Lawrence Berkeley National Laboratory

Abstract 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. Plain Language Summary Seismic hazard assessment requires a good understanding of earthquake source physics and how seismic energy scales with magnitudes. This information is encapsulated in source parameters such as the spectral stress drop, which is a measure of the static stress released during earthquakes. However, characterizing small earthquakes is challenging because of potential biases from wave propagation effects. The attenuation, which is often poorly known, is the main source of inconsistency in spectral stress drops for small earthquakes. In this study, we demonstrate the potential of using a fiber‐optic cable deployed in the borehole with Distributed Acoustic Sensing (DAS) to analyze shallow attenuation and microearthquakes in a geothermal reservoir. The downhole DAS provides an unprecedentedly dense seismic network in the subsurface and a closer recording of the small earthquakes. We obtain the first high‐resolution attenuation structure from seismic observation. The near‐surface high attenuation echoes the importance of accounting for shallow attenuation when modeling the seismograms. The spectral stress drop variation likely characterizes the subsurface heterogeneity or comes from more complex sources. We show that downhole DAS can provide new attenuation observations in the crust and improve our ability to model small earthquakes if we carefully account for the instrument response of the cable. Key Points We obtain the first high‐resolution attenuation structure in the top 2.5 km from seismic observation Borehole fiber‐optic cable captures a broad bandwidth, which is advantageous for modeling microearthquakes The channel‐dependent gauge length and directivity effects can introduce uncertainty in source characterization