Complex multi-scale preparatory processes of stick-slip events on rough laboratory faults

We discuss data of laboratory stick-slip experiments on Westerly Granite samples performed at elevated confining pressure and constant displacement rate on rough pre-fractured faults. The experiments produced complex slip patterns including fast and slow ruptures with large and small fault slips, as well as failure events on the fault surface producing acoustic emission bursts without externally-detectable stress drop. Preparatory processes leading to large slips were tracked with an ensemble of ten seismo-mechanical and statistical parameters characterizing local and global damage and stress evolution, localization and clustering processes, as well as event interactions. We decompose observable complex spatio-temporal trends in AE-derived seismic characteristics identifying effects of persistent and evolving fault roughness at different length scales, multi-scale damage, and local stress evolution approaching large events. The observed trends highlight localization processes on different spatial and temporal scales. The preparatory process of large slip events is facilitated by smaller events marked by confined bursts of AE activity that collectively prepare the fault surface for a system-wide failure by conditioning the large-scale stress field. Our results provide a set of reliable, physics-based characteristics of processes leading to large failure events that may allow for improved earthquake forecasting along natural faults.