Postseismic Relaxation Mechanisms of a Granular Fault: Laboratory Experiment and Numerical Simulation

Understanding the time-dependent postseismic relaxation that leads to afterslip and aftershocks is still an active research topic. Previous studies suggest that fault gouge plays a significant role in fault relaxation. We perform a time-lapse relaxation experiment on a sheared granular laboratory fault under X-ray micro-CT, with continuous monitoring of stress, acoustic emissions (AEs), and 2D radiographic images. A hybrid continuum-discrete numerical model is used to reproduce the experiment. Burger's contact model is applied to the grain contacts to describe the rheological behavior of the granular fault gouge. The evolution of the microstructure in the numerical specimen, including coordination number, contact force, and anisotropies of contact force chain, are monitored during postseismic relaxation. We also calculate the energy budget to study the energy release characteristics of AEs. The results show that the sheared laboratory fault exhibits stress relaxation accompanied by AEs. An exponential decay model could properly describe the long-term stress relaxation. Sudden stress variations with AE bursts are observed in laboratory and numerical simulation and exhibit a decay event rate that follows Omori's law. Two types of AEs with distinguished signal characteristics are captured both in the laboratory experiment and numerical simulation. We identified that high-frequency events are induced by contact failures within the granular layer, while low-frequency events are related to the interaction between fault walls and gouge particles.