Isotropic components of microseismic moment tensors at Utah FORGE reveal a diversity of fluid pathway creation processes in EGS development.

Full moment tensor (MT) inversion of induced microseismic events provides insight into stimulation processes during Enhanced Geothermal System (EGS) development. This study resolves the mechanisms of >180 microseismic events (local magnitudes 0.0-1.9) induced in two fracture zones during the 2024 stimulations at the Utah FORGE EGS test site. The remarkably homogeneous strike-slip mechanisms are generally consistent with the local stress field, but small rotations are observed between fracture zones of different stimulation stages. A significant proportion of events exhibit positive isotropic components, indicating simultaneous tensile opening. Within each fracture zone, the maximum isotropic component increases with injected volume. Interestingly, tensile opening associated with microseismic events is more prominent in fault reactivation than in zones where a hydraulic macrofracture is dominant. Non-double-couple MT components, particularly positive isotropic components, prove to be a powerful tool for characterizing reservoir development and differentiating between complex fracture networks and hydraulic macrofractures. These MT components may serve as an indicator of fault reactivation and a proxy for increased conductivity, highlighting the potential for improved reservoir characterization and management.