Influence of the Loading Stiffness on Sheared Granular Fault Gouge, and Applicability to Slip‐Weakening Theory

Abstract Tectonic faults can slip in a spectrum of fault slip behaviors, from aseismic slip and slow ruptures to elasto‐dynamic earthquakes. Following frictional theory, laboratory experiments have shown that the basic ingredient that may control this transition is the interaction between the fault stiffness and the surrounding elastic medium. We aim at investigating the role of the loading stiffness on the seismic cycles in granular fault simulations. For this purpose, we build a numerical model based on the Discrete Element Method, inspired by laboratory friction experiments on fault gouge in the presence of an elastic loading system. The coupling between fault granular rheology and surrounding rock elasticity leads to seismic cycles with properties that are strongly influenced by the loading stiffness. Stiff fault systems generally produce frequent compactional events with limited sliding distances and low to moderate stress drops, while soft fault systems generally produce rare dilatational events with large sliding distances and stress drops. We show that, on average, simulated events are well‐described by a simple linear slip‐weakening friction law, but the weakening rate that best describes the events is tightly coupled with the loading stiffness. This contradicts the idea of an intrinsic friction law for the granular gouge layer and demonstrates the need to consider a fault as a tribological system coupling the scales of the granular gouge and of the elastic surrounding medium.