3D Modeling of fluid-induced seismicity on fault with heterogeneous frictional asperities

A key challenge in induced seismicity is that fluid injection perturbs stress and pore pressure on faults with heterogeneous properties, leading to complex earthquake nucleation, migration and magnitude. The BedrettoLab (Bedretto Underground Laboratory for Geosciences and Geoenergies), located in the Swiss Alps, provides a unique natural testbed to study how these coupled hydro-mechanical processes interact with fault heterogeneity under controlled injection conditions, with direct access to well-characterized and densely instrumented fault zones. Previous characterization of the target MC fault zone at BedrettoLab show that layers of frictional velocity-strengthening (VS) fault gouge are embedded within velocity-weakening (VW) granitic bare rock, forming a strongly heterogeneous frictional architecture. However, how this frictional partitioning controls fault slip behavior and the magnitude of induced seismicity remains unclear. In this study, we use the newly developed 3D hydro-mechanical model HydroMech3D to explore the interplay between frictional heterogeneity and seismicity in fluid injection simulations, governed by rate- and state-dependent friction. We simulate injection scenarios using parameters and conditions derived from the ongoing FEAR (Fault Activation and Earthquake Rupture) experiments at Bedretto. Our simulations investigate how the spatial distribution of VS and VW patches control seismicity magnitude. By systematically changing the partition of VS and VW patches, we explore its influence on event size distributions and maximum magnitudes. Further simulations are conducted under various hydro-mechanical pre-conditioning conditions, by pre-determining the pressurized patch on the fault via the injection protocol prior to the main injection. These simulations allow us to understand whether fault pre-conditioning may influence the maximum magnitude of induced seismicity. Our results emphasize the critical role of frictional heterogeneity and injection strategy in fault dynamics, providing new insights into the hydro-mechanical behavior of complex fault zones during fluid injection and improving seismic risk assessment and mitigation strategies.