Evidence from seismology and related sources on the Earth's present internal structure

Abstract

A critical summary is given of evidence relating to the determination of the Earth's internal layering and the variation of certain of the main internal physical properties (density, incompressibility, rigidity, pressure, etc.) at the present time. The evidence provides, in effect, a set of boundary conditions which any plausible theory of the evolution of the Earth should satisfy.The summary takes account of such recent evidence as the revised estimate of the Earth's moment of inertia coefficient following analyses of satellite orbits, recent revisions of the P and S seismic velocity distributions, including the use of data from nuclear explosions and array stations, shock-wave experiments involving pressures equal to those reached in the Earth's deepest interior, special studies of compressibility and studies of aspects of finite-strain theory, and analyses of observations of free Earth oscillations. Special attention is paid to the assessment in various layers of the Earth of the coefficient η which occurs in an equation for density gradient which supersedes the Williamson-Adams equation.Numerical detail is given for the parametrically simplest Earth model which the authors have so far derived in their endeavours to fit, within the uncertainties, evidence which includes the above items. The model is a spherically symmetrical “average” model, the intention being to construct a suitable base model from which corrections for lateral variations may be set down in the form of auxiliary tables. The model marks an advance from previous work but is still capable of improvement. For example, certain difficulties connected with the estimate of the radius of the Earth's core and the fine structure just above the mantle—core boundary have yet to be resolved.Reference is made to questions of scientific method both in respect of the “average Earth” and in combining evidence from different sources.