Seismic Waves near 110°: is Structure in Core or Upper Mantle Responsible?

Abstract

An analysis is given of longitudinal waves which are observed at distances between 105° and 115° some minutes after the arrival of the P waves diffracted by the mantle‐core boundary. A number of seismograms which clearly show these wave trains is examined; recordings at WWSS stations from the Chilean shock of 1965 March 28 are specially studied. The observations show (i) that discernible PKiKP waves with wavelengths of order 10 km are reflected from the boundary of the Earth'sinner core back to distances of at least 105° and (ii) that many longitudinal wave onsets (the PdP phase) having travel‐times up to 60 s before PKiKP and 90 s before PP arrive near 110° by means of reflection from the lower side of physical discontinuities in the upper mantle of the Earth. The first result is consistent with a relatively sharp increase in P velocity between the transition zone and the inner core at a radius of about 1220 km. The second suggests the existence of a number of discrete shells of different elastic properties in the Earth'supper mantle above 400 km; in particular, one prominent group of PdP waves of order 2 s period is consistent with reflection from a discontinuity near 385 km. This result provides confirmation of the overall high velocity gradient near this depth inferred by L. Johnson; there is an indication, however, of first‐order discontinuities in the velocity function assumed to be smooth by Johnson. Copyright © 1968, Wiley Blackwell. All rights reserved