Electromagnetic Research on Structure of Graben and its implication to Volcanism

1997年在九州南部沿沿着东西走向的26个测点进行了大地电磁测量，1998年在宫崎县北方的13个测点进行了大地电磁测量。在ULF、ELF和VLF波段同时测量了磁场和电场的变化。实验发现了一些异常区域，表明有岩浆活动。火山区的特点是简单的四层结构;相对电阻的覆盖层，在几百米深处非常导电的第二层，电阻基底和深部导体。另一方面，非火山地区的特征是电阻率较高的第二层和缺乏深部导体。低阻第二层（浅层低阻区）分布在雾岛火山区及其它火山周围，这些火山有金矿或地热区。这一证据表明，SLRR是一个水饱和的多孔层或蚀变层。深层导体（深层低电阻率区）平均出现在地表以下几十公里处，但在目前活跃的火山下方较浅。这些SLRR和DLRR主要分布在串野至雾岛火山区，经过几百万年内有火山活动的宫之城和大口盆地，但部分向NE方向延伸至人吉盆地等非火山区。该方向可以对应于凹陷区域或旧雕刻的趋势的方向。

We carried out magnetotelluric survey at 26 sites along East-West lines across the southern Kyushu in 1997, and 13 sites in the northern part of Miyazaki Prefecture in 1998. Simultaneous magnetic and electric field variations were measured in the ULF, ELF and VLF band. The experiment found some anomalous regions suggesting magmatic activity. Volcanic area is characterized by a simple four-layer structure ; relatively resistive overburden, very conducting second layer at the depth of a few hundred meters, resistive basement, and deep conductor. On the other hand, a rather resistive second layer and lack of deep conductor characterize non-volcanic region. The low-resistivity second layer (Shallow Low Resistivity Region) is distributed around Kirishima volcanic region and other volcanoes, which have gold mines or geothermal areas. This evidence indicates that SLRR is a water-saturated porous layer or altered layer. The deep conductor (Deep Low Resistivity Region) appears about a few tens km below the surface in average, but shallower beneath the presently active volcanoes. These SLRR and DLRR mainly distribute from Kushikino to Kirishima volcanic region via Miyanojo and Okuchi Basins, which have volcanic activity within several million years, but partly elongates to the non-volcanic such as Hitoyoshi Basin in the NE direction. This direction may correspond to the direction of the trend of the depression region or old graven.