A Comparison of Seismicity Characteristics and Fault Structure Between Stick-Slip Experiments and Nature

Fault zones contain structural complexity on all scales. This complexity influences fault mechanics including the dynamics of large earthquakes as well as the spatial and temporal distribution of small seismic events. Incomplete earthquake records, unknown stresses, and unresolved fault structures within the crust complicate a quantitative assessment of the parameters that control factors affecting seismicity. To better understand the relationship between fault structure and seismicity, we examined dynamic faulting under controlled conditions in the laboratory by creating saw-cut-guided natural fractures in cylindrical granite samples. The resulting rough surfaces were triaxially loaded to produce a sequence of stick-slip events. During these experiments, we monitored stress, strain, and seismic activity. After the experiments, fault structures were imaged in thin sections and using computer tomography. The laboratory fault zones showed many structural characteristics observed in upper crustal faults, including zones of localized slip embedded in a layer of fault gouge. Laboratory faults also exhibited a several millimeter wide damage zone with decreasing micro-crack density at larger distances from the fault axis. In addition to the structural similarities, we also observed many similarities between our observed distribution of acoustic emissions (AEs) and natural seismicity. The AEs followed the Gutenberg-Richter and Omori-Utsu relationships commonly used to describe natural seismicity. Moreover, we observed a connection between along-strike fault heterogeneity and variations of the Gutenberg-Richter b value. As suggested by natural seismicity studies, areas of low b value marked the nucleation points of large slip events and were located at large asperities within the fault zone that were revealed by post-experimental tomography scans. Our results emphasize the importance of stick-slip experiments for the study of fault mechanics. The direct correlation of acoustic activity with fault zone structure is a unique characteristic of our laboratory studies that has been impossible to observe in nature.