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

Abstract

AbstractEffects of explosion depth in 1D velocity structure on the amplitude and character of Lg are examined. For shallow spherical explosions, the S* wave can be a significant contributor to Lg. 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 φd = sin−1(β/α)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 αsurface ≦ βmantle, the radiation peak is trapped in the crust and S* in Lg can dominate seismograms from spherical explosions at depths down to 1 km. In such structures, the character of Lg 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 Lg may be representative of observations from decoupled explosions, thus possibly providing a way of discriminating them from other shallow sources.