3-DIMENSIONAL SEISMIC MODELS OF THE EARTHS MANTLE

Accurate models of the distribution of elastic heterogeneity in the Ea rth's mantle are important in many areas of geophysics. The purpose of this paper is to characterize and compare quantitatively a set of rec ent three-dimensional models of the elastic structure of the Earth, to assess their similarities and differences, and to analyze their fit t o one class of data in order to highlight fruitful directions for futu re research. The aspherical models considered are the following: M84C (Woodhouse and Dziewonski, 1984), L02.56 (Dziewonski, 1984), MDLSH (Ta nimoto, 1990a), SH.10c.17 (Masters et al., 1992), and S12_WM13 (Su et al., 1994). Through much of the discussion, M84C and L02.56 are combin ed into a single whole mantle model, M84C + L02.56. The fit of each mo del to previously tabulated even degree normal mode structure coeffici ents taken from Smith and Masters (1989a) and Ritzwoller et al. (1988) for multiplets along the normal mode fundamental and first, second, a nd fifth overtone branches is also presented. Rather than concentratin g on detailed comparisons of specific features of the models, analyses of these models are general and statistical in nature. In particular, we focus on a comparison of the amplitude and the radial and geograph ical distribution of heterogeneity in each model and how variations in each affect the fit to the normal mode observations. In general, the results of the comparisons between the models are encouraging, especia lly with respect to the geographical distribution of heterogeneity and in the fit to the normal mode data sensitive to the upper mantle and lowermost lower mantle. There remain, however, significant discrepanci es in amplitude and in the radial distribution of heterogeneity, espec ially near the top of the upper mantle and near the top of the lower m antle. The confident use of these models to constrain compositional an d dynamical information about the mantle will await the resolution of these discrepancies. The factors that may be responsible for the diffe rences in the models and/or for the misfit between the observed and pr edicted normal mode data are divided into two types: intrinsic (or pro cedural) and extrinsic (or structural). We discuss only three extrinsi c factors at length here, including errors in the reference crustal mo dels, unmodeled topography on discontinuities in the interior of the m antle, and errors in the assumed relationships between shear (upsilon( s)) and compressional (upsilon(p)) heterogeneity.