TOKYO. Millions of people in greater Tokyo were stranded far away from home on Friday evening after Japan's biggest earthquake on record shut down the capital's massive subway system.
Countless workers, who had earlier fled violently swaying office blocks, found themselves stuck far from their families -- and unable to speak to them because the overloaded mobile phone system could not carry most calls.
Sirens wailed through Tokyo, television helicopters buzzed overhead and people rushed to the city's ubiquitous 24-hour convenience stores, quickly emptying shelves of bento boxes, sandwiches and instant noodle cups.
"I have no idea how I'll get home," said an 18-year-old woman waiting outside Ginza subway station. She described how ceramics shattered around her in a department store when the huge quake hit mid-afternoon.
"Telephone lines are not working and the subway has completely stopped. I think Tokyo is very fragile right now," said Shintoku Arita, 35.
The government used loudspeaker alerts and TV broadcasts to urge people to stay near their workplaces rather than risk a long walk home, as highways leading out of the city centre were choked and hotels quickly fully booked.
"Please do not try to force your way home when there is no means of transportation, but stay in your offices and other safe places," said an emergency advisory carried by national public broadcaster NHK.
"Night is falling," the NHK newscaster said as chilly darkness fell.
"If long-distance commuters try to cross prefecture borders on foot at night, they may fall victim to secondary accidents."
The greater Tokyo region -- a sprawl that takes in Yokohama and vast suburban areas across the Kanto plain -- is the world's largest urban area, with more than 30 million people, many of whom commute for hours every day.
The spaghetti-like railway grid of the Tokyo Metro System and Japan Railway lines criss-crossing the megacity remained shut down for hours after the 8.9-magnitude monster quake violently shook buildings across the city.
Some lines, including the Ginza line, reopened around 9:00 pm, but others remained shut.
The "shuto kosoku" Tokyo metropolitan expressway was blocked entirely, and parts of the major Tohoku and Chuo expressways were also closed.
Despite the scale of the disaster, Tokyo was spared the worst by the quake, which hit offshore and spawned a tsunami that devastated coastal areas.
Volcano-dotted Japan is located on the "Pacific Ring of Fire", and Tokyo is situated in one of its most dangerous areas.
Seismologists say that the "Big One" -- a huge quake below or near Tokyo, forecast to kill thousands -- is, statistically speaking, long overdue.
The city sits on the intersection of three continental plates -- the Eurasian, Pacific and Philippine Sea plates -- which are slowly grinding against each other, building up enormous seismic pressure.
The government's Earthquake Research Committee warns of a 70 percent chance that a magnitude-eight quake will strike within 30 years in the Kanto plain.
The last "Big One" to hit Tokyo was the 1923 Great Kanto Earthquake that claimed over 140,000 lives, many of them in fires that ripped through wooden buildings. In 1855 the Ansei Edo quake also devastated the city.