关键词: 天池火山, 空降碎屑物, 数值模拟, 扩散模型

Abstract: Tianchi is a central composite volcano with potential danger of eruption.Tephra of Tianchi eruption in 1215(±15)has diffused to the Japanese sea and southern Japan.However,the study about simulation of the tephra of Tianchi is little.This paper tries to recur to the ash distribution of this eruption by simulating the tephra dispersion of Tianchi volcano eruption.This paper adopts the mathematic model for tephra dispersion proposed by Suzuki(1983).Based on Suzuki's formula,it simulates the tephra dispersion of Tianchi eruption of 1215(±15).There are different densities of the grains from this eruption because of the difference of content of air bubbles.Wind speed and air parameters relate with the altitude.According to the wind speed,we adopt three models:30m/s,MW1 and MW2.The conclusions are as follows:1.The value of the dispersal parameter β of Tianchi eruption in 1215(±15)is 0.45,which indicates most of tephra congregating at the top of the volcano eruption column;2.The differences of settling rates for different diameter particles are very big.For the particles of diameter in 2～1cm,the settling rate achieved 8.95m/s,but for the particles of diameter in 0.005～0.001cm,the settling rate only 0.03m/s;3.There is a turning point in the relations chart of particle diameters and in the settling rate.It is speculated that this turning point possibly relates with transformation of the eruption column from the convective region to the diffusion region;4.The probability densities of diffusion are different for different diameter particles.Big particles start to diffuse at the bottom of the eruption column,but small particles only start to diffuse near the peak of eruption column.And the probability densities of diffusion of small particles are much bigger than that of big ones;5.In the three models the diffusion distance simulated by the first model is farther than that of MW1 model,but the width is smaller.MW2 model is not suitable for this region;6.There is an exponential relationship between the ash deposit thickness and the distance from the volcanic crater in the downwind direction 20km away from the crater.It is nearly 800 years after the eruption.We can only estimate that the downwind is 120°,so in this model it is regardless of the change of the wind direction along with altitude.In addition,there are also some problems about the numerical simulation which need to be solved,for examples,the shape of the volcanic ash particles,the error near the crater,and the second big deposit thickness and etc.

Key words: Tianchi volcano, tephra fallout, numerical simulation, diffusion model