TOKYO, Japan (AP) -- A strong earthquake struck northwestern Japan on Monday, causing a radioactive water leak and fire at one of the world's most powerful nuclear power plants and turning buildings into piles of lumber. At least seven people were killed and hundreds injured.
After an earthquake, fire broke out at a Japanese nuclear plant -- the world's largest in power output.
1 of 3 more photos » The plant leaked about a half-gallon of water in a building housing one of its reactors, said Katsuya Uchino, a Tokyo Electric Power Co. official.
Uchino said the water contained a tiny amount of radioactive material -- a billionth of the guideline under Japanese law -- and is believed to have flushed into the Sea of Japan.
A company statement said the leak had stopped and that there had been no "significant change" in the seawater under surveillance and no effect on the environment.
The reactor automatically shut down at the time of the leak, the report said. The quake triggered a fire at an electrical transformer at the plant, but Tokyo Electric Power Co. said earlier in the day that the reactor was not damaged.
Flames and billows of black smoke poured from the Kashiwazaki nuclear plant -- the world's largest in terms of power output capacity. See crumbled roads and homes after the killer quake »
Aileen Mioko Smith, of the environmentalist group Green Action, said the fire showed that some facilities at nuclear power plants such as electrical transformers were built to lower quake-resistance levels than other equipment such as reactor cores.
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"That's the Achilles heel of nuclear power plants," said Mioko Smith, who said it took the plant two hours to extinguish the fire. "Today's a good example of that... How prepared are they to put out fires when they happen?
The earthquake, which left fissures 3 feet wide in the ground along the coast, hit shortly after 10 a.m. local time and was centered off Niigata state. Buildings swayed 160 miles away in Tokyo. Sirens wailed in Kashiwazaki, a city of about 90,000, which appeared to be hardest hit.
Japan's Meteorological Agency measured the quake at a 6.8 magnitude. The U.S. Geological Survey, which monitors quakes around the world, said it registered 6.7.
"I was so scared -- the violent shaking went on for 20 seconds," Ritei Wakatsuki, who was on her job in a convenience store in Kashiwazaki. "I almost fainted by the fear of shaking."
Hours later, a meteorological agency reported a 6.6 magnitude quake off Japan's west coast. Watch CNN's Chad Myers offer a possible explanation for the second quake »
Tsunami warnings were issued along the coast of Niigata but later lifted.
A series of smaller aftershocks rattled the area, including one with a 5.8 magnitude. The Meteorological Agency warned that the aftershocks could continue for a week.
The quake hit on Marine Day, a national holiday in Japan, when most people would have been at home.
Four women and three men -- all either in their 70s or 80s -- were killed, according to the National Police Agency in Tokyo and NHK, which reported more than 800 people were hurt.
Nearly 300 homes in Kashiwazaki -- a city known mainly for its fishing industry -- were destroyed and some 2,000 people evacuated, officials said.
A ceiling collapsed in a gym in Kashiwazaki where about 200 people had gathered for a badminton tournament, and one person was hurt, Kyodo reported. The quake also knocked a train car off the rails while it was stopped at a station. No one was injured.
Several bullet train services linking Tokyo to northern and northwestern Japan were suspended.
Prime Minister Shinzo Abe -- whose ruling party is trailing in the polls -- interrupted a campaign stop in southern Japan for upcoming parliamentary elections, rushed back to Tokyo and announced he would head to the damaged area. He later arrived in a blue uniform to survey the damage.
"Many people told me they want to return to their normal lives as quickly as possible," Abe told reporters in Kashiwazaki. "The government will make every effort to help with recovery."
Japan sits atop four tectonic plates and is one of the world's most earthquake-prone countries. The last major quake to hit the capital, Tokyo, killed some 142,000 people in 1923, and experts say the capital has a 90 percent chance of suffering a major quake in the next 50 years.
In October 2004, a magnitude-6.8 earthquake hit Niigata, killing 40 people and damaging more than 6,000 homes. It was the deadliest to hit Japan since 1995, when a magnitude-7.2 quake killed 6,433 people in the western city of Kobe.
