Azimuthal Variation of Short-Period Rayleigh Waves from Cast Blasts in Northern Arizona

We have examined the generation and propagation of short-period (SP) Rayleigh waves (Rg) from two cast blasts of similar yield (~726,000 kg of explosives), delay sequence, and near-source geology at a coal mine in northern Arizona. The blasts were oriented approximately perpendicular to each other, allowing for radiation pattern studies at regional distances. Near-source seismic (<5 km) and videographic data were collected to determine the origin times and surficial effects of the two mining explosions. The local seismic data show the dominant surface-wave energy is delayed with respect to the blast initiation time and is better aligned with the onset of the spalled material impact with the ground. SP (4 sec) surface-wave amplitudes recorded at regional distances for both cast blasts differ by as much as a factor of 2.5, and these differences are attributed to the orientation of the casting. We completed a modeling study to investigate the source of the azimuthal variations in the SP surface-wave radiation. The overall effect of the horizontal spall is to create amplitudes as much as 2.5 times greater in the direction normal to the topographic bench than parallel to the free face. Modeling results for azimuths behind the topographic bench are consistent with the observed regional data. However, the observed data suggest that the amplitudes in front of the bench may be a factor of 2 less than the model predicts. Since equivalent radiation patterns were not observed for the P and Lg phases, the observation of these Rg radiation patterns at regional distances could act as a cast blast discriminant in regions of monitoring concern.