Abstract
The August 1977 eruption of Usu Volcano produced four major tephras and several minor ones. In this paper, the four major tephras were simulated by a model of two-dimensional diffusion in the atmosphere. The simulation leads to the following conclusions : (1) In the atmosphere with stable wind, the mean value of wind vectors between the height and the ground can be used as an effective parameter to determine the dispersion of tephra. (2) A combination of the diffusion model and mean wind vectors leads to good simulation of the dispersal of tephra. The simulation is good even if in the complicated wind vector field. (3) When the median size of the tephra deposit changes systematically with the direction of locality measured at the vent, the locality of fine size deposit shows leeward direction of wind in the lower atmosphere and the locality of coarse size deposit shows that in the upper atmosphere. (4) The size distribution of whole-deposit of tephra can be estimated by the diffusion model on the supposition of logarithmic probability distribution.
