Testing a local-distance Rg/Sg discriminant using observations from the Bighorn Region, Wyoming

This study explores the effectiveness of local-distance ( < 200 km) 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 M_L ∼ 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 M_L ∼ 1:5 if proper considerations are made to account for event size and near-source material.