Self-potential measurements in Satsuma-Iwojima

薩摩硫黄島で自然電位観測を行った結果, 硫黄岳では, 山麓から順に, 地形効果と考えられる低電位, 高電位勾配, 正異常の領域が見られ, 山頂火口付近をピークとする250mV程度のルート(√)字型の自然電位分布を示すことが明らかになった。山麓で見られた低電位は, 天水及び熱水の下降流に起因し, 高電位勾配は, 地質構造に深く関わっていると推測される。一方, 鬼界カルデラの境界付近では, 透水性の高い構造に起因すると考えられる負の異常が見られた。
Shin'you MORI, Wataru KANDA and Masato IGUCHISatsuma-Iwojima is located in the northwestern margin of the Kikai Caldera that erupted about 6300years before.Iwodake (rhyolitic lava dome) located at the eastern part of the island has continued its fumarolicactivity for hundreds years.Self-potential (S.P.) observation was aimed to reveal the hydrothermal systembeneath Satsuma-Iwojima volcano. A positive S.P. anomaly around the summit crater and a negative one at thefoot of Iwodake were observed. They can be interpreted as electrokinetic potentials assoceated with theascending and descending flows of the hydrothennal fluid, respectively. A large potential gradient observedaround 300m a.s.l. was suggested to be caused by the difference in geological structure. Compared with othervolcanoes, it is characteristic that magnitude of the positive anomaly is relatively smallHigh temperature vapor, which cannot carry the positive charge, and upward fluid flow beneath the active vent may contribute thispositive anomaly.