A magnitude 6.9 earthquake hit off the main Japanese island of Honshu, the US Geological Survey said on Tuesday 12 May. It was later estimated the magnitude was 6.6.
People in the capital Tokyo were said to have felt light shaking.
The quake was centred 78 miles (125 km) southeast of Morioka, Japan, at a depth of 20.5 miles (33 km), it said.
The USGS said residents should "expect aftershocks. These secondary shockwaves are usually less violent than the main quake but can be strong enough to do additional damage to weakened structures and can occur in the first hours, days, weeks, or even months after the quake".
Later, the Japanese Meteorological Agency said the quake was 6.6 on the Richter Scale and that no tsunami alerts had been issued.