Some Recent Work on Earth Models, with Special Reference to Core Structure

Abstract

Summary The earlier part of the paper gives background, relating to a sequence of Earth models A, HB1, B2, HB2 and HBs, from which we have inferred that various past interpretations of certain seismic PKP precursor readings in terms of regular ray theory are unsatisfactory. As a consequence, one of us (R.A.H.) has proposed a new interpretation of the precursors in terms of scattering near the mantle4ore boundary. The later part of the paper gives details of the new interpretation and discusses some of the preliminary findings. On this interpretation, a simple two-layer core can satisfy the entire body of data, including free Earth oscillation data, on the structure of the core, and the difficulties of arriving at a satisfactory overall Earth model appear to have been much reduced. 1. Introductioll The Earth models to be considered will be spherically symmetrical (apart from the scattering effects to be discussed) and concerned with distributions of the density p, incompressibility k, rigidity p, and related variables such as the P and S velocities a and 8, the pressure p, and gravitational intensity g. The symbols T and A will denote travel times along seismic paths (not necessarily rays) and the angles subtended by the paths at the Earth's centre. The problem of securing a completely satisfactory model has been lately complicated by difficulties in interpreting seismic data for the core. The present paper outlines a section of the writers' work on Earth models, tracing the sequence of events leading one of us (R.A.H.) to his latest interpretation of core bodily-wave data. 2. The model HBl