Abstract
A transect resistivity model was generated to a depth of 1.5 km around the Gomura Fault by conducting an audio-frequency magnetotelluric survey. The model clearly presents the structure of fault ruptures, and is characterized by three conductive regions and one moderately conductive region. One of the conductive zones and the moderately conductive zone are sub-horizontal and situated on the eastern side of the Gomura Fault; the former is shallow, whereas the latter is deep.
The resistivity structure of these layers and the surrounding area are well consistent both with their lithofacies and a resistivity log made to a depth of 1300 m near the survey line of this study. Pore water at the boundary between the weathered and weakly-weathered granitic rocks, combined with an increase in clay minerals, decreases the resistivity of the shallowest conductive layer. The second and third conductive regions are located beneath the surface traces of the Gomura and Go-seihou Faults, respectively. The second conductive region is interpreted as a fracture zone produced by fault movements, with a high water content and probably containing clay minerals. The observation that the third region is small and isolated may indicate that the Go-seihou Fault is a subsidiary fault. A comparison of the conductive fracture regions of the Gomura Fault and the Hijima Fault of the Yamasaki fault system suggest that the width and conductivity of a conductive region beneath the surface trace of an active fault are controlled by the mean slip rate and cumulative displacement, rather than the difference of ~1000 years since their most recent earthquake events.
