過去の降下火山灰分布公表資料から推測されるわが国の降灰確率予測図作成試案

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More than 500 volcanic ash fall units in Japan were summarized with the data base in the program of the "Research on volcanic ash fall hazard assessment and risk management for industrial location" and the "Impact analysis on the volcanic ash fall in the metropolitan area". The digital data, including the thickness of the ash fall deposit, for around each one kilometer mesh, which is authorized by the Third Digital National Land Information System in Japan, was used for analysis. The degree of flatness, which is shown as the ratio of the short axis and long axis of the distribution pattern, for each unit were from 0.05 to 1.0, and the average was 0.5. There was a minor difference of the degree of flatness depending on their volume. The larger the magnitude the smaller the degree of flatness, excepts the case of caldera forming gigantic eruption. The distribution direction for each unit was determined by the straight line, passing through the crater, which divides the volume of the deposit into halves. Major of the Japanese air fall ash tend to distribute to the east by the strong west wind. Almost 57% and 77% of the distribution direction are in the east plus or minus 20 degrees, and 40 degrees, respectively. The probability of the ash fall deposition was calculated using the data of the degree of flatness and the direction of ash fall units for each classified volume. For example, the probability of the deposition of 1mm and 1cm ash fall in central Tokyo by the same magnitude of the Hoei (1707) eruption of Fuji volcano, which volume was measured to be as 1.3km^3, were estimated as around 33% and 28%, respectively. And the probability maps of the volcanic ash fall deposit for all over Japan in the next ten thousand years were also shown using the same distribution model assumed that there should be eruptions as same size and frequency as the last ten thousand years from each volcano. This kind of probability map of the volcanic ash fall had not been published, and it is useful for the volcanic disaster mitigation staffs in each municipal office and people living in Japan.
2010-02-27

過去の降下火山灰分布公表資料から推測されるわが国の降灰確率予測図作成試案

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