Effects of gouge zone evolution on frictional and mechanical behavior of fault zones: insights from Distinct Element simulations

Both laboratory experiments and numerical simulations demonstrate that the presence of fault gouge and its evolution through time may influence frictional behavior and stability of fault zones. Understanding the entire earthquake process, however, requires a complete knowledge of the relationships between variations in friction and the accumulation of gouge. We carry out Distinct Element simulations to simulate the breakdown of wall rocks and gouge evolution, in an attempt to determine the parameters that control deformation, strength, and the spatial and temporal distribution of failure events during fault block loading and slip. Fault blocks composed of bonded particles are sheared past each other, either inhibiting or allowing for block fracture and fragmentation, for the purpose of comparison between these two distinct deformation processes. Results of experiments with inhibited fault block damage show that frictional strength of fault blocks increases when fault block slides forward by overcoming interlocked asperities, and it decreases when asperities become unlocked and the fault blocks slip. Variations in friction generally correspond to variations in dilation. In contrast, fault blocks with fracture and fragmentation show a lower peak friction, a result of decreased resistance force by the failure of asperities. Variations in friction with gouge zone evolution are much more complex, depending on many factors, such as initial fault surface roughness, fault block strength, loading rate, and normal stress.