A strong aftershock rattled northeastern Japan Monday, triggering new tsunami warnings and the evacuation of an already-crippled nuclear power plant. Authorities also urged more people living near the plant to prepare to leave the area, citing concerns about long-term health risks from leaking radiation.
The U.S. Geological Survey initially reported the magnitude of the aftershock at 7.1, the same strength as one that hit on Thursday. But authorities later revised it downward to 6.6. A tsunami warning was issued for portions of the northeast coast, but was lifted less than one hour later.
Monday's aftershock struck just hours after people across Japan bowed their heads in silence to mark the one-month anniversary of the massive March 11 earthquake and tsunami that killed up to 25,000 people and triggered the crisis at the Fukushima-Daiichi nuclear power plant on the country's northeastern coast.
More than 13,000 bodies have been recovered and more than 14,000 remain missing, many still lying under the rubble or washed out to sea.
Officials at Japan's nuclear safety agency said workers were temporarily evacuated Monday from the Fukushima nuclear plant, which has been leaking radiation since the March 11 quake. They said outside electrical power to three of the plant's units was cut off, disrupting the injection of cooling water into the reactor cores, but that the electricity was restored after less than an hour. The officials said they had detected no change in radiation levels.
Authorities also announced preparations to expand the 20-kilometer evacuation zone around the Fukushima plant.
Chief Cabinet Secretary Yukio Edano said residents should prepare to evacuate from certain towns and villages outside the zone as soon as radiation levels exceed pre-determined levels.
He said the government will coordinate with municipal officials in those areas and that residents should wait for further instructions.
Edano also said residents living between 20 and 30 kilometers from the stricken Fukushima plant should be packed and ready to leave on short notice if the situation at the plant deteriorates. He said those who are ill or have small children should leave the zone in advance of any emergency.
Edano said the decision to evacuate specified communities outside the 20-kilometer radius is based on radiation concentrations caused by geological and weather factors. He said residents must also be prepared for a new emergency at the plant, even though the likelihood of that is considerably lower than before.
The Greenpeace environmental group issued a statement earlier Monday calling for the government to evacuate residents from several radiation hotspots and to remove the most vulnerable people from Fukushima city. It said its own monitoring showed residents of the city could receive the annual maximum dose of radiation within a few weeks.
Earlier Monday, repair crews at the Fukushima plant had begun using remote-controlled power shovels and bulldozers to clear rubble from two nuclear reactors that were damaged by hydrogen explosions.
Officials said they were also preparing to transfer highly radioactive water from a tunnel next to the plant's number two reactor into a condenser. Technicians have been pumping less radioactive water into the ocean since last week to make room in a temporary storage facility for the more dangerous water.
The crisis has made many Japanese distrustful of nuclear power. About 2,000 to 3,000 people took part in a protest march Sunday demanding that the government shut down all nuclear plants in the country.
Some information for this report was provided by AP, AFP and Reuters.