Mapping Svelocity heterogeneities in the D″ region, from SmKS differential travel times
Abstract
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 coremantle boundary (CMB). Thus, the differential travel time anomalies are interpreted as the result of lowermantle heterogeneity. We compiled all the available published data, and added new observations of travel times from GEOSCOPE stations. We thus obtained 807 S2KSSKS and 209 S3KSS2KS 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 3dimensional Swave velocity model SH12WM13. This model gives a 10% variance reduction of the residuals. The remaining variance of residuals is higher for S2KSSKS than for S3KSS2KS, which is consistent with a D″ origin for the heterogeneities. The data are inverted for regional variations using a leastsquares analysis, assuming a singlelayer D″ structure 300 km thick. The Earth is divided in 150 equalarea 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 longwavelength 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 highvelocity anomalies coincides with places where subducted slab material may have accumulated at the CMB. These highvelocity heterogeneities are also generally correlated with places where a discontinuous velocity increase of several per cent has been detected at the top of D″.
 Publication:

Physics of the Earth and Planetary Interiors
 Pub Date:
 March 1996
 DOI:
 10.1016/00319201(95)03093X
 Bibcode:
 1996PEPI...94....1S