Japan's Meteorological Agency says it has upgraded the magnitude of Friday's catastrophic earthquake to 9.0.
The agency earlier measured it at 8.8. The quake was already the biggest to hit Japan since record-keeping began in the late 1800s and one of the biggest ever recorded in the world.
The agency warned Sunday of more strong aftershocks following the quake, which unleashed massive tsunamis in northern Japan and killed at least 763 people.
The U.S. Geological Survey has measured the quake at magnitude 8.9, and that number remained unchanged Sunday.RELATED: Search the Red Cross "Safe and Well" site to see if loved ones in the quake zone have checked in to say they are okay
A partial meltdown was likely under way at a second nuclear reactor, a top Japanese official said Sunday, as authorities frantically tried to prevent a similar threat from nearby unit following a catastrophic earthquake and tsunami.
Some 170,000 people have been ordered to evacuate the area covering a radius of 12 miles (20 kilometers) around the plant in Fukushima near Iwaki. A meltdown refers to a very serious collapse of a power plant's systems and its ability to manage temperatures. A complete meltdown would release uranium and dangerous byproducts into the environment that can pose serious health risks.
The Japanese government said radiation emanating from the plant appeared to have decreased after Saturday's blast, which produced a cloud of white smoke that obscured the complex. But the danger was grave enough that officials pumped seawater into the reactor to avoid disaster and moved 170,000 people from the area.
Japan's nuclear safety agency then reported an emergency at another reactor unit, the third in the complex to have its cooling systems malfunction. To try to release pressure from the overheating reactor, authorities released steam that likely contained small amounts of radiation, the government said.
Japan dealt with the nuclear threat as it struggled to determine the scope of the earthquake, the most powerful in its recorded history, and the tsunami that ravaged its northeast Friday with breathtaking speed and power. The official count of the dead was 686, but the government said the figure could far exceed 1,000.
Teams searched for the missing along hundreds of miles (kilometers) of the Japanese coast, and thousands of hungry survivors huddled in darkened emergency centers that were cut off from rescuers and aid. At least a million households had gone without water since the quake struck. Large areas of the countryside were surrounded by water and unreachable.
The explosion at the nuclear plant, Fukushima Dai-ichi, 170 miles (274 kilometers) northeast of Tokyo, appeared to be a consequence of steps taken to prevent a meltdown after the quake and tsunami knocked out power to the plant, crippling the system used to cool fuel rods there.
The blast destroyed the building housing the reactor, but not the reactor itself, which is enveloped by stainless steel 6 inches (15 centimeters) thick.
Inside that superheated steel vessel, water being poured over the fuel rods to cool them formed hydrogen. When officials released some of the hydrogen gas to relieve pressure inside the reactor, the hydrogen apparently reacted with oxygen, either in the air or the cooling water, and caused the explosion.
"They are working furiously to find a solution to cool the core," said Mark Hibbs, a senior associate at the Nuclear Policy Program for the Carnegie Endowment for International Peace.
Nuclear agency officials said Japan was injecting seawater into the core - an indication, Hibbs said, of "how serious the problem is and how the Japanese had to resort to unusual and improvised solutions to cool the reactor core."
Officials declined to say what the temperature was inside the troubled reactor, Unit 1. At 2,200 degrees Fahrenheit (1,200 degrees Celsius), the zirconium casings of the fuel rods can react with the cooling water and create hydrogen. At 4,000 F (2,200 C), the uranium fuel pellets inside the rods start to melt, the beginning of a meltdown.
Chief Cabinet Secretary Yukio Edano said radiation around the plant had fallen, not risen, after the blast but did not offer an explanation. Virtually any increase in dispersed radiation can raise the risk of cancer, and authorities were planning to distribute iodine, which helps protect against thyroid cancer. Authorities ordered 210,000 people out of the area within 12 miles (20 kilometers) of the reactor.
It was the first time Japan had confronted the threat of a significant spread of radiation since the greatest nightmare in its history, a catastrophe exponentially worse: the 1945 atomic bombings of Hiroshima and Nagasaki by the United States, which resulted in more than 200,000 deaths from the explosions, fallout and radiation sickness.
Officials have said that radiation levels at Fukushima were elevated before the blast: At one point, the plant was releasing each hour the amount of radiation a person normally absorbs from the environment each year.
The Japanese utility that runs the plant said four workers suffered fractures and bruises and were being treated at a hospital. Nine residents of a town near the plant who later evacuated the area tested positive for radiation exposure, though officials said they showed no health problems.
As Japan entered its second night since the magnitude-8.9 quake, there were grim signs that the death toll could soar. One report said no one could find four whole trains. Others said 9,500 people in one coastal town were unaccounted for and that at least 200 bodies had washed ashore elsewhere.
The government said 642 people were missing and 1,426 injured.
Atsushi Ito, an official in Miyagi prefecture, among the worst-hit states, could not confirm the figures, noting that with so little access to the area, thousands of people in scores of towns could not yet be reached.
"Our estimates based on reported cases alone suggest that more than 1,000 people have lost their lives in the disaster," Edano said. "Unfortunately, the actual damage could far exceed that number considering the difficulty assessing the full extent of damage."
Japan, among the most technologically advanced countries in the world, is well-prepared for earthquakes. Its buildings are made to withstand strong jolts - even Friday's, the strongest in Japan since official records began in the late 1800s. The tsunami that followed was beyond human control.
With waves 23 feet (7 meters) high and the speed of a jumbo jet, it raced inland as far as six miles (10 kilometers), swallowing homes, cars, trees, people and anything else in its path.
"The tsunami was unbelievably fast," said Koichi Takairin, a 34-year-old truck driver who was inside his sturdy, four-ton rig when the wave hit the port town of Sendai. "Smaller cars were being swept around me. All I could do was sit in my truck."
His rig ruined, he joined the steady flow of survivors who walked along the road away from the sea and back into the city Saturday.
Smashed cars and small airplanes were jumbled against buildings near the local airport, several miles (kilometers) from the shore. Felled trees and wooden debris lay everywhere as rescue workers in boats nosed through murky waters and around flooded structures.
The tsunami set off warnings across the Pacific Ocean, and waves sent boats crashing into one another and demolished docks on the U.S. West Coast. In Crescent City, California, near the Oregon state line, one person was swept out to sea and had not been found Saturday.
In Japan early Sunday, firefighters had yet to contain a large blaze at the Cosmo Oil refinery in the city of Ichihara. Four million households remained without power. The Russian news agency RIA Novosti reported that Japan had asked for additional energy supplies from Russia.
Prime Minister Naoto Kan said 50,000 troops had joined the rescue and recovery efforts, helped by boats and helicopters. Dozens of countries offered to pitch in. President Barack Obama said one American aircraft carrier was already off Japan and a second on its way.
Aid had just begun to trickle into many areas. More than 215,000 people were living in 1,350 temporary shelters in five prefectures, the Japanese national police agency said.
"All we have to eat are biscuits and rice balls," said Noboru Uehara, 24, a delivery truck driver who was wrapped in a blanket against the cold at a shelter in Iwake. "I'm worried that we will run out of food."
The transport ministry said all highways from Tokyo leading to quake-stricken areas were closed, except for emergency vehicles. Mobile communications were spotty and calls to the devastated areas were going unanswered.O
ne hospital in Miyagi prefecture was seen surrounded by water, and the staff had painted "SOS," in English, on its rooftop and were waving white flags.
Around the nuclear plant, where 51,000 people had previously been urged to leave, others struggled to get away.
"Everyone wants to get out of the town. But the roads are terrible," said Reiko Takagi, a middle-aged woman, standing outside a taxi company. "It is too dangerous to go anywhere. But we are afraid that winds may change and bring radiation toward us."
Although the government played down fears of radiation leak, Japanese nuclear agency spokesman Shinji Kinjo acknowledged there were still fears of a meltdown - the collapse of a power plant's systems, rendering it unable regulate temperatures and keep the reactor fuel cool.
Yaroslov Shtrombakh, a Russian nuclear expert, said it was unlikely that the Japanese plant would suffer a meltdown like the one in 1986 at Chernobyl, when a reactor exploded and sent a cloud of radiation over much of Europe. That reactor, unlike the reactor at Fukushima, was not housed in a sealed container.RELATED: Watch videos of the earthquake and tsunami as they happened in Japan
The magnitude-8.9 offshore quake unleashed a 23-foot (seven-meter) tsunami and was followed for hours by more than 50 aftershocks, many of them of more than magnitude 6.0.
Dozens of cities and villages along a 1,300-mile (2,100-kilometer) stretch of coastline were shaken by violent tremors that reached as far away as Tokyo, hundreds of miles (kilometers) from the epicenter. A large section of Kesennuma, a town of 70,000 people in Miyagi, burned furiously into the night with no apparent hope of being extinguished, public broadcaster NHK said.
"The earthquake has caused major damage in broad areas in northern Japan," Prime Minister Naoto Kan said at a news conference.
The government ordered thousands of residents near a nuclear power plant in the city of Onahama to move back at least two miles (three kilometers) from the plant. The reactor was not leaking radiation but its core remained hot even after a shutdown. The plant is 170 miles (270 kilometers) northeast of Tokyo.
Trouble was reported at two other nuclear plants as well, but there was no radiation leak at either of them.
Japan's coast guard said it was searching for 80 dock workers on a ship that was swept away from a shipyard in Miyagi.
Even for a country used to earthquakes, this one was of horrific proportions because of the tsunami that crashed ashore, swallowing everything in its path as it surged several miles (kilometers) inland before retreating. The apocalyptic images on Japanese TV of powerful, debris-filled waves, uncontrolled fires and a ship caught in a massive whirlpool resembled scenes from a Hollywood disaster movie.
Large fishing boats and other vessels rode high waves ashore, slamming against overpasses or scraping under them and snapping power lines along the way. Upturned and partially submerged cars bobbed in the water. Ships anchored in ports crashed against each other.
The tsunami roared over embankments, washing anything in its path inland before reversing directions and carrying the cars, homes and other debris out to sea. Flames shot from some of the homes, probably because of burst gas pipes.
Waves of muddy waters flowed over farmland near Sendai, carrying buildings, some of them ablaze. Drivers attempted to flee. Sendai airport was inundated with thick, muddy debris that included cars, trucks, buses and even light planes.
Highways to the worst-hit coastal areas buckled. Telephone lines snapped. Train service in northeastern Japan and in Tokyo, which normally serve 10 million people a day, were suspended, leaving untold numbers stranded in stations or roaming the streets. Tokyo's Narita airport was closed indefinitely.
President Barack Obama said the U.S. "stands ready to help" Japan.
Jesse Johnson, a native of the U.S. state of Nevada who lives in Chiba, north of Tokyo, was eating at a sushi restaurant with his wife when the quake hit.
"At first it didn't feel unusual, but then it went on and on. So I got myself and my wife under the table," he told The Associated Press. "I've lived in Japan for 10 years, and I've never felt anything like this before. The aftershocks keep coming. It's gotten to the point where I don't know whether it's me shaking or an earthquake."
NHK said more than 4 million buildings were without power in Tokyo and its suburbs.
As night fell, Tokyo's streets were jammed with cars, buses and trucks trying to get around and out of the city. Pedestrians swarmed the sidewalks to walk home, or at least find a warm place to spend the night as the temperatures dropped.
Tomoko Suzuki and her elderly mother stood on a crowded downtown corner, unable to get to their 29th-floor condominium because the elevator wasn't working. They unsuccessfully tried to hail a taxi to a relative's house and couldn't find a hotel room.
"We are so cold," said Suzuki. "We really don't know what to do."
A large fire erupted at the Cosmo oil refinery in the city of Ichihara and burned out of control with 100-foot (30-meter) flames whipping into the sky.
"Our initial assessment indicates that there has already been enormous damage," Chief Cabinet Secretary Yukio Edano said. "We will make maximum relief effort based on that assessment."
He said the Defense Ministry was sending troops to the hardest-hit region. A utility aircraft and several helicopters were on the way.
Also in Miyagi prefecture, a fire broke out in a turbine building of a nuclear power plant, but it was later extinguished, said Tohoku Electric Power Co.
A reactor area of a nearby plant was leaking water, the company said. But it was unclear if the leak was caused by the tsunami or something else. There were no reports of radioactive leaks at any of Japan's nuclear plants.
Jefferies International Ltd., a global investment banking group, estimated overall losses of about $10 billion.
Hiroshi Sato, a disaster management official in northern Iwate prefecture, said officials were having trouble getting an overall picture of the destruction.
"We don't even know the extent of damage. Roads were badly damaged and cut off as tsunami washed away debris, cars and many other things," he said.
The U.S. Geological Survey said the 2:46 p.m. quake was magnitude 8.9, the biggest to hit Japan since record-keeping began in the late 1800s and one of the biggest ever recorded in the world.
The quake struck at a depth of six miles (10 kilometers), about 80 miles (125 kilometers) off the eastern coast, the agency said. The area is 240 miles (380 kilometers) 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.
A tsunami warning was extended to a number of areas in the Pacific, Southeast Asia and Latin America, including Japan, Russia, Indonesia, New Zealand and Chile. In the Philippines, authorities ordered an evacuation of coastal communities, but no unusual waves were reported.
Thousands fled homes in Indonesia after officials warned of a tsunami up to 6 feet (2 meters) high, but waves of only 4 inches (10 centimeters) were measured. No big waves came to the Northern Mariana Islands, a U.S. territory, either.
The first waves hit Hawaii about 9 a.m. EST (1400 GMT). A tsunami about 7 feet (2.1 meters) high was recorded on Maui and a wave at least 3 feet (a meter) high was recorded on Oahu and Kauai. Officials warned that the waves would continue and could get larger.
Japan's worst previous quake was a magnitude 8.3 temblor in 1923 in Kanto that killed 143,000 people, according to USGS. A 7.2-magnitude quake in Kobe in 1996 killed 6,400 people.
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 nations. A magnitude-8.8 temblor that shook central Chile in February 2010 also generated a tsunami and killed 524 people.
For emergency information, assistance, and locating family in connection with earthquake in Japan: http://www.facebook.com/l/6b2e3a9CLMNbUwBsOw1jOL8d5aw/www.jhelp.com
Phone numbers in US and Japan:
202 559 4683
800 373 1110
0570 000 911
011 81 90 7170 4769
011 81 90 3080 6711