Kristin S. Vogfjord
Effects of explosion depth and Earth structure on the excitation of L _g waves; S (super *) revisited
Bulletin of the Seismological Society of America (October 1997), 87(5):1100-1114

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 (super *) wave can be a significant contributor to L _g . Its amplitude, however, decays exponentially with source depth, and for that reason, SK) has usually been dismissed as insignificant for all but the shallowest source depths. However, the radiation pattern of S (super *) has a narrow peak (<10 degrees ) at phi a = sin (super -1) (beta /alpha ) _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 approximately 1-Hz range remain significant for depths to at least 1 km. In favorable velocity structures, where alpha _surface < or =beta _mantle , the radiation peak is trapped in the crust and S (super *) 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 (super *) -dominated L _g may be representative of observations from decoupled explosions, thus possibly providing a way of discriminating them from other shallow sources.