The fault gouge response to abrupt changes of normal stress

The frictional response of fault rock to abrupt changes of normal stress has significant implications for earthquake nucleation and rupture propagation on geometrically complex faults, where fault slip can change the on-fault normal stresses. The empirical constitutive modeling framework of Rate- and State-dependent Friction (RSF) is commonly used to describe the time-dependent frictional response of fault gouge to perturbations from steady sliding. In a previous study, we used a granular-physics-based model to explore the extent to which RSF behavior, as observed in rock and gouge friction experiments, can be explained by the response of a granular gouge layer with time-independent properties at the contact scale. We examined slip histories for which abundant lab data are available, and found that the granular model can reproduce laboratory data in velocity-step and slide-hold loading protocols, outperform current empirical rate-state laws, and potentially pave the way for developing a physical basis for rate-state behavior. Here, we use the same model to explore the response of gouge to large and abrupt changes of normal stress. Despite the growing number of laboratory normal-stress-stepping experiments, uncertainty remains regarding the proper constitutive formulation for the frictional response, including the extent to which friction changes abruptly or only with the passage of slip or time. In our simulations, we apply nearly instantaneous stress step increases of 25-100%, at fault relevant normal stress and sliding velocities, using a range of loading stiffnesses. One of these stiffnesses mimics that of experiments on the Brown University rotary shear apparatus, to which we compare our numerical simulations. In our Brown-like simulations, we find a transient behavior similar to the majority of the lab experiments: In response to abrupt changes of normal stress, rock shear strength evolves gradually and over a sliding distance not very different from that following velocity-step experiments. To our surprise, simulations with very high machine stiffnesses show an evolution of shear stress that is nearly instantaneous with changes of normal stress. We are currently exploring the micromechanical origins of these observations, using data from the simulations.