Examination of P/S Spectral Ratios for Small Explosions at Local Distances and Interpretation of Moment Tensors Estimated from Near-Source Data.

摘要

The ratio of 6-8 Hz P/S energy is a robust discriminant between earthquakes and explosions at regional distances. We have examined the P/S discriminant at regional and local distances for simultaneously-detonated chemical explosions conducted for the Non-Proliferation Experiment (NPE), Source Phenomenology Experiments in Arizona (SPE Black Mesa and Morenci), Alaska Frozen Rock Experiments (FRE Frozen and Unfrozen), the New England Damage Experiment (NEDE) and other datasets. The shots were conducted at various scaled depths of burial in lithologies ranging from granite to alluvium. Several of these datasets have delay-fired mining explosions detonated in close proximity to the dedicated shots for comparison. At regional distances, the discrimination analysis includes identifying and picking the onset times of the local and regional phases Pn, Pg, and Lg, making amplitude measurements in a variety of passbands, normalizing those measurements for the effects of source and path using the Magnitude and Distance Amplitude Correction (MDAC) methodology, and calculating P/S ratios for each event. We find that the MDAC-corrected SPE Black Mesa ratios fall in the middle of the nuclear explosion population at station KNB with very little variation due to the differing shot conditions. At local distances, we windowed the P- and S- waves based on travel times and particle motion. We then estimated smoothed spectra for the P- and S-waves and formed P/S spectral ratios. The signal quality and difficulty in picking the shear arrival varied between the data sets. The NEDE dataset had the best S-wave development, while other datasets were complicated by P-coda, small amplitude shear-wave generation, and additional arrivals that were not shear waves based on velocity and polarization analyses. Some of the variability in the P/S results could be related to error in the phase picking and analysis.