Mapping S-velocity heterogeneities in the D'' region, from SmKS differential travel times

Seismic velocity heterogeneities in the D'' region are studied using differential travel time residuals between SmKS waves. Previous studies indicate that regional variations in the propagation times of these waves are not due to the propagation through the liquid core. Furthermore, they are only weakly influenced by the topography of the core-mantle boundary (CMB). Thus, the differential travel time anomalies are interpreted as the result of lower-mantle heterogeneity, We compiled all the available published data, and added new observations of travel times from GEOSCOPE stations. We thus obtained 807 S2KS-SKS and 209 S3KS-S2KS differential times. Differential travel time residuals with respect to the mean Earth model PREM (Preliminary Reference Earth Model) are corrected for ellipticity and for mantle heterogeneity down to 2590 km depth (300 km above the CMB) using the 3-dimensional S-wave velocity model SH12-WM13. This model gives a 10% variance reduction of the residuals. The remaining variance of residuals is higher for S2KS-SKS than for S3KS-S2KS, which is consistent with a D'' origin for the heterogeneities. The data are inverted for regional variations using a least-squares analysis, assuming a single-layer D'' structure 300 km thick. The Earth is divided in 150 equal-area sectors, about 115 of which are sampled by four rays or more, We performed inversions using 18 different geographic grids and averaged the results, so as to minimize the artefacts induced by imposing artificial block boundaries. The velocity heterogeneity map, though affected by the poor sampling of the oceans, reveals significant long-wavelength structure, with velocity perturbations exceeding +/-3% with respect to a reference velocity, which is slightly lower than the PREM one. This heterogeneity map is consistent with the results from most of previous studies, global as well as regional. The location of some of the high-velocity anomalies coincides with places where subducted slab material may have accumulated at the CMB, These high-velocity heterogeneities are also generally correlated with places where a discontinuous velocity increase of several per cent has been detected at the top of D''.