More than 10,000 people sheltered in evacuation centres on Tuesday in Japan's northwest after a strong earthquake flattened hundreds of houses, killing at least nine people and injuring more than 900.
As aftershocks continued, rain began to fall, and forecasts for two days of wet weather raised fears of mudslides that could add to the devastation.
Houses collapsed and water, gas and electricity supplies were cut by the 6.8 magnitude quake in Niigata prefecture on Monday, which also caused a small radiation leak and fire at the world's biggest nuclear plant.
Nine elderly people were killed by the tremor, which hit at 10:13 a.m. local time on Monday, police said.
"I am worried about the aftershocks," said 80-year-old Toshiko Kojima, who said she had spent a mostly sleepless night in a crowded elementary school gymnasium in Kashiwazaki.
The quake halted gas service to about 35,000 homes and disrupted the water supply to all of Kashiwazaki, a city with a population of around 95,000 that was hardest hit by the quake. About 25,000 homes in Niigata prefecture were without electricity, local officials and media said.
The country was rattled late on Monday evening by a deep tremor under the Sea of Japan estimated at magnitude 6.6 to 6.8 that caused buildings in Tokyo to sway, but there were no immediate reports of further damage.
Tokyo Electric Power Co (TEPCO) said 1.5 litres of water containing radioactive materials had leaked from a unit at its Kashiwazaki-Kariwa nuclear power plant -- the world's largest.
A similar amount of contaminated water had been released into the ocean and had had no effect on the environment, the company said in a statement, adding that the quake was stronger than its reactors had been designed to cope with.
A fire in an electrical transformer at the plant was quickly extinguished but it was unclear when TEPCO could restart three power units there.
Japan is one of the world's most earthquake-prone countries, with a tremor occurring at least every five minutes.
Houses, many wooden with traditional heavy tile roofs, collapsed and roads cracked in Monday's quake, centered in the same northwestern area as a tremor three years ago.
Troops and extra emergency teams helped with rescue and relief efforts, including distributing water and rice. Prime Minister Shinzo Abe cut short campaigning for parliamentary elections to inspect damage.
"We need to take every step to take lives. It's supposed to rain tomorrow (Tuesday) in the area so we have to take every step to save lives, secure lifelines and reassure people," Abe told reporters.
The government set up an emergency office to deal with the quake, which officials said had damaged about 500 buildings.
"I was sitting on the balcony and was scared to death," said Kiyono Fujisawa, a 70-year-old farmer, who lives with five others including her daughter and grandchildren in a house that was partly destroyed. "Look at my house. I'm too scared to go back in."
Bullet trains stopped services in northern Japan for a time after the quake and a local train toppled from the rails, but media said no one was injured.
Niigata was hit in October 2004 by a quake with a matching magnitude of 6.8 that killed 65 people and injured more than 3,000. It was the deadliest quake in Japan since a magnitude 7.3 tremor hit Kobe city in 1995, killing more than 6,400.