“Thickness-Isopach Area” Curve of Tephra

Abstract

“Thickness-isopach area” curves are drawn on the basis of data from 49 recent eruptions and 10 Usu 1977-1978 eruptions. 1) “Thickness-isopatch area” relation of tephras follows the equation (1) ; Y=AXⁿ, (X=log₁₀(h_max/h), Y=log₁₀(S/S_o). S : the area enclosed by isopach of thickness h. S_o : that of maximum thickness h_max). 2) Volume of tephra is calculated using the equation (1). 3) S_e (0.1 h_amx isopach area) or S_e (characteristic area) is more significant than Walker’s D (0.01 h_max isopach area) as a dispersal index of tephra and either of them relates with A and n as follows : A=log₁₀(S_e/S_o), n=log₁₀(h_max/hh_e)/log₁₀(S_c/S_o), and h_c×S_c=(h×S)_max. 4) Volume percentage of deposits inside S_c is about 35%, and M_d size of deposits on h_c (characteristic thickness) isopach indicates the size index of tephra. 5) S_e (0.1 h_max isopach area) is named an equi-distribution area since S_e×h_max indicates volume of tephra.