Intermittent Criticality Multi-Scale Processes Leading to Large Slip Events on Rough Laboratory Faults

We discuss data of three laboratory stick-slip experiments on Westerly Granite samples performed at elevated confining pressure and constant displacement rate on rough fracture surfaces. 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 complex spatio-temporal trends in the lab-quake characteristics and identify persistent effects of evolving fault roughness and damage at different length scales, and local stress evolution approaching large events. The observed trends highlight labquake localization processes on different spatial and temporal scales. The preparatory process of large slip events includes smaller events marked by confined bursts of acoustic emission activity that collectively prepare the fault surface for a system-wide failure by conditioning the large-scale stress field. Our results are consistent overall with an evolving process of intermittent criticality leading to large failure events, and may contribute to improved forecasting of large natural earthquakes. We discuss failure events in laboratory experiments on a rough fault performed at pressures existing in the Earth's crust. The laboratory faults were subjected to constant displacement resulting in short-lasting slips of their fault surface. We observe complex slip patterns including fast/slow ruptures with large/small fault slips. Very small slips on the fault surface were observed only with acoustic emission activity, representing tiny earthquakes of sub-mm size that produce elastic waveforms that can be recorded with piezo sensors. Using parameters derived from acoustic emission data, we analyzed physical processes leading to large slip events of the lab fault surface, an equivalent of a large earthquake in nature. Our parameters characterize local and global damage, stress, as well as interactions of small fractures before the labquake. We identify evolving fault roughness at different length scales, and find that the preparatory processes preceding lab quakes are facilitated by small earthquakes marked with bursts of acoustic emission activity. These bursts indicate ruptures of individual fault patches, which then interact and collectively prepare the fault surface for the labquake. Our results provide a set of physics-based parameters describing complex processes leading to lab slip events that may allow to improve earthquake forecasting along natural faults. We study preparatory processes preceding large slip events on rough laboratory faults using seismo-mechanical features derived from AE data The analysis highlights multi-scale rapidly evolving damage, roughness and stress changes along the fault surface Intermittent criticality marked by evolving stress correlations on different length scales can explain the observed patterns leading to large labquakes