Its epicenter was about 10 km deep under water off Fukushima Prefecture.

Tsunami warning was issued to prefectures including Ibaraki, Miyagi and Chiba immediately after the occurrence of the quake.

It was followed by four aftershocks of magnitude of above 5.0 in the area on the same day.

The continued quake left two persons dead and 200,000 households without electricity.

Right after the occurrence of the quake the Tokyo Electric Power Co. temporarily cut off the supply of power to Reactors Nos. 1, 2 and 3 of the Fukushima Nuclear Power Plant from outside and asked those who worked around the power plant to evacuate.

Magnitude 6.3 quake hit the area northeast of Chiba Prefecture Tuesday. Its epicenter was about 30 km deep under water off the area.

The Japanese government began considering the raise of the accident's severity level to 7, the worst on an international scale, from the current 5.

In the meantime, the U.S. Department of Energy made public the amounts of radioactive substances detected in the area within the radius of 80 km centering around the power plant.

According to them, radiation of 125 microsieverts per hour was measured in the compound of the power plant.

They said the area of more than 21 microsieverts spread 30~40 km northwest of the power plant in the shape of a band.