Electrical conductivity structures estimated by thin sheet inversion, with special attention to the Beppu-Shimabara graben in central Kyushu, Japan

Abstract

It is proposed that the Beppu-Shimabara graben (BSG), characterized by the continuous negative Bouguer anomaly, runs NE to SW in central Kyushu. In order to clarify the BSG, the shallow electrical conductivity structure in central Kyushu is obtained by thin sheet modeling using the inversion of a conjugate gradient relaxation method and also using the forward method. The data used are the single-site transfer functions at 75 sites for the periods of 77, 97 and 122 sec. The inverted model indicates three highly conductive areas: these are the Saga-Chikugo plain covered with thick alluvium and the eastern and the western parts of central Kyushu including the BSG. Both end zones of this BSG are highly conductive in our model, which is closely related to conductive layers shallower than about 1 km. In contrast, the central part of the BSG is rather highly resistive. The BSG, thus, is not revealed as a continuous high-conductive belt and seems to be separated into three parts; the eastern and the western high-conductive zones and the central resistive area in our geoelectrical model of the shallow crust.