Variations in the Fine Structure at the Core-Mantle Boundary Under Siberia

A compilation of mostly short period data recordings from Soviet nuclear explosions, both weapons tests and Peaceful Nuclear Explosions (PNEs), shows significant variation in the character of PcP reflections beneath Siberia from 300 bounce points. These data include Russian PNE recordings from within Russia, with PcP arrivals identified on 3 different PNE shots on two different profiles (BATHOLITH and KRATON). Station intervals for these profiles are 15 to 20 km with maximum offsets of up to 4400 km giving an effective reflection profile at the core-mantle boundry (CMB). The PcP arrivals typically have durations of up to 2 seconds and as such are longer than expected from a simple-half space reflection from the CMB suggesting the existence of a ULVZ at the CMB. This long duration can be best explained as a result of structure at the CMB, most simply as a combination of the CMB arrival (PcP) and an arrival from the top of the ULVZ above the CMB (PdP). Furthermore, the waveform characteristics vary along the profiles with clear changes in the waveform shape of binned and stacked data over distances of 300 to 400 km (equivalent to about 150 to 200 km at the CMB). The PcP arrivals are relatively consistent between adjacent stations suggesting that local crustal scattering is not responsible for the variations in waveform shape at the CMB. This leaves variations in fine structure (velocity, density or thickness) at the CMB as the most likely cause of the lateral changes in waveform shape. In addition to the PNE data, we have identified PcP arrivals in records from stations outside Russia and from arrays such as Norsar and the Grafenberg array. First arrivals from most PNE and weapons tests are easily identified but PcP arrivals are extremely variable. Stations which have clear PcP arrivals from some shot locations have no arrivals from other shot locations while different stations have very different PcP characteristics from the same shot reflecting the differing locations of the bounce points at the CMB. The PcP arrivals at some offset ranges have amplitudes which are close to that of the primary P arrival. The variations in PcP character suggest a large degree of variability in the nature of the CMB under Siberia.