The mechanism of regional shear wave excitation by underground nuclear explosions (UNE), a long-standing issue in nuclear seismology, is investigated by studying the phase composition of regional waves from clustered UNE. Regional seismic waves from 67 UNE at the Balapan test site of Kazakhstan, recorded at station Borovoye, are analyzed with a source array technique under the reciprocity theorem. This analysis allows one to obtain the phase velocity composition of the plane waves leaving the source region. Source locations and the original times are obtained from geodetic measurements, a scaling law between event magnitude and depth, and a calibrated Pn travel time curve. Between frequencies of 0.5 and 3.0 Hz the Pn slowness power spectra are concentrated at a phase velocity (v(h)) of 8.0 km/s. The expected Sn window contains two dominant phases: a scattered P wave with a v(h) of around 7.1 km/s and a mantle shear wave with a v(h) of 4.8 km/s. The Lg waves are coherent between 0.5 and 2.0 Hz, with a dominant v(h) of 4.2 km/s, which is typical in SmS-type waves. The Rg wave at frequencies between 0.5 and 0.8 Hz is dominantly composed of a coherent fundamental mode Rayleigh wave with a vh of 3.0 km/s. At higher frequencies (> 0.8 Hz) this coherent Rayleigh wave is not observed in the Rg window because of attenuation during wave propagation. The slower vh of Rg and faster vh of Lg, at which the strong coherent peaks in slowness power spectra are observed, suggest that the dominant components of these two waves are different and they are originated differently in the source region. The proposed Rg-to-S scattering does not appear to be a dominant mechanism for Lg excitation in the Balapan test site. It is also found that the strength of shear waves contained in the expected Sn window varies with local geology much more than the strength of Lg does. Since the Sn is enriched in high-frequency (> 1 Hz) content as compared to the Lg, this observation suggests that the local geology influences shear wave excitation at high frequencies more than at lower frequencies.
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