EVIDENCE FOR NORMAL AND INHOMOGENEOUS LOWERMOST MANTLE AND CORE-MANTLE BOUNDARY STRUCTURE UNDER THE ARCTIC AND NORTHERN CANADA

Seismic P waves from a total of about 200 nuclear explosions in the US A, the former USSR and China, observed at 10 arrays and four networks in Europe, Canada and the USA, are: used to analyse the structure at t he base of the mantle and the core-mantle boundary (CMB). The simple w aveforms and well-controlled source parameters of nuclear explosions a llow one to use the events as source arrays in addition to the usual r eceiver array configuration. A new array technique (double beamforming ; Kruger er al. 1993) integrating both concepts is applied, which incr eases the slowness resolution considerably. A total of 56 source-recei ver combinations (i.e. reflection points in the lower mantle or on the CMB) could be analysed. In five regions, anomalous arrivals (PdP) wit h slowness and arrival times between those of P and PcP are observed. One of these five areas (Svalbard region) shows short-period PcP/P amp litude ratios, which are about three times higher than those predicted by standard earth models. In the Severnaya Zemlya region, where PdP a nd PcP precursors were observed previously (Kruger et al. 1993), PcP s hows azimuth deviations of up to 10 degrees. For some other regions, d eviations of the PcP waveform from the direct P waveform are also obse rved. These anomalous phases and the PcP waveform distortions cannot b e explained with standard 1-D earth models. They are probably produced by inhomogeneities in the lowermost mantle. The observed variations i n the waveforms are strong indications of a laterally heterogeneous st ructure in two depth ranges. The first is the CMB and its immediate vi cinity of a few tens of kilometres; the second region is the depth ran ge between about 200 and 300 km above the CMB. Maps of the North Pole region, giving the distributions of inhomogeneities in the lower mantl e and on the CMB, are presented. These maps show evidence of strong he terogeneity of the D '' boundary layer and possibly also of the CMB in the same area.