桜島における火山灰放出量予測に関する研究

Abstract

桜島火山の昭和火口から放出される火山灰放出量を観測坑道内に設置された伸縮計の爆発直後のひずみ変化を用いて予測することを試みた。爆発的噴火直後には火口直下浅部の圧力源の収縮に伴うひずみが観測される。収縮体積量と放出火山灰量はおおよそ比例することが昭和火口噴火においても確かめられたので，マグマ溜まり-火道のマグマ供給系を理想気体が占めているとするモデル(Nishimura，1998)を用いて初期ひずみ変化から最終ひずみを予測した。収縮継続時間の1/6のデータを用いて，10%の誤差で予測ができた。

Amount of volcanic ash ejected by explosive eruption at Showa crater of Sakurajima volcano is estimated by using initial strain record immediately after the beginning of the explosion. Extensometers record extension strain in radial component from the crater and contraction strain in tangential one induced by deflation of a shallow part beneath the crater. Deflation volume of the pressure source is proportional to weight of volcanic ash ejected by the eruption. Final strain change is predicted by short-term strain change after the beginning of the explosion by fitting a model of magma reservoir-conduit (Nishimura, 1998) to the strain record. The magma reservoir is filled with perfect gas, which is ejected through a conduit after failure of a cap-rock at the top of the conduit. The final strain is predicted with an error of 10% by using 1/6 data of the whole contraction strain record.