Abstract

We develop local to near-regional (0–200 km) amplitude discriminants using earthquakes, single-fired explosions, and delayed-fired explosions. We use the Source Phenomenology Experiment (SPE) broadband data, collected across the Colorado Plateau during the summer of 2003, which recorded explosions in hard- and soft-rock mines. We optimized a local surface wave magnitude scale derived from a stable regional surface wave magnitude (Russell, 2006) for the sources. However, the magnitude of the source and the distance to the station limit the surface wave energy, which can cause instability in the surface wave magnitudes. Magnitude scales for the local phases (Pg, Lg/Sg, and Rg) were also tested and compared. When an azimuthal station correction was applied to each magnitude scale, there was an average reduction of standard deviation by 16%–58%. Regional and teleseismic discrimination techniques were optimized for local distance, and our results show that lithology of the source is a critical component in discriminating events at local distances. We also found that magnitude and amplitude ratios are effectively used to discriminate between small earthquakes and explosions at local distances, with each performing the best in the hard-rock lithology. Furthermore, the ratio techniques perform equally in both lithologies if multiple stations are used to establish the ratio values; we find that an average of four or five stations will generally have a 90% or better correct identification for both types of ratios. Finally, we find that the source lithology, local site effects, and large-scale geologic features control most of the variability in the amplitude measurements.

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