Spectral ratios between normal-depth and over-buried nuclear explosions are investigated for purposes of detecting the effects of spall on the amplitude spectra of regional seismic signals. Lg and Pg spectral ratios in the frequency band 0.2 to 2.0 Hz are found to correlate with the strength of spall for explosions on Yucca Flats. The Lg ratios are characterized by spectral scalloping and a pronounced null at 0.55 Hz. The spectral null does not shift in frequency for explosions with significantly different spall dwell times. This observation, and the fact that a corresponding null is absent in the spectral ratios of Pg waves, are strong evidence against an interference effect in the spall time function as the cause of the spectral null. We examine the effects of Green's function excitation and spall source mechanism for models in which a normal-depth explosion is assumed to be a linear superposition of a spherical source (monopole) and spall. A tensile crack model for spall cannot reproduce the salient features of the spectral ratios. The only linear, equivalent elastic model in general agreement with the observations is one where spall is represented by a compensated linear vector dipole (CLVD), and Lg is generated by near-source scattering of Rg waves into body waves, which become trapped in the crust. In this model, the Rg spectrum is imprinted onto the scattered P and S waves. A spectral null at 0.55 Hz is caused by an excitation null for Rg waves for a CLVD source buried in the upper 500 m of the crust. While monopole and spall sources can excite Lg waves directly, the results of this study suggest that scattering of large-amplitude Rg waves in the near field is the most important source of Lg waves with frequencies near 1 Hz for explosions on Yucca Flats. This model is discussed in light of several outstanding observations documented for regional discriminants using Lg waves.