The Density Ratio at the Boundary of the Earth’s Core

A theoretical study is made of the behaviour of a P wave incident from the mantle on the boundary of the earth’s core, with special reference to phase reversals. The results are given graphically in Fig. 2, which gives the conditions for given values of the angle of emergence e and the density ratio ϱʹ/ϱ or the ratio of the elastic parameters λʹ/λ+2μ at the core boundary. The dependence of the phase changes on the density ratio is clear; for instance for ϱʹ/ϱ = 1 (no density discontinuity) there is a change of phase both for e<10° and for e<55°, whereas for ϱʹ/ϱ ⩾ 1.7 there is a change of phase only for e<6.2°. Observations of PcP waves around epicentral distance = 30° (Fig. 3) prove conclusively that there is a first-order density discontinuity at the core boundary. They also show that λʹ/λ+2μ > 0.34;. A method for determining the density ratio by more complete observations of PcP is indicated.