DAS Moment Magnitude Calculation in the Time Domain for General Dislocations: Application at Utah FORGE

ABSTRACT We developed a new formulation for the estimation of seismic moment using far-field, time-domain strain measurements from distributed acoustic sensing (DAS). The method takes into account the axial component nature of DAS measurements, thus removing bias related to an incomplete sampling of the strain field. For the implementation, we computed new spherical coefficients that account for the effect of compressional and shear radiation patterns in the distributed strain formulation. For comparison, we also extended the estimation of the spherical averages of displacement radiation patterns to non-double couples. For such a purpose, we apply a general dislocation model, for which pure double couples are a particular instance. We utilize this new formulation to analyze microseismic events observed during the 2023 circulation tests at the Utah FORGE site. We also evaluated different aspects that can bias the magnitude estimations, including background noise, the sampling of the focal sphere provided by borehole DAS arrays, and the assumption of pure double-couple source mechanisms. In addition, we also compared the DAS estimations with independent results obtained with a seismometer and from the moment tensor inversion of the DAS records. Our analysis shows consistency between different magnitude estimation approaches when various biasing factors are taken into consideration.