami risk exists based on historical earthquake and tsunami data".
However it said that quakes of this size sometimes generate local tsunamis.
"Authorities in the region of the epicentre should be aware of this," the centre said in a statement.
The Japan Meteorological Agency said that there might be some changes in sea levels due to the quake but that there were no reports of damage or casualties and none were expected, according to the Kyodo news agency.
It said the quake jolted the Iwate prefecture, an area badly hit by the 9.0-magnitude earthquake and tsunami on 11 March that left almost 20,000 dead or missing and sparked the worst nuclear crisis since Chernobyl.