A powerful magnitude-7.1 quake rattled Japan's northeast on Monday, forcing evacuation of workers from a radiation-leaking atomic plant around which the exclusion zone was also widened.
The latest tremor, which struck at 5.16 pm local time, shook buildings in Tokyo as well, hours after people across the country fell silent in the memory of nearly 30,000 people killed or missing in the March 11 mega quake and tsunami.
The US Geological Survey put the magnitude of Monday's onshore quake at 7.1 and said it struck at a depth of just 13 km. Its epicentre was 81 km south of Fukushima city near the crippled nuclear plant.
Japan's Meteorological Agency said the quake measured 7.0 on the Richter Scale and hit at a depth of 10 km.
At one time, it issued a tsunami warning of about three-foot high waves for the coastal areas of Ibaraki Prefecture, which was lifted 50 minutes later. A tsunami advisory for the
coastal areas of neighbouring prefectures was also lifted.
Several minor quakes followed today's massive tremor and the agency also warned of possible aftershocks with intensities of 6 plus or 6 minus.
Last Thursday, a 7.1-magnitude aftershock, which was the most powerful tremor since the March 11 magnitude-9 quake and tsunami, had rattled the region, claiming four lives and leaving millions of homes without power.
The operator of the crippled Fukushima nuclear plant, Tokyo Electric Power Company, which briefly evacuated its workers from the facility following Monday's quake, said
that radiation figures at monitoring posts around the power station remained unchanged.
The new quake came as Japan announced that it was extending the evacuation zone around the plant because of radiation fears.
"The government has designated for evacuation areas where the radiation exposure level is expected to reach 20 millisieverts per year," Chief Cabinet Secretary Yukio Edano said.
With the crisis at the Fukushima plant dragging on, Edano said some municipalities within the 20-to30-km radius of the plant would now be designated as additional evacuation areas.
The municipalities which will be part of the new evacuation zone include Katsurao, Namie and Iitate, all located in Fukushima prefecture. Residents in these municipalities will be expected to move to different areas within one month, Edano was quoted as saying by Kyodo.
The evacuation zone was widened exactly a month after the twin disaster ravaged Japan's northeast, in which 13,100 people were confirmed dead and over 14,300 remained missing.
The entire nation mourned the victims of the massive quake and tsunami, with people across the country observing a moment of silence at 2:46 pm local time. The natural disaster had struck Iwate, Miyagi and Fukushima prefectures at the same time on March 11.
A 46-year-old man was killed and five others injured in Ibaraki and Tochigi prefectures in Monda's quake, Kyodo said. The temblor was followed by another earthquake in the same area a minute later with a preliminary magnitude of 6.0.
The restoration work at the Fukushima plant was affected by the latest quake, but the government's nuclear safety agency said no major safety problem is believed to have occurred at the troubled reactors.
The nuclear plant, which was crippled by the twin disaster causing a full-blown nuclear crisis, continues to leak radiation into the air and sea, with little likelihood of it being brought under control anytime soon.
TEPCO has been struggling hard to restore reactor cooling systems since the March 11 disaster knocked out electricity at the plant. Heavily contaminated water in turbine buildings and a concrete tunnel has been hampering restoration efforts and preventing workers from even inspecting the pumping systems.
TEPCO on Sunday began removing debris from the plant using unmanned heavy machinery. It also plans to start moving highly radioactive water from the concrete tunnel to another storage facility soon, national broadcaster NHK reported. The power company said it is looking into using air instead of seawater for cooling.
It is also considering cooling the reactors' containment vessels with water, but could not say when it would be able to control the radiation. TEPCO said the water injection into the crippled reactors was briefly suspended after outside power lines were shut down by today's earthquake. It said that outside power was restored for Nos. 1, 2 and 3 reactors and water injection resumed after a suspension of about 50 minutes.
The government's Nuclear and Industrial Safety Agency admitted that it had failed to properly manage the accidents at the Fukushima nuclear power plant. Its spokesman Hidehiko Nishiyama said the agency could not clearly address the problems at the plant, as one emergency followed another.
He said the agency will thoroughly review what it has done so far.
The government on Monday established a panel to study how to compensate residents, farmers, fishery workers and other people for damage inflicted by the crisis at the Fukushima nuclear plant.
TEPCO is 'primarily' liable for damages, said Economy, Trade and Industry Minister Banri Kaieda, who is the head of the panel.
His remarks came a day after Prime Minister Naoto Kan promised the tsunami survivors of Japan that his government would "never abandon" them, as he surveyed the disaster-hit zone for the third time.