Whole Earth tomography from delay times of P, PcP, and PKP phases: Lateral heterogeneities in the outer core or radial anisotropy in the mantle?

We perform tomographic inversions of compressional wave travel time measurements, in order to image the laterally varying radial anisotropy of P wave velocity in the entire mantle. In order to achieve an adequate sensitivity to both horizontal and vertical P velocity throughout the mantle, we include in our data set travel time measurements corresponding to ray paths of very different geometries. Specifically, we use data associated with the phases P, PcP(core-reflected) and PKPbc, PKPdf (core-refracted), all extracted from the bulletins of the International Seismological Centre, 1964-1993, PcP, PKPbc, and PKPdf data are also sensitive to the topography of the core-mantle boundary (CMB) and (PKP only) to the velocity structure of the Earth's core. These circumstances require that we also allow for CMB topographic anomalies as free parameters in our inversions and investigate the possibility of the existence of lateral heterogeneities in the outer core [e.g., Ritzwoller et al., 1986; Stevenson, 1987; Wahr and de Vries, 1989; Vasco and Johnson, 1998]. The solution models that we propose include alternatively a laterally heterogeneous outer core, a radially anisotropic mantle, or both, so that trade-offs between those features can also be evaluated. We conclude that only minor, and very localized, radially anisotropic anomalies are present in the middle and lower mantle, the most prominent being located in the deepest 400 km, under the western Pacific. We find a discrepancy, particularly evident by a comparison between different models of the CMB topography, between the travel time measurements that are sensitive td the Earth's core and those that are not; we show that it is not possible to explain this discrepancy only in terms of radial anisotropy of the mantle and/or lateral heterogeneity of the outer core.