Consistent precursors to PKKP from Novaya Zemlya explosions have been detected at the LASA seismic array in Montana. The precursory wave train is at least 65 sec long, and up to seven distinct and correlatable arrivals can be observed in the train. A similar investigation of E. Kazakh explosions, however, showed no evidence of precursors.
A hypothesis of scattering on reflection at the core-mantle boundary provides a qualitative explanation of the observed precursors. The source of the scattered waves cannot be established with certainty, but the simplest interpretation is that they are generated by irregularities (“bumps”) on the boundary itself. The absence of precursors from E. Kazakh explosions is at least partly explicable in terms of the lower magnitudes of these events, but could be a result also of regional variation in the scattering properties of the core-mantle boundary.