Comparison of rock outcrop permeability with increasing distance from the Rokko fault zone

Abstract Fault zones, which have relatively high permeability compared to country rock, can act as hydraulic conduits between subsurface and deep groundwaters, and their outcrop areas can be recharge or discharge areas for deep groundwater systems. Therefore, the characterization of fault zone permeability is essential to investigate deep groundwater systems, which are necessary for the assessment of geological waste disposal. Fault zone media is not merely damaged by fault activity but is altered by the other processes, such as weathering and hydrothermal alteration, whose influences vary with the distance from the fault core and the depth from the ground surface. Consequently, the permeability of fault zone media varies by location even within a fault zone. However, the quantitative evaluation of the fault zone permeability has been rarely attempted because it not only requires surface surveys and experiments but also information on both hydrothermal-alteration and weathering processes. Herein, we present a quantitative investigation of the effects of weathering and hydrothermal alteration on the outcrop permeability of the Rokko active fault zone with Arima hot spring upwelling in southwestern Japan. The changes in the permeabilities of granite outcrops around the Rokko active fault zone were captured by on-site and laboratory experiments. The results show that the outcrop permeability increases on approaching the fault zone; however, this increase cannot be characterized by a conventional empirical model describing the effects of mechanical damage on permeability near the fault zone. Mineralogical analyses and inspections of outcrops and rock samples reveal that minerals altered or formed by hydrothermal alteration and/or weathering change the connectivity of conductive fractures attributed to mechanical damage and thus, affect outcrop permeabilities in this area. This suggests that the permeability variation in surface granite subjected to both fault activity and weathering can be indexed by determining the alteration mineral assemblage, which serves as information on the areal, hydraulic influences on the fault zone.