Oral Presentation

Seismic hazard assessment in fluid injection settings has traditionally focused on critically stressed faults, treating understressed faults as benign. We present field observations and models from the world's largest enhanced geothermal system (EGS) demonstrating that this assumption breaks down when fault slip enhances permeability. A downhole pressure gauge in a vertical monitoring well, which we interpret to lie within the damage zone of an initially understressed fault, directly records the fault's pressure response to hydraulic stimulation. The record indicates that slip enhanced fault permeability by 3–4 orders of magnitude, while the absence of seismicity near a minor casing deformation implies that much of this slip was aseismic. The evidence for permeability enhancement on faults with aseismic slip has significant implications for natural swarms. Both the analysis of seismic hazard of understressed faults in subsurface injection settings and interpretations of seismic swarms must therefore treat permeability as an evolving, slip-dependent quantity rather than a fixed hydraulic property. We support this interpretation with a 3D hydro-mechanical model, implemented in HBI (Ozawa et al., 2023, 2024), coupling hydraulic fracture growth, fracture–fault hydraulic interaction, leak-off from the hydraulic fractures and faults into the reservoir, and slip-induced permeability evolution on the fault. The same physics that amplifies hazard also presents an opportunity: the enhanced conductivity of slipped fault damage zones can form central components of engineered geothermal circulation networks. We argue that fault slip should be understood not solely as a hazard to be mitigated, but as a controllable process to be optimized — a shift in perspective that positions physics-based models of slip, seismicity, and permeability evolution at the center of next-generation geothermal design.