Monitoring Seismic Moment Release and Focal Mechanisms at Cape and FORGE EGS Sites, Utah

Abstract

ABSTRACT Accurate estimation of seismic magnitudes and focal mechanisms is a key component of seismic monitoring in enhanced geothermal systems (EGS) for evaluating the state of stress in the subsurface, efficiency of geothermal activities, and induced seismicity hazard. We present a reference catalog of point-source, double-couple moment tensors for 123 seismic events in the moment magnitude range ∼0.9 to 3.0 at and in the vicinity of the Cape and Utah Frontier Observatory for Research in Geothermal Energy EGS fields in Milford Valley, southwest Utah, for the time period 1 February 2024 to 9 March 2025. We estimate moment tensors by inverting low-frequency waveforms recorded by local permanent seismic stations at distances ∼1 to 8 km, in passbands ranging from ∼0.6 to 1.5 Hz to ∼0.1 to 0.2 Hz using Green’s functions calculated assuming path-averaged 1D velocity models between event and station locations. We obtain reasonable quality waveform fits between observed waveforms and synthetic waveforms predicted by the best-fitting moment tensors (variance reduction ∼34% to 73%, median ∼53%) at ∼5 to 11 stations (median ∼8). The estimated moment magnitudes differ from the catalog magnitudes by >0.3 magnitude units for ∼29% of the events. We find the primary focal mechanisms in the geothermal field to be strike-slip faulting (∼40% to 50%), followed by oblique faulting with strike-slip faulting being the major component (∼45%) and the minor component comprising roughly similar proportions of normal and reverse faulting. The dominant strike-slip component is consistent among events, with the possible fault planes striking either north-northwest (NNW) (right-lateral) or west-southwest (left-lateral), with ∼10° to 15° of variability (standard deviation) among events. The fault plane striking NNW is consistent with the orientations of linear trends of seismicity and the most dominant set of natural fractures identified in image logs in the reservoir in previous studies.