The Tokyo area infrastructure, particularly public and private transportation, doesn't hold up very well under stress. Typhoons that pass over too closely; the increasingly common localized, violent "guerrilla rains"; or even a few centimeters of snow are likely to cause disruption, and often the degree of disruption seems disproportionate to the cause. The number of people living and working in a relatively small area, and the distances that many of them have to commute--I've seen averages figures of 26 km and 68 minutes claimed for people working in Metropolitan Tokyo--contribute greatly to this, as do many other factors including road conditions and the layout of the city. Even a single slightly delayed train, along with a certain amount of ripple effect, can inconvenience a lot of people if it's anywhere around rush hour, for example, and if the delay demands switching passengers to buses, the already chronically congested streets don't make things any smoother.
Yesterday's first snowfall of the season was the heaviest in six years, but it only amounted to between two and six centimeters around Tokyo and caused fewer train delays than I've seen in the past. Nevertheless, there were at least 53 people taken to hospitals with snow-related injuries (mostly pedestrians who slipped and fell), 393 traffic accidents in Tokyo, and well over 1000 traffic accidents if the immediate surrounding area is included. There almost certainly would have been more trouble if the snow had started falling more heavily and earlier: many people seem to have managed to start for home in time to avoid getting stuck, and much of the snow fell after the commuters and their trains, buses, and cars were mostly done for the day.
Earthquakes, being destructive as well as obstructive, obviously can be even more disruptive, as those of us who have lived here for awhile already thought we knew, before last March's catastrophe demonstrated to us just how much nature can interfere with civilization. I think it's safe to say that most Tokyo or Southern Kanto dwellers tended to think of Tohoku as being pretty far away, even while being intellectually aware that Sendai is only two hours from Tokyo by Shinkansen. The direct, and indirect--and continuing--effects of the 3/11 disaster have disabused us of that notion.
Now a team of University of Tokyo researchers has published a report saying that a Magnitude 7 earthquake (I believe that's roughly comparable to the 1906 or 1989 earthquakes that struck my home town of San Francisco) has a 70% chance of striking Tokyo within the next four years. The Japanese government's previous claim has been a 70% chance within 30 years. Earthquake prediction, even when old records are considered more carefully and less skeptically than they had been, is a very inexact science. It's also not the only branch of science that doesn't really lend itself well to speaking of percentage chances over time...likelihood can be a tricky thing to quantify. Still, the Meteorological Agency reports that an average of 1.48 earthquakes per day ranging from Magnitude 3 to 6 have occurred in and near Tokyo since last March's quake, roughly five times as many as before it. The increased seismic activity is thought by at least some researchers to increase the likelihood of a big quake striking Tokyo, sooner than was previously expected.
Depending on the time of day, on wind velocity, and on the season, the casualties and damage that would be caused by a Magnitude 7 quake hitting near Tokyo would be huge. Even pretty conservative estimates suggest between 7,000 and 12,000 deaths, 6.5 million people unable to return home, and something between a half and 1.5 million buildings destroyed completely by earthquake, liquefaction, and fire combined. It doesn't require too much imagination to see how the disruption of Japan's highly centralized national government and financial/adminstrative/communications center would affect rescue services and the logistics of getting essential services back up.
Personally, I would want to be neither in one of the deeper subways nor in a high-rise condominium or office when a quake of that magnitude or larger strikes Tokyo. Both are likely to be fairly quake-resistant (at least the newer ones), but neither offer very attractive options for getting to any other relatively safe place with food and water available in the short term. I concede that either might be safer than, for example, being caught riding my motorcycle under the expressway or a railway trestle, but I think I'd prefer the probably illusory feeling of freedom that being at ground level with my own vehicle would give me.
I'm also well aware that a 70% chance of a massive quake within the next 30 years or within the next four means that it could be tomorrow either way. That's not a thought that anyone wants to dwell on very much, including me. That shouldn't prevent us from doing what we can to prepare as much as is practical for the eventuality, however. Next time, I'll go into some of my thoughts on that.