Abstract
　In this paper, a theoretical model which gives the amount and the distribution of grain size of erupted volcanic ash around active volcano is obtained. Int he derivation of the model, a particle erupted from the crater is assumed to be conveyed by wind and fall at its terminal velocity. Probability density functions for the height of eruption column, wind velocity and grain size of erupted volcanic ash are also introduced in the model. By the identification of the model parameters, the amount of volcanic ash erupted from Sakurajima Volcano is estimated to be l43 million tons in these 11 years. The theory shows that the standard deviation of settling velocity of deposited volcanic ash is independent of distance from the crater, while the mean value of the settling velocity reduces as the distance conforming to a power function.

【キーワード】：活火山, 桜島火山, 噴出火山灰量, 堆積火山灰量, 火山灰の粒度特性

*九州大学工学部水工土木学科
Department of Civil Engineering Hydraulics, Faculty of Engineering, Kyushu Univ.
**鹿児島工業高等専門学校土木工学科
Department of Civil Engineering, Kagoshima National College of Technology