ALTERNATIVES TO CORE-MANTLE BOUNDARY TOPOGRAPHY

Nuclear explosions at the French test sites in the South Pacific have been recorded at the regional seismic networks in the western United S tates, A total of approximately 600 good-quality, short-period seismog rams were selected from an initial set of 900. The dense receiver spac ing, particularly in California, allows the division of the networks i nto numerous subarrays and the data from each subarray can be analysed using conventional array techniques such as beam-forming and frequenc y-wave-number analysis. The seismic rays provide dense sampling of a s mall area of the core-mantle boundary (CMB) beneath the eastern Pacifi c from 5 degrees to 13 degrees N and 133 degrees to 126 degrees W. PcP has a shallow incidence angle on the CMB at an epicentral distance of 65 degrees and D '' is sampled over a large swathe from 2 degrees S t o 24 degrees N and from 134 degrees to 123 degrees W. Beam traces have been formed for the P and PcP phases for each of the subarrays, using slowness and azimuth values derived from array methods. Good-quality beams have been selected for further analysis, By applying a matched f ilter to the beamed traces, an accurate set of PcP-P differential trav el times has been determined and compared with IASP91. The residuals s how a smooth trend over the area sampled, For the given epicentral dis tances, P and PcP have similar paths through the crust and upper mantl e so we make the assumption that near-source and near-receiver structu res have negligible effect on the differential times. We also assume t hat the midmantle is relatively homogenous. Many previous studies have interpreted PcP-P residuals in terms of CMB topography. By mapping th e residuals directly to topography, a range in elevation of 40 km over a lateral extent of about 700 km results. Alternatively, we attempted to interpret the residuals in terms of velocity heterogeneity in D '' . A range of D '' models have been tested including lateral velocity v ariation in a D '' layer with constant thickness, The PcP-P residuals may be explained by allowing the P-wave velocity to vary laterally by a factor of 2% over the region sampled, with most of the region being marginally fast, This suggests that D '' heterogeneity has large spect ral power at relatively short wavelengths. In this scenario, previous tomographic inversions for CMB topography will have large errors since in these inversions, corrections for D '' structure are based on low- order spherical harmonic models. (C) 1997 Elsevier Science B.V.