Volcanic hydrothermal system imaged from electrical resistivity structure

Abstract

Knowing where magma and groundwater are located and in what state they exist is highly important to understand volcanic activity. We investigate the distribution of electrical resistivity within the volcanic edifice and are trying indirectly to know the difference in constituent materials or in the state. So far, we have clarified the shallow resistivity structure of volcanoes that have repeated phreatic eruptions from dense electromagnetic surveys around the active crater. As a result, hydrothermally altered, poorly permeable rock layers (cap rocks) with low resistivity values and areas with rather high resistivity values beneath them were commonly detected in shallow areas. We believe that the area showing this slightly high resistivity value is a gas reservoir, and could be a preparation zone for a phreatic eruption. In addition, at the Kusatsu-Shirane volcano, we have succeeded in imaging the high salinity region that developed above the deep partially molten region. Multiple low-permeability barriers are thought to control volcanic activity.