A 6.4-magnitude earthquake hit northern Japan early today, but there was no tsunami warning and no immediate reports of damage. US and Japanese seismologists said, the underwater quake happened at 2156 GMT Sunday off Iwate prefecture, 500 kilometers north of Tokyo. Its depth was estimated at 24 kilometers. Around 20 per cent of the world's most powerful earthquakes strike Japan, which has developed one of the most sophisticated tsunami warning systems.Residents routinely hold drills to prepare for major earthquakes.