ISSN:
1365-246X
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
Seismic P waves from a total of about 200 nuclear explosions in the USA, 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 the base of the mantle and the core-mantle boundary (CMB).The simple waveforms and well-controlled source parameters of nuclear explosions allow one to use the events as source arrays in addition to the usual receiver array configuration. A new array technique (double beamforming; Krüger et al. 1993) integrating both concepts is applied, which increases the slowness resolution considerably.A total of 56 source-receiver combinations (i.e. reflection points in the lower mantle or on the CMB) could be analysed. In five regions, anomalous arrivals (PdP) with slowness and arrival times between those of P and PcP are observed. One of these five areas (Svalbard region) shows short-period PcP/P amplitude ratios, which are about three times higher than those predicted by standard earth models. In the Severnaya Zemlya region, where PdP and PcP precursors were observed previously (Krüger et al. 1993), PcP shows azimuth deviations of up to 10°. For some other regions, deviations of the PcP waveform from the direct P waveform are also observed.These anomalous phases and the PcP waveform distortions cannot be explained with standard 1-D earth models. They are probably produced by inhomogeneities in the lowermost mantle. The observed variations in the waveforms are strong indications of a laterally heterogeneous structure in two depth ranges. The first is the CMB and its immediate vicinity of a few tens of kilometres; the second region is the depth range between about 200 and 300 km above the CMB. Maps of the North Pole region, giving the distributions of inhomogeneities in the lower mantle and on the CMB, are presented. These maps show evidence of strong heterogeneity of the D″ boundary layer and possibly also of the CMB in the same area.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-246X.1995.tb07017.x
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