TOKYO, Japan — It is the seismic shift that Tokyoites dread most of all: a powerful earthquake that kills thousands and plunges Japan's political and financial nerve center into chaos in the space of minutes.

A catastrophe that size has caused low-level anxiety for decades, but the massive earthquake and tsunami that struck the Tohoku region in Japan's northeast almost a year ago has brought the prospect of devastation in the capital into even sharper relief.

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While Tokyo escaped serious damage on March 11, images of swaying skyscrapers, panicked office workers and gridlocked public transport have spawned fevered speculation of when, and precisely where, the Big One might strike.

This week, the local media devoted masses of space and air time to a warning from experts at Tokyo University's Earthquake Research Institute that the chances of a big earthquake striking Tokyo in the next four years could be as high as 70 percent, a more alarming scenario than that offered by the government.

The researchers said that in the worst case, a quake of magnitude 7 would hit the southern part of metropolitan Tokyo by 2016, while the chances of a similar disaster occurring within 30 years were as high as 98 percent.

The government, by contrast, estimates the possibility of an earthquake that size striking the capital at 70 percent in the next three decades.

The new warning comes less than a year after a magnitude 9 earthquake off the country's northeast coast triggered a tsunami that left about 20,000 dead or missing. The dramatic rise in the frequency of smaller quakes in the Tokyo area since then has led researchers to assume that disaster will strike the capital sooner rather than previously forecast.

According to researchers, the number of moderate quakes in the region of magnitude 3 or bigger rose to 343 in the six-month period after the March quake, compared with just 47 in the six months before.

The blanket coverage, which came on the same day as a magnitude 5.1 quake shook Fukushima prefecture, was generated by an interview one of the experts gave with the Japanese daily the Yomiuri Shimbun, and appeared to catch the institute by surprise. In fact, the findings were presented to experts last October, and the time frame in which a major earthquake could strike Tokyo has since lengthened, according to one of the authors of the study who spoke to Global Post.

"The data is from a group of individual researchers, it's not an official report," said Shinichi Sakai, an associate professor at the institute. "There is less seismic activity now than there was after the March 11 disaster so I would say — although this is only a loose estimate — that the chances of a magnitude 7 earthquake striking Tokyo within four years have receded. It's more like 10 years."

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That doesn't mean metropolitan Tokyo's 35 million residents can breathe easy. Big earthquakes are generally thought to strike particular areas in cycles of about 100-150 years; the last such disaster to affect Tokyo, the Great Kanto Earthquake, struck in 1923, killing more than 140,000 people. According to the Central Disaster Management Council, a magnitude 7.3 earthquake in the northern Tokyo area could kill 11,000 people, destroy 850,000 buildings and cause about US$1 trillion in damage.

The government said the institute's researchers had reached their more worrying conclusion because they had used a different computer model to calculate the risks. While the institute's study factored in the post-March 11 increase in moderate seismic activity, the government's estimate analyses quakes of between magnitude 6.7 and magnitude 7.2 that have occurred over the past 150 years. No quake that big has occurred in the Tokyo area since March, so the government estimate remains unchanged, Sakai said.

But Robert Geller, a professor of seismology in the department of earth and planetary sciences at Tokyo University and a critic of earthquake prediction, says the latest warnings should not be taken seriously.

"It's completely meaningless, just numbers cranked out by inputting other numbers into an unverified forecasting model," Geller told Global Post. "These are the same people who kept saying the Tokai region (along central Japan's Pacific coast) was at great risk and that the risk in Tohoku was minimal. We all know how that worked out. It baffles me why anyone would pay attention to these guys after 3.11. I suppose they have to say stuff like this to justify their continued funding, but it's really sad."

Geller says history bears out his disdain for the government's "hazard map" showing the likely locations of big quakes. Since 1979, earthquakes that caused 10 or more fatalities in Japan occurred in places that were apparently low risk — proof that the hazard map and the methods behind it are flawed.

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In a recent paper for Bulletin of the Atomic Scientists, co-authored with Johannis Noggerath and Viacheslav Gusiakov, Geller argues that the obsession with a quake centered on or near Tokyo may have lulled people in the devastated Tohoku region into a false sense of security.

Repeated warnings about a possible earthquake near Tokyo, they said, "may have led the population of the Tohoku area to believe that they were not at risk of a large earthquake and a subsequent tsunami."

Days before the media reported on the latest doomsday scenario, Tatsuo Hirano, the minister in charge of the recovery from the March disaster, said he was confident Tokyo could withstand a powerful quake. "Please rest assured and don't flee Tokyo," he said.

Hirano cited the lack of serious damage to buildings and transport infrastructure in the capital last year as proof that it would be able to resist a large earthquake striking directly beneath or nearby.

Tens of millions of Tokyo residents are hoping he is right.