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Investigations of the core reflection PcP at distances in the neighbourhood of the core shadow zone are especially appropriate for the study of isolated regions of D" because PcP for these distances has a large angle of incidence at the core-mantle boundary and is strongly influenced by the structure immediately above the core. A striking result of this investigation of PcP in the distance range 70°–84° is that PcP could be detected only in four cases (from a total of 16 earthquakes and 29 nuclear explosions) in the NORSAR- and GRF-array beams for PcP. This result is found although the P-wave-coda noise levels in the PcP beams are very low, mostly below the amplitude level of PcP predicted by standard Earth models. In the cases where PcP could not be identified, the PcP/P amplitude ratio, which is estimated from the beams with the aid of a cross-correlation procedure, must be regarded as an upper limit for the actual PcP/P ratio. The interpretation of the PcP/P amplitude ratios with one radially symmetric, elastic model proved to be impossible. The generally small PcP/P ratios can be explained by lateral variations in seismic wave absorption within D". Qα values for short-period P waves in D", between 800 (below Usbekistan) and 100 (below the northern Hudson Bay and Central Siberia), were found. Several recent seismological investigations of the P- and S-wave velocity of the lowermost mantle have suggested velocity models with first-order discontinuities (with velocity increases of the order of 1.5%–3.0%) 150–300 km above the core-mantle boundary. Through the calculation of synthetic short-period seismograms and comparison with the array data compiled for the main part of this study, it is shown that such models are very unlikely to be a global feature if the P velocity jump is of the order of 2.5%–3.0%. Even models with reduced P velocity jumps of 1.3% produce clear onsets in short-period seismograms. They are not observed for P waves with ray paths bottoming beneath Central Siberia.
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