Testing a Local‐Distance $Rg/Sg$ Discriminant Using Observations from the Bighorn Region, Wyoming

This study explores the effectiveness of local‐distance (⁠$<200km$⁠) seismic discriminant to distinguish between surface mine blasts, single‐shot borehole explosions, and earthquakes in the Bighorn Mountains region, Wyoming. We focus on the ratio between local‐distance fundamental‐mode surface waves (⁠$Rg$⁠) and the crustal shear‐wave (⁠$Sg$⁠) signals. The observed spectral amplitude measurements are fit to propagation models that account for distance‐dependent geometrical spreading and attenuation, and site amplification factors. The results support previous observations that $Rg$ attenuates rapidly, is amplified in sedimentary basins, and has suppressed amplitudes in isolated mountainous terrain. $Sg$ attenuates less rapidly than $Rg$ but exhibits a similar spatial site amplification pattern. We compute an $Rg/Sg$ source discriminant by taking the ratio between site‐ and distance‐corrected $Rg$ and $Sg$ amplitude measurements. The results suggest that the site‐ and distance‐corrected $Rg/Sg$ ratios can distinguish events larger than $ML∼1.5$ (in the Bighorn region). The discriminant may also be sensitive to explosion emplacement conditions, where the ratios are higher for borehole shots in sedimentary strata and lower for explosions within the basement. The analysis shows that the $Rg/Sg$ discriminant is effective for events in the Bighorn region for events larger than $ML∼1.5$ if proper considerations are made to account for event size and near‐source material.