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Effects of explosion depth and earth structure on the excitation of L_g waves: S* revisited

Department of Geosciences Princeton University, Princeton, New Jersey 08544

Abstract

Effects of explosion depth in 1D velocity structure on the amplitude and character of L_g are examined. For shallow spherical explosions, the S* wave can be a significant contributor to L_g. Its amplitude, however, decays exponentially with source depth, and for that reason, S* has usually been dismissed as insignificant for all but the shallowest source depths. However, the radiation pattern of S* has a narrow peak (<10°) at _d = sin^–1(ß/)_surface, where amplitudes are larger than in the original P wave. The width and amplitude of this peak also decrease with increasing source depth, but amplitudes at frequencies in the 1-Hz range remain significant for depths to at least 1 km. In favorable velocity structures, where _surface ß_mantle, the radiation peak is trapped in the crust and S* in L_g can dominate seismograms from spherical explosions at depths down to 1 km. In such structures, the character of L_g may also be determined by whether the radiation peak ends up as turning waves or Moho reflections. Decoupled nuclear explosions may be most spherical-explosion-like, and the characteristics of S*-dominated L_g may be representative of observations from decoupled explosions, thus possibly providing a way of discriminating them from other shallow sources.

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