HIGH-FREQUENCY SPECTRA OF REGIONAL PHASES FROM EARTHQUAKES AND CHEMICAL EXPLOSIONS

We analyze the high-frequency (1 to 50 Hz) spectra of chemical explosi ons and earthquakes at local and regional distances in the northeaster n United States and in Norway to understand the seismic signal charact eristics of single explosions, multiple-hole instantaneous explosions, ripple-fired quarry blasts, and earthquakes. Our purpose is to evalua te 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 othe r discriminators based on spectral estimates, the spectrogram method r equires data with high signal-to-noise ratios at high frequencies for unambiguous discrimination. In addition, banding is not seen in spectr ograms for shots with small delay times (less than 8 msec) and short t otal durations. We have successfully modeled the observed high-frequen cy spectral bands up to about 45 Hz of the regional signals from quarr y blasts in New York and adjacent states. Using information on shot-ho le patterns and charge distribution, we find that ripple firing result s 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 topograp hy (ridges, benches) in which the shots are emplaced. To discriminate instantaneous explosions from earthquakes, we find the P/S spectral am plitude ratio at high frequencies is complementary to the use of spect rogram methods. A high P/S spectral ratio above 10 Hz is a stable char acteristic of instantaneous explosions.