Fracture Orientation Inferred from Correlation-Based Microseismicity Focal Mechanism Using Integrated Arrays at Utah FORGE

ABSTRACT Characterizing fracture networks is essential for developing enhanced geothermal systems, but determining focal mechanisms for low-magnitude microseismicity is often hindered by insufficient data coverage or low signal quality. In this study, we characterize fracture network orientations at Utah Frontier Observatory for Research in Geothermal Energy during the April 2022 stimulation by integrating downhole geophones and surface nodal arrays. This combined approach overcomes the limitations of individual datasets, specifically, the limited azimuthal coverage of high-fidelity downhole sensors and the lower signal quality of comprehensive surface arrays. We apply array processing techniques to enhance direct P-wave arrivals and employ correlation-based clustering to link small magnitude events (M < 0) with reference events (M > 0) for focal mechanism inversion. Compared to previous studies that considered only high-quality data, this approach extends the usable microseismicity to the lowest magnitudes around M −1.5 and expands our dataset by tenfold to approximately 400 events. Our results reveal predominantly strike-slip mechanisms with spatially varying orientations across the stimulation volume. Northern areas exhibit northeast–southwest to north-northeast–south-southwest strikes with steep fault dips (80°–90°), transitioning southward to north–south and north-northwest–south-southeast orientations with gentler dips (65°–75°). These orientations align with both drilling-induced fractures and natural fracture trends observed in Formation Microimager logs, indicating that microseismicity originates from both newly developed and preexisting fractures reactivated during stimulation. The spatial and temporal evolution of microseismicity further suggests a mixed stimulation mechanism involving both new fracture propagation and the reactivation of preexisting fractures. Distinct seismicity patterns and fracture orientations across the stimulation volume indicate spatial variability in fracture network transmissivity, likely related to localized reservoir heterogeneity on hundred-meter scales.