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

本研究旨在阐明包括热液循环系统在内的火山水文环境，并利用地电方法探测侵入的岩浆。为此，我们对USU(英安岩岩浆作用)进行了大地电磁观测和自然电位测量。宫家岛火山(玄武岩岩浆作用)和北海道小马多火山(安山岩浆作用)1)三座火山建筑物下的电阻率结构有一个共同的特征：第一层(约100m左右)是由于火山岩和沉积物的阻性(约1000欧姆-m)。第二层为导电岩石(几个到几十个欧姆)，表明存在电离水。这一层覆盖了相对阻性的第三层(几百欧姆)。第三层为前第三系岩石。2)在山顶陨石坑(USU，400 mV；三宅岛，800 mV；北海道小松，600 mV)上普遍观察到正自然电位异常。这些异常可能源于深层岩浆提供的热量所加热的热水上涌。这些热液循环的发现对于考虑岩浆冷却过程是重要的。3)USU上的大地电磁测深揭示了峰顶陨石坑下方相对高阻体(400欧姆-m左右)。考虑到英安岩的电阻率与温度的关系，以及苏北地区(5 0 0~6 0 0℃)的火成岩温度，该岩体被认为是1977年至1978年喷发时的侵入岩浆。

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.