A tsunami warning was issued for Eastern Japan after a powerful quake hit the country with an epicentre close to the nuclear plant crippled by a huge tsunami a month ago.
The US geological survey said the 7.1 magnitude onshore quake hit with a depth of just 10 kilometres (six miles), its epicentre 86 kilometres south-southeast of Fukushima city.
Japan's meteorological agency issued a tsunami warning saying a one-metre (three feet) wave could hit Ibaraki prefecture, the area worst hit by last month's massive tsunami.
A wave half a metre high was expected in Fukushima prefecture.
Narita airport temporarily closed a runway following the quake for safety checks.
Earlier,
Sirens wailed and people bowed their heads and wept along Japan's devastated northeast coast on Monday as they marked a month since the tsunami that killed up to 25,000 people and unleashed a persistent nuclear crisis.
The tsunami-flooded Fukushima Dai-ichi nuclear plant is still leaking radiation after its cooling systems were knocked out by the tsunami, and the government Monday urged even more people living around the complex to leave within a month, citing concerns about long-term health risks from radiation as the crisis wears on.
People living within 12 miles (20 kilometers) already have been ordered to leave because of concerns about radiation in the air. Other people farther out had been advised to stay indoors.
Chief Cabinet secretary Yukio Edano said Monday that residents of five more communities, some more than 20 miles (30 kilometers) from the plant, are being urged to leave because of high levels of radiation.
"This is not an emergency measure that people have to evacuate immediately," Edano said. "We have decided this measure based on long-term health risks."
It was one more reminder of how long it could take to resolve the nuclear crisis. With that still ongoing, thousands of bodies yet to be found and more than 150,000 people living in shelters, there was little time on Monday for reflection on Japan's worst disaster since World War II.
"My chest has been ripped open by the suffering and pain that this disaster has caused the people of our prefecture," said Yuhei Sato, the governor of Fukushima, which saw its coastal areas devastated by the tsunami and contains the damaged plant at the center of the nuclear crisis. "I have no words to express my sorrow."
People in hard-hit towns gathered for ceremonies at 2:46 p.m., the exact moment of the magnitude-9.0 quake that spawned the tsunami March 11.
In a devastated coastal neighborhood in the city of Natori, three dozen firemen and soldiers removed their hats and helmets and joined hands atop a small hill that has become a memorial for the dead. Earlier, four monks in pointed hats rang a prayer bell there as they chanted for those killed.
The noisy clatter of construction equipment ceased briefly as crane operators stood outside their vehicles and bowed their heads.