Investigations of volcanic structure and intruded magma by means of the geoelectrical method - Usu, Miyake-jima and Hokkaidou Komagatake -

本研究旨在利用地电方法阐明包括热液循环系统在内的火山水文环境，探测侵入岩浆。为此，我们对乌苏（英安岩）岩浆活动进行了大地电磁观测和自电位测量。三宅岛火山（玄武岩岩浆作用）和北海道Komagatake火山（安山岩岩浆作用）。1)三个火山大厦下面电阻率结构的共同特征是：第一层（100m左右）由于火山岩和沉积物的作用，电阻率约为1000欧姆。第二层是导电岩石（几至几十欧姆），表明电离水的存在。这一层覆盖了电阻率相对较高的第三层（几百欧姆）。第三层被认为是前第三纪的岩石。2)峰顶陨石坑多见自电位正异常（臼臼400 mV，三宅岛800 mV，北海道Komagatake 600 mV）。这些异常可能源于深部岩浆提供的热量加热的热地下水上涌。这些热液循环的发现对考虑岩浆冷却过程具有重要意义。3)乌苏的大地电磁测深显示，在山顶火山口下方有一个相对较高的电阻体（400欧姆左右）。结合英安岩电阻率与温度的关系，以及乌苏火山喷发温度（500 ~ 600℃），确定该岩体为1977 ~ 1978年火山喷发时的侵入岩浆。

Present study has aimed to elucidate the volcano-hydrological environment including the hydrothermal circulation system and to detect the intruded magma by using the geoelectrical methods. For these purposes, we made magnetotelluric observations and self-potential measurements on Usu (dacite magmatism). Miyake-jima (basaltic magmatism) and Hokkaido Komagatake (andesitic magmatism) volcanoes.1) The common feature of the resistivity structure is seen beneath the three volcanic edifices: The first layer (100m or so) is resistive (about 1000 ohm-m) due to the volcanic rocks and sediments. The second layer is composed of the conductive rocks (several to several tens of ohm-m), suggesting the existence of the ionized water. This layer covers the relatively resistive third layer (several hundreds of ohm-m). The third layer is assumed to be the pre-Tertiary rocks.2) Positive self-potential anomalies are commonly observed on summit craters (Usu, 400 mV; Miyake-jima, 800 mV; Hokkaido Komagatake, 600 mV). These anomalies can originate from the upwelling of hot groundwater warmed by the heat supplied from deep-seated magma. These findings of hydrothermal circulation are important to consider the magma cooling process.3) Magnetotelluric soundings on Usu revealed the relatively highly resistive body (400 ohm-m or so) beneath the summit crater. Considering the relation between the resistivity of dacite rock and temperature, and the fumarolic temperature of Usu (500〜600℃, this body is identified as the intruded magma at the time of the 1977-1978 eruptions.