Self-Potential Changes and Subsurface Hydrothermal Activity Accompanying the 1990-1995 Eruption of Unzen Volcano

The evolution of a hydrothermal system beneath the Unzen Volcano, one of the dacitic volcanoes in Shimabara peninsula, southwest Japan, was investigated by self-potential (SP) observations during the 1990-1995 eruption events. SP changes accompanying the eruption can be divided into three stages. In the first stage, March through June of 1991, the rapid growth of a distinct positive SP anomaly was detected by continuous observation in the vicinity of a lava dome which began extrusion in May 1991. The positive anomaly is thought to be caused by hydrothermal upflows induced around the intruded magma through electrokinetic coupling. In the second stage, July 1991 through December 1993, the lava dome erupted in an exogenous manner. The growth of a dipolar SP anomaly was detected by repeated surveys during this stage. The cause of this SP change is thought to be electrokinetic potentials associated with shallow hydrothermal circulation which grew at the west of the lava dome for over two years. In the third stage, from the beginning of 1994, the dome growth became endogenous. SP showed concentric increase around the dome. This SP change probably suggests the expansion of upflow zone at high temperature in shallow levels.