Fault Network Activation During Controlled Hydraulic Stimulation Experiments in the BedrettoLab

Understanding how faults are activated and earthquakes are triggered is still a central challenge in seismology and seismic hazard assessment. Controlled hydraulic stimulation experiments offer a valuable opportunity to study these processes under well-constrained conditions and at spatial and temporal resolutions that are rarely achievable in natural settings. In this study, we present the results of three hydraulic stimulations experiments conducted at the Bedretto Underground Laboratory and monitored by a dense, high-sensitivity seismic network.

These experiments revealed a complex spatio-temporal evolution of induced seismicity, characterized by the activation of a multi-segment fault network. Two dominant seismic clusters were activated early on and show a clear spatial connection to the injection borehole, suggesting that pore pressure is the main driver of seismicity within these clusters. At later stages, a third cluster with a different orientation was activated, despite showing no obvious direct hydraulic connection to the injection interval. Seismicity within this cluster occurred with a temporal delay compared to the other two clusters. This suggests that the fault activation was likely driven by indirect processes such as aseismic deformation, stress transfer, and delayed fluid migration.

The observed fault network activation closely resembles patterns commonly reported in natural earthquake sequences. These findings suggest that the physical mechanisms controlling fault reactivation and earthquake triggering are largely independent of scale, linking controlled field experiments and natural earthquakes. Our results emphasize the importance of fault network geometry and stress interactions in understanding induced and natural seismicity.