Audio-frequency magnetotelluric imaging of the electrical resistivity structure around the Biwako Fault of the Yamasaki fault zone, southwest Japan

Abstract

The subsurface structure of active faults can be investigated using electrical resistivity data. We conducted an audio-frequency magnetotelluric survey across the western segment of the Biwako Fault in the Yamasaki fault zone, southwestern Japan, yielding the two-dimensional resistivity structure to a depth of 1.5 km. Our resistivity structure model is characterized by a near-surface conductive layer, laterally continuous conductive zones (350-500 m depth), and two conductive zones (900-1400 depth). The deeper conductive zone is located between the western segment of the Biwako Fault and the northwestward extension of the surface trace of the eastern segment of the fault, suggesting that the eastern segment extends northwestward, as a concealed fault, nearly parallel to the western segment. Our model for the Biwako Fault is significantly different to the resistivity structure model of the Hijima Fault, which shows shallower conductive zones beneath the surface fault traces and a different spatial distribution of deeper conductive zones. The discrepancy is likely associated with different subsurface geology, different patterns of fluid supply, and local changes in the stress field governed by fault geometry.