Laboratory Acoustic Emissions Reveal Stress Rotation From Preparation Processes Toward Fault Slip on Varying Surface Roughness in Granular Materials

We investigate the influence of fault roughness on physical damage prior to large laboratory rock failure and the evolution of the local stress field surrounding the fault zone as macroscopic shear slip approaches. To achieve this, we analyze acoustic emission (AE) data from displacement-driven rock friction experiments conducted on porous sandstone samples containing either a saw-cut (smooth) or a rough fault. Using high-quality AE-derived focal mechanisms and two stress tensor inversion approaches–one considering double-couple (DC) components and the other one incorporating non-DC components, we examine the temporal evolution of the local stress tensor for both smooth and rough faults. Our results show no significant differences between the two stress inversion methods, indicating that non-DC components have no significant influence on the resulting stress tensors in our experiments. As macroscopic shear slip approaches, the principal stress axes surrounding the fault zone gradually rotate, regardless of the initial fault roughness. The observed evolution of stress tensors correlates with the evolving partitioning between volumetric and shear deformation, as derived from moment tensor inversion of AEs. Compared to the smooth fault, the rough fault exhibits higher local stress heterogeneity and more erratic fluctuations in AE source-related parameters as loading progresses.