From Kyung Lah, CNN. March 12, 2011 3:10 p.m. EST
Shirakawa, Japan (CNN) -- A frantic scramble for survivors of Japan's strongest-ever earthquake -- which unleashed a wall of seawater that decimated entire neighborhoods -- stretched into a third day Sunday as rescuers dug through mud and rubble to find the buried, both alive and dead.
On Saturday, dazed survivors were plucked from collapsed homes, muddy water and burning debris. Japanese Prime Minister Naoto Kan said more than 3,000 people were rescued, according to the country's Kyodo News Agency.
While the official death toll from Japan's National Police Agency was at 686, with 642 missing, the actual toll is thought to be much higher, with Japanese public broadcaster NHK reporting more than 900 dead and Kyodo News Agency saying the death toll could top 1,800.
The number is expected to rise as rescuers reach more hard-hit areas. In one costal town alone -- Minamisanriku, in Myagi Prefecture -- some 9,500 people, half the town's population, were unaccounted for, Kyodo reported.
"We'd first like to focus on saving lives and secondly the comfort of the evacuees," Kan said. "There will be many resources that will be needed for this evacuation process."
Friday's 8.9-magnitude quake was centered about 130 kilometers (80 miles) from Sendai. The city, with a population of about a million, is located in a farming region already battling youth population losses to big cities, leaving aging residents struggling to keep up with the global economy.
Aftershocks continued to jolt the island nation Saturday, including two at 6.1 and 6.4 magnitude.
Kan visited areas of northeastern Japan affected by the quake and tsunami Saturday, according to Kyodo News Agency. He visited a crippled nuclear plant in Fukushima, and inspected other areas from the air.
In the city of Shirakawa, south of Sendai, rescuers dug through rubble with shovels to try to reach 13 neighbors trapped when the earth opened up and swallowed their homes.
Relatives and friends stood in the cold, quietly watching, praying and waiting. Others wept.
In other affected areas, military choppers plucked people from rooftops. In some cases, rescuers trudged in muddy water, carrying survivors on their backs. Weary, mud-spattered residents wandered through streets filled with crumpled cars and other debris.
The original quake struck Friday and left towns and villages along the northeastern coast devastated. Scores of aftershocks followed Saturday.
"The earth shook with such ferocity," said Andy Clark, who experienced the main earthquake at Narita Airport near Tokyo, about 370 kilometers (230 miles) southwest of the quake's epicenter. "I thought things were coming to an end. ... It was simply terrifying."
The quake also disrupted rail service and affected air travel. Flight cancellations left at least 23,000 people stranded in two Tokyo airports, Kyodo said. Departing and arriving flights resumed Saturday. Limited rail service also was back in operation Saturday.
Six million households, more than 10% of the total in Japan, were without electricity as of Saturday, said Ichiro Fujisaki, the nation's ambassador to the United States.
Meanwhile, experts worked to ensure the country's nuclear power plants are safe.
A blast at the Fukushima Daiichi plant injured four workers and crippled the plant's cooling systems, according to Japan's nuclear and industrial safety agency. The explosion sent a plume of white smoke into the air.
The explosion was not caused by the nuclear reactor but by "water vapor that was part of the cooling process," Chief Cabinet Secretary Yukio Edano said Saturday. He said no harmful gases were emitted by the explosion, which buckled the walls of a concrete building surrounding one of the reactors but did not damage the reactor itself.
"A tsunami far exceeded what had been expected," Kan said, and "some backup operation problems occurred" at the reactor, according to Kyodo News Agency.
Authorities have evacuated people living 20 kilometers (12 miles) from the reactor, according to the prime minister. However, Edano said radiation levels appeared to be falling after the blast.
On Saturday night, three people at a nearby hospital tested positive for radiation exposure, NHK reported, citing the prefecture as saying.
The three were randomly selected from a group of 90 hospital workers and patients who were already at the medical facility -- about three kilometers from the Daiichi plant -- before Friday's massive quake. The patients were outside of the hospital awaiting evacuation at the time of the explosion.
While the three showed signed of exposure, "no abnormal health conditions have been observed," NHK quoted the prefecture as saying.
Kan said the government is committed to preventing health problems among area residents, the news agency said.
Tokyo resident Nicky Washida said she was on the way to work, bicycling over a bridge, when the quake struck.
"The ground was heaving and rolling, and I looked around but there was just nowhere to run," she wrote in an iReport. "The noise was terrible -- metal clanging and scraping, and banging noises like concrete being dropped from a great height. I was ... going over part of Tokyo Bay, and I just cycled as hard as I could to get off the bridge because it felt absolutely like it was going to collapse (at) any moment."
The Japanese government made a formal request for U.S. aid, including military support, and full planning for deployment is in effect, with the U.S. military in Japan taking the lead, according to Sgt. Maj. Stephen Valley with U.S. Forces Japan.
The III Marine Expeditionary Force, based on the island of Okinawa south of Japan, said it was sending support staff, a cargo aircraft and transport helicopters. "Additional aircraft and supplies will continue to be moved in the next several days," the unit said in a statement.
The U.S. Agency for International Development has sent two search and rescue teams, from Virginia and California. Those teams, of about 150 people and 12 rescue dogs trained to find survivors, were expected to arrive Sunday morning and immediately begin working alongside Japanese and international teams.
At least 48 other countries and the European Union also have offered relief to Japan, and supplies and personnel are already on the way.
Online search engine Google said it has launched an online database to help people in Japan find their friends and relatives. The service collects information about the location and condition of people affected by the disaster.
The United States said that while Japan has officially sought its help in dealing with the earthquake's aftermath, it is not involved with efforts at Fukushima and Japan has not made requests for help at the nuclear plant. The International Atomic Energy Agency's Incident and Emergency Centre has offered assistance in dealing with the plant.
The impact of the quake was felt far and wide.
On the U.S. mainland, wave heights from Alaska to California ranged from less than a foot to more than 8 feet. The highest measurement, 8.1 feet, was at Crescent City, California.
In McKinleyville, California, three men were swept away by waves while taking pictures of the Pacific Ocean, according to the U.S. Coast Guard. Two returned to shore, but one died.
Hawaii Gov. Neil Abercrombie signed a "state of disaster proclamation" after the tsunami caused millions of dollars in damage. The proclamation will allow the state to get federal funds to rebuild, the governor said in a statement.
The quake was the latest in a series around Japan this week.
On Wednesday, a 7.2-magnitude earthquake struck off the coast of Honshu, the country's meteorological agency said. Early Thursday, an earthquake with a preliminary magnitude of 6.3 struck off the same coast.
Friday's quake is the strongest earthquake in recorded history to hit Japan, according to U.S. Geologic Survey records that date to 1900.
The world's largest recorded quake took place in Chile on May 22, 1960, with a magnitude of 9.5, the USGS said.
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