We analyze the high-frequency (1 to 50 Hz) spectra of chemical explosions and earthquakes at local and regional distances in the northeastern United States and in Norway to understand the seismic signal characteristics of single explosions, multiple-hole instantaneous explosions, ripple-fired quarry blasts, and earthquakes. Our purpose is to evaluate practical discriminants, and to obtain a physical understanding of their successes and failures.
High-frequency spectra from ripple-fired blasts usually show clear time-independent frequency bands due to the repetitive nature of the source and are distinctively different from the spectra of instantaneous blasts or earthquakes. However, like other discriminators based on spectral estimates, the spectrogram method requires data with high signal-to-noise ratios at high frequencies for unambiguous discrimination. In addition, banding is not seen in spectrograms for shots with small delay times (less than 8 msec) and short total durations.
We have successfully modeled the observed high-frequency spectral bands up to about 45 Hz of the regional signals from quarry blasts in New York and adjacent states. Using information on shot-hole patterns and charge distribution, we find that ripple firing results in an enrichment of high-frequency S waves and efficient excitation of the Rg phase. There is an azimuthal dependence of P-wave amplitude associated with orientation of the path with respect to local topography (ridges, benches) in which the shots are emplaced.
To discriminate instantaneous explosions from earthquakes, we find the P/S spectral amplitude ratio at high frequencies is complementary to the use of spectrogram methods. A high P/S spectral ratio above 10 Hz is a stable characteristic of instantaneous explosions.