PdP and PKiKP Waves and Diffracted PcP Waves

Abstract

Travel times, listed in the I.S.S., of longitudinal waves at distances of 100° < Δ < 115° from 16 earthquakes with depths near 100km give rather precise information on properties of the Earth (a) in the upper third of the mantle, (b) at the inner core boundary, and (c) at the bottom of the mantle. The sources are relocated using the 1968 tables of Herrin et al. Precursors to PP, called PdP, are widely reported up to about 85s before PP; few are reported at earlier times. These independent observations provide further evidence that, above a depth of at least 400 km, the upper mantle contains structural discontinuities (perhaps due to phase changes) on a global scale. PKiKP waves (reflections from the inner core) are traceable in this data sample back to 106°. The result is consistent with a sharp inner core boundary at 1220km radius. Observed PKP travel times near 113° are closely consistent with PKIKP times derived by Bolt for use with the 1968 P tables of Herrin et al.; the observed times for Δ < 113° are about 0.5 s later than predicted. Travel times (for 100 km focal depth and 102° < Δ < 110°) of the PcP wave which is diffracted into the shadow of the core fit the curve t= (14m 02.8±;0.14s)+(4.60±0.05 s/deg)(Δ–106°.45). This result leads to a radius for the Earth's core of 3479±2km; there is an indication that the P velocity may decrease by a few per cent at the base of the mantle. Copyright © 1970, Wiley Blackwell. All rights reserved