The relative density-to-shear velocity scaling in the uppermost mantle

We perform inversions of gravity data (geopotential model EGM96) and seismic tomography model (SlBRLBM) for the scaling factor (zeta), which relates relative density anomalies to relative S-wave velocity anomalies. The gravity data and tomographic model are anti-correlated below continents down to a depth of z = 200 km. This anti-correlation is not present below oceans. Except for smoothness, which is controlled by a damping factor, no a priori information is added to the inversion. Data are filtered between degrees l = 11 and l = 16 of the spherical harmonic expansion. This spectral window is well suited for the study of intermediate-size (2000-4000 km) anomalies in the uppermost mantle. Calculations are made separately for sub-continental and sub-oceanic mantle. The sub-continental and sub-oceanic scaling factors are significantly different at depths shallower than 260 km. In both cases, the magnitude of zeta is around 0.05. The sub-continental scaling factor has a positive root down to,z = 220 km, whereas the sub-oceanic scaling factor yields positive values down to z = 140 km only. At depth shallower than 350 km, models of zeta do not depend on the damping factor or the viscosity model. At depths greater than 350 km, the resolution of zeta (z) decreases significantly and low degrees (l = 2-1) add information from large-scale anomalies and from the lower mantle. As a result, the shape and values of zeta for l = 2-16 and l = 11-16 are significantly different at depths greater than 350 km. A possible explanation of the discrepancies between the sub-continental and sub-oceanic scaling factor is that intermediate-scale anomalies are more important in the continental uppermost mantle than in the oceanic uppermost mantle. (C) 2001 Elsevier Science B.V. All rights reserved.