| The deformation along tectonic plate boundaries is associated with the storage and release of elastic energy. The abrupt release of strain energy results in seismic energy emission during fault slip, i.e., earthquake ruptures. The dynamics of these earthquakes, including the nucleation, propagation and arrest of ruptures, is closely tied to local fault zone properties. However, the details of how fault zone properties influence small and large earthquakes is insufficiently understood, partially due to limited observations of fault zone structure at seismogenic depths, and necessarily incomplete seismic catalogs. Here, we investigate the connection between micro-seismicity, and variations in fault stress and structure during series of stick-slip experiments on structurally-complex fault zones generated in natural granite samples. Within the present experimental series, we strove to mimic the natural faulting process by creating series of stick-slip events under seismogenic stress conditions on complex faults. Throughout the experiments, we monitored variations in stress, strain, and seismic activity. The latter was compared to post-experimental micro-structure of faults observed in computer tomography images and thin sections. Our laboratory-created faults exhibited many of the structural hallmarks of faults in nature. Moreover, the observed seismic event distributions show many similarities to natural seismicity, including Omori-Utsu type aftershock decays, Gutenberg-Richter frequency magnitude distributions, power-law off-fault activity decays, and fractal hypocenter distributions. The spatial and temporal variations in seismic event distributions in our experiments can be explained through variations in fault structure and stress. At high stresses and in the proximity to fault asperity regions, micro-seismic events tend to cluster, and show low b-values as well as high seismic moment release. Furthermore, our laboratory experiments provide insight into the creation of off-fault damage and resulting variations in micro-seismicity distributions. The off-fault micro-seismic events decay as a power-law at larger fault-normal distance with an exponent that is connected to fault roughness and normal stress. Our laboratory faults showed rapid structural evolution toward less complexity, however, evolutionary processes were predominantly limited to initial seismic cycles. Our results emphasize that small seismic events contain essential information about fault properties, e.g., roughness, structural heterogeneity and stress level, which in turn may control the dynamics of large earthquakes. A detailed analysis of micro-seismicity statistics and inferred fault properties therefore have the potential to significantly advance the seismic hazard assessment of active tectonic regions. |
