A powerful earthquake with a preliminary magnitude of 6.3 has stricken Japan ’s northeastern prefecture of Fukushima , which was devastated by a 9.0 magnitude quake and subsequent tsunami on March 11.

The earthquake hit Fukushima at 2:07 p.m on April 12, just six hours after another quake, with a magnitude of 6.3, jolted the central prefecture of Chiba , shaking the capital city of Tokyo and neighbouring prefectures, reported the Japan Meteorological Agency (JMA).

JMA said it had seen surge waves, but no tsunami warning was issued.

Workers at the Fukushima Nuclear Power Plant No. 1 were ordered to evacuate. However, the plant operator, Tokyo Electric Power Co., said no abnormality was found at pumps used to cool its reactors.

In another move, Japan raised the severity level of the ongoing emergency at the crippled Fukushima Daiichi nuclear plant on April 12 from level 5 to the maximum 7 on an international scale, recognising that the tsunami-induced accident ranks with the world's worst nuclear catastrophe in 1986 at Chernobyl .

The government's Nuclear and Industrial Safety Agency upgraded its provisional evaluation based on an estimate that radioactive materials far exceeding the criteria for level 7 have so far been released into the external environment, but added the release from the Fukushima plant is about 10 percent of that from the former Soviet nuclear plant.

Chief Cabinet Secretary Yukio Edano said the government was ''sorry to Fukushima residents, the Japanese people and the international community'' over the nuclear disaster caused by the massive March 11 quake and tsunami. However, Edano affirmed there was no "direct health damage" so far from the crisis./.