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,**3 2 0 3 + 456 ,**3 2 +, 3 ,, 457Trial to Make Up the Probability Map of the Volcanic Ash Fall Deposit,
Derived from the Published Ash Fall Data, in Japan
Shigeru SUTO῍, Takayuki INOMATA῍῍, Hisashi SASAKI῍῍ and Sakae MUKOYAMA῍῍More than /** 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 andlong axis of the distribution pattern, for each unit were from *.*/ to +.*, and the average was *./. There was aminor di#erence of the degree of ﬂatness depending on their volume. The larger the magnitude the smaller thedegree 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 /1ῌ and 11ῌ
of the distribution direction are in the east plus or minus ,* degrees, and .* degrees, respectively. The probabilityof 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 +mm and +cm ash fall in central
Tokyo by the same magnitude of the Hoei (+1*1) eruption of Fuji volcano, which volume was measured to be as+.- km-, were estimated as around --ῌ and ,2ῌ, 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 eachvolcano. This kind of probability map of the volcanic ash fall had not been published, and it is useful for thevolcanic disaster mitigation sta#s in each municipal o$ce and people living in Japan.
Key words : volcanic ash, ash fall disaster, probability map, Japanese volcano, Fuji volcano+ῌ ῐ ῎ ῑ ῏,*** 89 :;<=9>?@AB"CDEF GHI

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Kokusai Kogyo Co. Ltd., ,῍,.῍+, Harumi, Fuchu,
Tokyo +2-῍**/1 Japan.
Corresponding author : Shigeru Sutoe-mail : shigeru.suto@aist.go.jp