Electromagnetic Research on Structure of Graben and its implication to Volcanism

1997年在九州南部东西线26个测点，1998年在宫崎县北部13个测点进行大地电磁测量。同时在极低频、极低频和极低频波段测量磁场和电场的变化。实验发现了一些暗示有岩浆活动的异常区域。火山区具有简单的四层构造特征；电阻性较强的覆盖层，在几百米深的第二层，电阻性基底和较深的导体。另一方面，非火山地区的第二层电阻率较高，缺乏深层导体。低电阻率第二层（浅低电阻率区）分布在雾岛火山和其他火山周围，这些火山有金矿或地热区。这表明SLRR为含水多孔层或蚀变层。深导体（深低电阻率区）平均出现在地表以下几十公里处，但在目前活跃的火山下面较浅。这些SLRR和DLRR主要分布于Kushikino -雾岛火山区，途经宫野城盆地和大口盆地，这些盆地在数百万年内有火山活动，但在NE方向部分延伸到仁吉盆地等非火山盆地。这一方向可能对应于坳陷地区或老凹陷的走向方向。

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.