Selection and characterization of the target fault for fluid-induced activation and earthquake rupture experiments

Abstract. Performing stimulation experiments at approximately 1 km depth in the Bedretto Underground Laboratory for Geosciences and Geoenergies necessitates identifying and characterizing the target fault zone for on-fault monitoring of induced fault slip and seismicity, which presents a challenge when attempting to understand seismogenic processes. We discuss the multidisciplinary approach for selecting the target fault zone for experiments planned within the Fault Activation and Earthquake Ruptures (FEAR) project, for which the aim is to induce the fault slip and seismicity for an earthquake magnitude of up to 1.0 while enhancing the monitoring and control of fluid-injection experiments. Structural geological mapping, remote sensing, exploration drilling and borehole logging, ground-penetration radar, and laboratory investigations were employed to identify and characterize the target fault – a ductile–brittle shear zone several meters wide with an intensely fractured volume spanning over 100 m. Its orientation in the in situ stress field favors reactivation in normal to strike-slip regimes. Laboratory tests showed slight velocity strengthening of the fault gouge. The fault's architecture, typical for crystalline environments, poses challenges for fluid flow, necessitating detailed hydraulic and stress characterization before each of the FEAR experiments. This multidisciplinary approach was crucial for managing rock volume heterogeneity and understanding implications for the dense monitoring network. Successfully identifying the fault sets the stage for seismic activation experiments commencing in spring 2024.