Shear Band Development and Permeability Anisotropy Evolution in Fault Gouge

ABSTRACT: The fault zone generally undergoes grain breakage during shear slip events, resulting in the change of shear mode and pore structure in fault gouge. We establish a discrete elemental model of shear tests of granular fault gouge under different normal stresses at the constant velocity, to investigate the effects of grain breakage on fault friction strength evolution and shear band development as well as permeability. With the increase of normal stress, the fault friction strength increases by about 20% due to grain contact area evolution, accompanied by many small slip events triggered by grain breakage. Additionally, the fault gouge has a high tendency to dilation under low normal stress, causing an increase in permeability; while the fault gouge compacts rapidly due to grain breakage under high normal stress, leading to the decrease in permeability. Consequently, a constitutive model about porosity evolution is developed through relating to volumetric strain. Further analyses show that the heterogeneous grain breakage leads to the local reduction in porosity, promoting shear band development and changing the microstructural characteristics: P shears dominate microstructure of shear bands at low normal stress. With the increase of normal stress, more R1 shears with larger angle develop. 1. INTRODUCTION A large amount of fine-grained gouge materials usually exists in the fault cores, which are mainly evolved from the abrasion and fragmentation of the rock mass on both sides of the fault during the fault sliding process (Billi et al., 2003). The presence of fault gouge affects the friction strength magnitude and stability of the fault. In mature fault zones, most of the deformation from sliding locates at the fault gouge, with undergoing a long-term process of grain breakage (Scuderi et al., 2017). This continuous evolution of grain size distribution and morphology during the shearing process profoundly alters the friction mechanical properties and permeability of the fault (Zhang et al., 2020). This also impacts the microstructure of the faults and induces the development of shear bands within the fault gouge (Balsamo and Storti, 2010). These macro-micro features resulting from grain breakage are often correlated with the deformation and strength of the fault and can be used to further understand the slip behavior of the fault (Kuo et al., 2014).