TOKYO (Reuters) - The impact on Tokyo from a major quake could be much more devastating than the government has predicted, a new study shows.
The study by the Ministry of Education, Culture, Sports, Science and Technology comes just over a year after one of the biggest tremors on record struck Japan's northeast coast, triggering a massive tsunami and the world's worst nuclear crisis in 25 years.
The study, reported by Japanese media on Saturday, will likely prompt the government to review its assumptions for damage.
While Tokyo buildings shook violently when last year's 9.0 magnitude quake devastated the northeast, there was virtually no lasting structural damage in a city which, with surrounding areas, is home to some 35 million people.
The five-year study found 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.
The Shindo scale measures ground motion at a specific place and points to the likely impact on people and structures.
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.
However, the study concluded that the tectonic plates seen as the focal point in a quake were 10 kms (6.2 miles) shallower than previously estimated, making any impact more severe.
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.
The Great Kanto earthquake of 1923 had a magnitude of 7.9 and killed more than 140,000 people in the Tokyo area.
(Reporting by Nathan Layne, Editing by Jonathan Thatcher)