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The most damaging earthquake in Japan’s history was the Great Kanto Earthquake in 1923. It measured 7.9 on the Richter Scale and devastated much of downtown Tokyo. More than 140,000 people died, most of them from fires, as the city was then made up of much more wooden structures than now. Occurring exactly at Noon, the shaking tipped over numerous hibachi, or charcoal fires, preparing lunch.

A similar quake today would probably result in far fewer deaths, but would still cause enormous damage and disruption. Tokyoites got a taste of that one year ago, when the earthquake shut down Tokyo’s entire rail and subway system, leaving millions to get home by other means, often by foot. More than a few spent seven, eight, nine hours walking home from work.

Japan has invested considerable effort in attempting to predict the next 'Big One,' but most of the effort has been focused first on the capital itself, and then on the area further south where the Philippine Plate pushes under the Eurasian Plate. This area also boasts another nuclear power plant complex, known as the Hamaoka site. Former Prime Minister Naoto Kan ordered the three plants closed in the weeks following the Fukushima crisis, fearing they would be exposed to another massive quake.

The Tohoku quake occurred in a different geologic neighborhood, where the Pacific Plate meets the Okhotsk Plate. Almost forgotten is that the mammoth March 11 quake was preceded by a shaking that registered 7.2 on the Richter Scale two days previously. It is almost unheard of for a quake of that magnitude to be followed by an even larger one, scientists say. 

Meanwhile, new worries about another Tohoku earthquake arose when, on Feb. 14, a team from the European Geosciences Union warned that the seismic risk to the Fukushima plants had increased because the underlying plates had readjusted to a more precarious position. The worry was that another 7+ quake might occur closer to the plant site.

Those in stricken nuclear power plants are now considered to be in a state "equivalent to a cold shutdown," which means that the fuel is no longer overheating. But it is a fragile cold shut down as the site manager Takeshi Takahashi has acknowledged. The cooling system was put together last summer in a hurry and is almost literally held together by duct tape and bailing wire.

Additionally, the spent fuel pools of several containing radioactive fuel rods are located above ground and exposed to the elements. If the cooling system was interrupted by another power outage or the cooling displaced, a radiological fire could spew more radiation into the environment.

Tepco, the utility that owns the Fukushima plants, says it has prepared various contingency plans in the event that another quake hits the plant. “If the cooling system is knocked out, we’re prepared and will be able to handle it,” Takahashi said.

Time will tell.