Subsurface structure around the Gomura fault zone revealed from electromagnetic imaging and possibility of time-variation of the fault zone conductor

Abstract

The Gomura fault zone in the Tango Peninsula, southwestern Japan, comprises the Chuzenji (CZJ) and Gomura (GMR) faults and also includes the Go-seihou (GS) fault, which is shorter but has a similar strike to the CZJ and GMR faults. We conducted an audio-frequency magnetotelluric survey at 27 stations along a survey line crossing all of these faults and constructed a two-dimensional resistivity model (GMR2019 model) extending to a depth of 1.5 km. The GMR2019 model can be divided into three areas (Areas 1 to 3). Area 1 is located in the northeastern part of the survey line and shows a typical resistivity structure of the Miyazu granite body, which is almost unaffected by active faulting, and represents the background resistivity structure of the survey area. Area 2 is located in the central part of the survey line and contains the CZJ and GMR faults at its northeastern and southwestern ends, respectively. A highly conductive and subvertical zone is identified just beneath the surface trace of the GMR fault up to ~1 km depth and is interpreted as a fault zone conductor (FZC) formed by fault activity. In contrast, no pronounced FZC is found below the surface trace of the CZJ fault. As both faults have developed in the same granite body under the same tectonic conditions, this difference in FZC can be ascribed to the difference in elapsed time since the last earthquake along each fault; namely, ~100 yr for the GMR fault and 12,000-18,000 yr for the CZJ fault. This shows the possible temporal change in an FZC over a prolonged period of ~10,000 yr. Area 3 is located in the southwestern part of the survey line and contains the GS fault. The resistivity structure and surface displacement related to fault movement of the GS fault are both obscure, suggesting that this fault is likely a secondary fault associated with the adjacent GMR fault.