Comment on “An Upper Bound on Rg to Lg Scattering Using Modal Energy Conservation” by Jeffry L. Stevens, Heming Xu, and G. Eli Baker

A recent short note by Stevens et al. (2009a) presents upper bound estimates of Rg -to- Lg scattering amplitudes using an energy conservation approach for plane-layered Earth models. Their results are based on calculations for two velocity models, one for low-velocity structures at the Nevada Test Site (NTS) and the other for high-velocity structures at the Semipalatinsk Test Site (STS). Stevens et al. (2009a) conclude that for low-velocity structures Rg -to- Lg scattering is a viable mechanism only for Rg waves excited by the spherical source below 0.3 Hz; direct Lg from the spherical source will dominate above 0.3 Hz; and scattering of Rg excited by the compensated linear vector dipole (CLVD) source never dominates Lg . For high-velocity structures, direct Lg from the CLVD will dominate above 1 to 3 Hz, while at lower frequencies, Rg -to- Lg scattering amplitudes from the explosion exceed amplitudes of direct Lg and dominate over scattered Rg amplitudes from the CLVD source. In this comment, we point out deficiencies in scattering estimates based on plane-layered velocity models, and we illustrate the existence of bias in the results of Stevens et al. (2009a) because their single-source spectra calculations do not account for wave-field interactions between sources and their velocity models are not representative of test site structures in the upper 2 km where Rg excitation and propagation take place. We show that, contrary to the conclusions of Stevens et al. (2009a), upper bound estimates of Rg scattering exceed amplitudes of direct Lg above 0.5 Hz for NTS velocity models with low-velocity surface layers. Also in contradiction to Stevens et al. (2009a), we find that, compared to Rg excitation by a spherical explosion source, CLVD excitation for shallow depths in low-velocity surface layers can be rich in high frequencies and an important factor in upper bound estimates of Rg -to- Lg scattering. Stevens …