Signatures of flow path creation in isotropic components of microseismic moment tensors at Utah FORGE

The Utah Frontier Observatory for Research in Geothermal Energy (FORGE) is a field-scale laboratory for the study of enhanced geothermal systems (EGS) in low-permeable granitic and metamorphic basement rocks. Utah FORGE comprises a highly deviated injection–production well pair reaching a depth of ~2.5km and temperatures above 220°C. The site is monitored by multiple comprehensive microseismic networks with sensors installed at the surface, in shallow boreholes, and in deep boreholes at reservoir level. Following high-pressure hydraulic stimulation campaigns in 2022 and 2024, the wells were successfully connected through at least two principal fracture zones.

We study the induced microseismicity and its relation to flow path creation processes by performing waveform-based full moment tensor (MT) inversions for >180 events (local magnitude ML 0.0–1.9) recorded during the 2024 stimulations. Including non–double–couple (non-DC) or, more specifically, isotropic components helps characterize a complex reservoir development. Locally, most events exhibit highly similar strike-slip mechanisms consistent with the regional stress field, though minor rotations are observed between different fractured zones. We interpret well-resolved positive isotropic components as indicators for tensile opening components in the microseismic events. The maximum isotropic component increases with cumulative injected volume. Interestingly, the tensile components are more pronounced in areas dominated by fault reactivation compared to zones characterized by the opening of new hydraulic fractures and fracture networks. Our analysis highlights the complex interplay between the hydraulic activation of pre-existing fractures and the hydraulic opening of newly formed macrofractures during the stimulations at Utah FORGE. While resolving microseismic non-DC components requires a thorough, challenging analysis of resolution and uncertainties, their inclusion in routine monitoring can help illuminate not only where the reservoir is breaking but also how the hydraulic connection is established.