Abstract

In August and September of 1997, three 25-ton chemical explosions were detonated at nominal depths of 550, 300, and 50 m in boreholes at the former Soviet test site at Balapan, Kazakhstan. One objective of this experiment was to evaluate the effect of differing source depth on the regional wave field. Analysis of regional seismic phases lead to the observation that regional P/S wave amplitude ratios in the 1- to 5-Hz band increase as a function of source depth. However, at frequencies greater than about 5 Hz, the relative amplitudes of P and S waves remain approximately constant for the differing depth shots. Similarly, regional coda spectra are amplified in the 1- to 5-Hz band for the shallow shots. At local distances, Rg is the dominant seismic phase, with peak amplitudes in the 0.7- to 5-Hz range, and Rg is strongest for the shallower shots. Within the short distance spanned by the local stations (<20 km), Rg is rapidly attenuated, and the attenuation is accompanied by a significant shift in peak amplitude toward lower frequency. At regional distances, Rg is below the noise level. The coincident frequency band in which local Rg rapidly loses energy and regional S phases are amplified points toward Rg scattering as the dominant mechanism causing the discrepancies between P/S amplitude ratios in this study. These observations are particularly relevant to the understanding of physical processes affecting regional P/S discriminants and may lead to improvements in discriminant methods.

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