Characterizing Shallow Attenuation and Microearthquake Source Parameters Using a Downhole DAS Array at the Cape Modern Geothermal Field

Earthquake source parameters are crucial for assessing seismic hazard and understanding earthquake source physics. However, source parameter estimation is challenging for small earthquakes because they suffer notorious trade‐offs with shallow attenuation. Borehole Distributed Acoustic Sensing (DAS) uses fiber‐optic cables to provide unprecedented spatial resolution for attenuation analysis and a closer recording of the earthquake source. In this study, we use DAS in a vertical well in the Cape Modern geothermal field, near Milford, Utah, to characterize shallow attenuation and estimate source parameters of microearthquakes that occurred during a stimulation. We directly correct the DAS instrument response and directional sensitivity. We use along‐depth spectral ratios to obtain a high‐resolution Quality Factor ( Q ) profile for P waves in the top 2,500 m. We find low Q at the near surface, and that attenuation primarily decreases with depth, with less dependence on rock types, consistent with previous studies in California and Switzerland. We use a single‐spectral approach to estimate corner frequency, source radius, slip, magnitude, and spectral stress drop over the limited magnitude range. The spectral stress drops do not show magnitude dependence, while their variability might relate to fault heterogeneity or rupture variability. The DAS response‐correction is channel‐dependent and can introduce biases in source parameters. We reduce potential biases by using channels with similar responses and checking measurement consistency. We use the Empirical Green's Function for two event pairs to benchmark the results. This work shows the potential of using DAS for understanding crustal attenuation and characterizing microearthquakes.