Six people were lightly injured, including an 18-year-old boy who was hit by his falling stereo speaker, according to the disaster management agency.

There were no reports of damage to houses, it said.

Japan's meteorological agency warned that more moderate aftershocks could strike, although there were no fears of a tsunami.

The strongest quake hit at 1:45am in the Pacific Ocean off Ibaraki prefecture, some 100km north-east of Tokyo.

Power was cut off to some 4,500 households in Tsukuba City, north-east of Tokyo, but has since been restored, the industry ministry said.

Tokyo Electric Power (Tepco) said on Thursday it had discovered a small leakage of radioactive water within one of its nuclear units north of Tokyo.

Tepco, the world's biggest private electric power company, added that the leak had no impact on the environment outside of the unit, and that it had found no other irregularities at any of the plant's other units.

A 6.8-magnitude earthquake rocked central Japan in July last year, killing 11 people and shutting down the world's largest nuclear power plant owned by TEPCO.

The impact of the latest tremors was strongest in Ibaraki and adjacent Tochigi prefecture where it cracked holes in weak buildings.

'We felt a strong jolt, but there are no reports as of now of any major damage,' an Ibaraki police spokesman said.

The strongest earthquake, which struck at a depth of 40km, followed a series of tremors off the Pacific coast early Thursday, including one measuring 6.3 on the Richter scale. - AFP