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TOKYO (Reuters) - Japan's defenses against a major tsunami and the safety of its nuclear plants were thrown into further doubt after two official studies predicted much higher waves could hit and that Tokyo quake damage could be bigger than it was prepared for.
The reports, carried in the media over the weekend, are likely to intensify the debate about whether to restart Japan's 54 nuclear reactors, all but one of which are shut amid public fears about nuclear safety sparked by the Fukushima disaster in March 2011.
One report said a quake as big as the one that rocked Japan in 2011 could trigger waves topping 34 meters (112 feet), almost double its previous estimate made in 2003 when its worst scenario forecast tsunami of no more than 20 meters (66 feet).
The Cabinet Office panel which authored the report, revised its predictions after one of the biggest tremors on record struck Japan last year, setting off a tsunami that topped 20 meters in the worst-affected areas and triggering the world's worst nuclear crisis in 25 years.
"We won't be able to contain a massive tsunami with the (current) embankments," said Masaharu Nakagawa, disaster prevention minister on a news conference on Saturday evening.
"We will have to work the (changes regarding) the city planning, disaster prevention education and evacuation into the policies," he said.
Waves at the now off-line Hamaoka nuclear plant in Shizuoka prefecture, operated by Chubu Electric Power, could reach 21 meters, breaching the 18-metre breakwater that the operators are currently building, the report said.
The government is keen to get some of the reactors running after surging fuels imports resulted in a rare trade deficit, raising worries about its declining ability to fund a huge public debt with domestic savings. But it must first persuade wary locals that the plants are safe.
Another official report suggested that the direct impact of a major quake has been underestimated.
It said that if a 7.3 magnitude quake hit Tokyo, some parts of the city and surrounding areas would likely be shaken at level 7 on Japan's seven-point "Shindo" scale of seismic activity, it said.
The Shindo scale measures ground motion at a specific place and points to the likely impact on people and structures and the study concluded that the tectonic plates seen as the focal point in a quake were 10 kilometers (6.2 miles) shallower than previously estimated, making any impact more severe.
The government has put the chances of a magnitude 7.3 quake centered in the north of Tokyo Bay at 70 percent over the next three decades, and has estimated there would be about 11,000 casualties and 850,000 buildings destroyed.
Japan, situated on the "Ring of Fire" arc of volcanoes and oceanic trenches that partly encircles the Pacific Basin, accounts for about 20 percent of the world's earthquakes of magnitude 6 or greater.
A magnitude 7.3 quake hit central Japan in 1995, devastating the port city of Kobe. It killed more than 6,400 people and caused an estimated $100 billion in damage.
Reporting by Antoni Slodkowski, Editing by Jonathan Thatcher