23 octobre 2014 4 23 /10 /octobre /2014 21:38

October 23, 2014

Study: 1% chance of catastrophic eruption striking Japan in 100 years




The probability of a large volcanic eruption bringing catastrophic damage to Japan over the next 100 years is 1 percent, a Kobe University research team said.

In a worst-case scenario, a huge eruption in the Kyushu region would emit pyroclastic flows covering widespread areas of the island and engulf the entire archipelago in ash, according to the team’s estimates announced Oct. 22.

Large-scale volcanic eruptions have taken place in Japan on average once every 10,000 years. The last major eruption occurred at a seafloor volcano off Kagoshima Prefecture 7,300 years ago.

Such massive blasts are called caldera-forming eruptions because they emit a large amount of magma and cause mountains to collapse, forming caldera craters.

The team, led by magmatology professor Yoshiyuki Tatsumi, examined 4,500 volcanic eruptions that took place in Japan over the past 120,000 years.

The researchers statistically examined the frequency of large eruptions that emitted volcanic ejecta of 100 billion tons or more and concluded there is a 1-percent chance of a similar volcanic eruption taking place in the next 100 years.

Extremely large eruptions that emit more than 1 trillion tons of volcanic ejecta have a probability of 0.25 percent over the same period, the researchers said.

Among such extremely large-scale eruptions was the eructation of Kagoshima Bay’s Aira caldera volcano 28,000 years ago, which emitted pyroclastic flows and volcanic ash covering extensive areas. The volcanic ejecta are still found in geological layers around the country.

If an extremely large-scale eruption took place in the central Kyushu region, pyroclastic flows would inundate a 30,000-square-kilometer area inhabited by 7 million people. Volcanic ash would pile as high as 50 centimeters in regions of western Japan where 40 million people reside, and 20 cm in eastern Japan.

Ashfall of just 1 cm to 2 cm can cripple traffic networks, and ashfall exceeding 30 cm can cause buildings to collapse.

In its report submitted last year, an expert panel of the Cabinet Office pointed out that knowledge about large-scale eruptions remains limited, and that structural efforts to study eruption prediction and countermeasures are lacking.

Toshitsugu Fujii, the head of the government’s Coordinating Committee for Prediction of Volcanic Eruptions, urged the government to start discussing measures to prepare for a large-scale eruption, saying that one could take place in Japan at any moment.

The article by the Kobe University researchers will be published in the November edition of Transactions of the Japan Academy.

(This article was written by Koji Kitabayashi and Chikako Kawahara.)

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