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The death toll from Friday's massive magnitude 8.9 quake stood at more than 200, but an untold number of bodies were believed to be lying in the rubble and debris, and Japanese were bracing for more bad news as authorities tried to reach the hardest-hit areas.
Aerial footage showed military helicopters lifting people on rescue tethers from rooftops and partially submerged buildings surrounded by water and debris. At one school, a large white "SOS" had been spelled out in English.
The earthquake that struck off the northeastern shore was the biggest recorded quake ever to hit Japan. It ranked as the fifth-largest earthquake in the world since 1900 and was nearly 8000 times stronger than one that devastated Christchurch last month, scientists said.
Chief Cabinet Secretary Yukio Edano said an initial assessment found "enormous damage," adding that the Defence Ministry was sending troops to the hardest-hit region.
The official casualty toll was 236 dead, 725 missing and 1028 injured, although police said 200-300 bodies were found along the coast in Sendai, the biggest city in the area. Authorities said they weren't able to reach the area because of damage to the roads.
Black smoke could still be seen in the skies around Sendai, presumably from gas pipes snapped by the quake or tsunami.
Early today, Atsushi Koshi, a 24-year-old call centre worker in the coastal city of Tagajo, about 16km east of Sendai, said his cousin remained trapped on the roof of a department store with about 200 to 300 other people awaiting rescue. The store wasn't far from the port of Sendai, where the tsunami had washed ashore.
The rest of his family was safe, but he wondered what to do, since the house he shared with his parents was tilting from the quake and a concrete block wall had fallen apart.
"If we clean up our house it might be livable, but we're discussing what to do next," he said.
The quake shook dozens of cities and villages along a 2100km stretch of coast and tall buildings swayed in Tokyo, hundreds of miles from the epicenter. Minutes later, the earthquake unleashed a 7m tsunami that washed far inland over fields and smashed towns.
The town of Rikuzentakada, population 24,700, in northern Iwate prefecture, looked largely submerged in muddy water, with hardly a trace of houses or buildings of any kind.
The entire Pacific had been put on alert - including coastal areas of South America, Canada and Alaska - but waves were not as bad as expected.
The US Geological Survey said that after the initial huge quake, there were 123 aftershocks off Japan's main island of Honshu, 110 of them of magnitude 5.0 or higher
President Barack Obama pledged US assistance following what he called a potentially "catastrophic" disaster. He said one US aircr
aft carrier was already in Japan and a second was on its way. A US ship was also heading to the Marianas Islands to assist as needed, he added.
Japan also declared its first-ever states of emergency for five nuclear reactors at two power plants after the units lost cooling ability in the aftermath of the earthquake, and workers struggled to prevent meltdowns.
The earthquake knocked out power at the Fukushima Daiichi plant, and because a backup generator failed, the cooling system was unable to supply water to cool the 460-megawatt No. 1 reactor. Although a backup cooling system is being used, Japan's nuclear safety agency said pressure inside the reactor had risen to 1.5 times the level considered normal.
Authorities said radiation levels had jumped 1000 times normal inside Unit 1 and were measured at eight times normal outside the plant. They expanded an earlier evacuation zone more than threefold, from 3km to 10km. About 3000 people were urged to leave their homes in the first announcement.
Tokyo Electric Power Co. warned of power shortages and an "extremely challenging situation in power supply for a while."
The utility, which also operates reactors at the nearby Fukushima Daini plant, later confirmed that cooling ability had been lost at three of four reactors there, as well as a second Fukushima Daiichi unit. The government promptly declared a state of emergency there as well. Nearly 14,000 people living near the two power plants were ordered to evacuate.
The level outside the 40-year-old plant in Onahama, a city about 270 northeast of Tokyo, is still considered very low compared to the annual exposure limit, said Ryohei Shiomi, an official with the Japan Nuclear and Industrial Safety Agency. It would take 70 days of standing at the gate to reach the limit, he said.
The Defence Ministry said it had sent troops trained to deal with chemical disasters to the plants in case of a radiation leak.
A large fire erupted at the Cosmo oil refinery in the city of Ichihara and burned out of control with 30m flames whipping into the sky.
Most trains in Tokyo started running again today after the city was brought to a near standstill yesterday. Tens of thousands of people were stranded with the rail network down, and the streets were jammed with cars, buses and trucks trying to get out of the city.
The city set up 33 shelters in city hall, on university campuses and in government offices, but many planned to spend the
night at 24-hour cafes, hotels and offices.
The quake struck at a depth of 10km, about 125km off Japan's east coast, the USGS said. The area is 380km northeast of Tokyo. Several quakes hit the same region in recent days, including one measured at magnitude 7.3 on Wednesday that caused no damage.
"The energy radiated by this quake is nearly equal to one month's worth of energy consumption" in the United States, USGS scientist Brian Atwater told The Associated Press.
Japan's worst previous quake was a magnitude 8.3 in Kanto that killed 143,000 people in 1923, according to the USGS. A magnitude 7.2 quake in Kobe killed 6400 people in 1995.
Japan lies on the "Ring of Fire" - an arc of earthquake and volcanic zones stretching around the Pacific where about 90 percent of the world's quakes occur, including the one that triggered the Dec. 26, 2004, Indian Ocean tsunami that killed an estimated 230,000 people in 12 countries. A magnitude-8.8 temblor that shook central Chile in February 2010 also generated a tsunami and killed 524 people.
