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A mechanism for generation of short-period transverse motion from explosions

TELEDYNE GEOTECH ALEXANDRIA LABORATORIES, 314 MONTGOMERY STREET, ALEXANDRIA, VIRGINIA 22312

Abstract

A large amount of short-period three-component data from explosions and earthquakes are examined for sites covering source-receiver distances 100 m to 2000 km. In general, the spatial attenuation and spectral composition of the maximum amplitude of transverse motion from explosions are found to differ considerably from those of the other two orthogonal components. A mechanism for the generation of the maximum amplitude short-period transverse motion from explosions is proposed in terms of scattering by heterogeneities encountered by seismic waves as they propagate from the source to the receiver. The transverse component coming out of the source region is generally small and "grows" with distance due to progressively more scattering. We suggest that at short distances from explosions, there is first near-source scattering of P into mostly SV (P SV). The SV is then additionally scattered by small heterogeneities perturbing a nearly plane-stratified medium. This results in transverse motion with small amplitudes but rich in high frequencies. As the source-to-receiver distance increases, continuous interaction among the three orthogonal components of ground motion (via SV SH and increasingly important SH SV) results in increasingly isotropic polarization, i.e., equal amplitude and spectrum for shear phases such as Lg. The effect of the free surface, where observations are generally made, is shown to make the observed transverse component larger than the radial and vertical components alone. At several sites, the spectra of all components of Lg from an explosion appear to be significantly deficient in higher frequencies so as to be potentially useful for source discrimination. This we believe to be due to the dominant generation of explosion shear waves by near-source P SV scattering (including "scattering" from horizontal layers) which is more efficient at low frequencies. If near-source scattering of P did not occur, then we would not expect the discriminant to work. We also present evidence that the transverse component of P arrivals is of higher frequency than the radial and vertical P, implying that the near-receiver scattering process responsible for transverse motion is more complete at high frequencies. In addition to the observed data, theoretical investigations of scattering appear to qualitatively support the proposed mechanism.

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