Analysis of regional data from East Kazakh, USSR, and Nevada Test Site (NTS) underground nuclear explosions suggests possible mechanisms for the generation of both low- and high-frequency Lg. Spectral characteristics of Pn and Lg phases from 25 Soviet nuclear shots recorded at the Chinese Digital Seismic Network (CDSN) station WMQ are analyzed for their dependence on parameters such as mb (generally related to shot depth) and spatial location (Shagan versus Degelen test sites). The observed results are compared with those from the Nevada Test Site (NTS), where the near-source conditions are better known. The average amplitude ratio Pn / Lg is found to be stable with mb but to vary strongly with frequency. For both Shagan and Yucca Flat explosions of similar yield, the reduction in amplitude over the frequency range of about 1 to 5 Hz is considerably larger for Lg than for Pn. The low-frequency Lg appears to be generated by the scattering of explosion-generated Rg into S, especially for the Shagan River explosions for which good agreement between observation and theory is observed. This mechanism also provides a possible explanation for the stability of Lg as an estimator of explosive yields because the scattering should be expected to occur over a large region of the shallow crust. Another important result, supported by theory, is that the higher-frequency Lg is strongly dependent on the source medium velocity. At higher frequencies (3 to 7 Hz), the amplitude ratios Pn / Lg for explosions from Shagan, Degelen, Pahute Mesa, and Yucca Flat test sites show significant differences that appear related to differences in their source medium velocities. The observed large differences in the relative spectral content of Pn and Lg from East Kazakh and NTS explosions are mainly due to the vast differences in the crustal structure of the two test sites.