Parametrically simple earth models consistent with geophysical data

Abstract

We present a set of three parametric earth models (PEM) in which radial variations of the density and velocities are represented by piecewise continuous analytical functions of radius (polynomials of order not higher than the third). While all three models are identical below a depth of 420 km, models PEM-O and PEM-C are designed to reflect the different properties of the oceanic and continental upper mantles, respectively. The third model PEM-A is a representation of an average earth. The data used in inversion consist of observations of eigenperiods for 1064 normal modes, 246 travel times of body waves for five different phases and regional surface-wave dispersion data extending to periods as short as 20 seconds. Agreement of the functionals derived for the PEM models with the appropriate observations is satisfactory. In particular, the fit of free-oscillation data is comparable to that obtained in inversion studies in which constraints imposed on the smoothness of structure were not as severe as in our study. Our density distribution for all depths greater than 670 km is consistent with the Adams-Williamson equation to within 0.2% maximum deviation, and these minute departures result only from the limitations imposed by the parametric simplicity of our models. We also show that the velocities in the lower mantle are consistent with the complete third-order finite-strain theory to within 0.2% for V P and 0.4% for V S (r.m.s. relative deviations). The derived pressure derivatives of the velocities are very similar to those obtained for corundum structures in laboratory experiments. We conclude that any departures from homogeneity and adiabaticity within the inner core, outer core or lower mantle must be very small, and that introduction of such deviations is not necessary on the basis of the available observational evidence.