An Earth Model based on free oscillations and body waves

Abstract

Several recent inversion studies have clearly indicated the lack of resolving power of the normal mode data set and the possible trade-offs among the various parameters. These studies have also shown that the final model is as dependent on the starting model as on the data set. It is therefore important to incorporate body wave data into any inversion scheme not only to gain resolution but also to reduce trade-offs between density and velocity. An earth model based on special studies of the structure of the mantle and core is inverted to be consistent with both body wave data and a representative set of normal mode observations (437 modes). The resulting model has a 40-km-thick upper mantle lithospheric lid terminating at 61 km, with high density (3.5 g/cm^3) and seismic velocities (8.38 and 4.71 km/s), a pronounced upper mantle low-velocity zone (LVZ) of 180-km thickness, and transition regions of rapid velocity increase at 375–425, 500–550, and 650–675 km. There are also anomalous gradients between 700 and 1200 km. This model, C2, is slow by about 0.6 and 2–4 s for P and S waves, respectively, in comparison with body wave solutions which have a greater continental bias. The major features of the upper mantle can be explained by partial melting (LVZ) and the successive transformation of an olivine-pyroxene mantle to β spinel, γ spinel, and garnet and further phase changes below 750 km. In addition to the radial inhomogeneities in the upper mantle there is evidence for inhomogeneity at the base of the mantle, the top of the core, and the regions on each side of the outer core-inner core boundary.