2021 年 66 巻 2 号 p. 101-117
Tephra simulation codes were originally developed to estimate tephra fall hazards; however, existence of several unknown parameters inhibits accurate calculation without elaborate parameter tunings. One of the most sensitive unknown parameters is source magnitude distribution (SMD), which describes amount of particle release as a function of distance from the vent along the plume axis. SMD can be obtained using inversion technique from real eruption products. Inversion techniques are also required to obtain other parameters; plume height of the eruption and the wind system at the time of the eruption are the most important ones among them. Although plume height and wind system can be observable for the recent eruptions, they could have some uncertainties and require further refinements. Additionally, they are totally unknown for unobserved ancient eruptions; however, they are essential parameters to describe eruptions. There are two types of relationship between amounts of tephra deposition and eruptive parameters; linear and non-linear. Inversions for linear and non-linear parameters need different approaches and they are briefly reviewed here. Since SMD (linear) and other parameters (non-linear) are often needed to be obtained at a time, combined inversion approach, which is a hybrid of linear and non-linear inversions, was proposed and discussed in this review. The results of inversion can be evaluated using satellite data and other plume models including 3D simulations, and help to understand structure of eruption plumes. Since accurate inversions highly rely on granulometric data of the surface deposit, further developments of techniques to obtain granulometric data with lesser time and effort are also required. The SMDs obtained by recent studies show logarithmic decay and a classic theory of particle segregation from turbulent plume can be applicable; however, more case studies are needed, especially to evaluate effect of particle aggregation and in-situ observation of falling particle is critically important.