A Simple Earth Model

A simple earth model consistent with the current physical and petrological theories of the mantle is constructed. The density of different layers is estimated in different ways, depending on the relative reliability in the seismic velocities in the layers. In the upper mantle, where the representative seismic velocities are somewhat uncertain, velocities and density are calculated from a petrological model; D. H. Green and A. E. Ringwood's (1967) model is taken as a convenient starting point. In the transition zone where VP alone is reliable, Birch's empirical relation between VP and density is used to estimate density. In the lower mantle where both VP and VS are reliable, an empirical relation between the bulk sound velocity and density is used to estimate density. Adams and Williamson's relation is used in the core. The various properties of this model, such as mass, moment of inertia, travel times, periods of free oscillations, and velocities of the surface waves, are calculated and compared with the observations. With the provision of a low-velocity zone in the upper mantle, this model satisfies the observations within the widely accepted uncertainties.