A test on the Earth's core‐mantle boundary structure with antipodal data: example of Fiji‐Tonga earthquakes recorded in Tamanrasset, Algeria

Abstract

Conflicting models of the core‐mantle boundary (CMB) structure have been derived from tomographic inversion of bulletin data: their long wavelength topography ranges from ±3 km to ±7 km according to the model considered. A simple test of these models is provided by the analysis of PKP(AB) phases observed at the antipode of a seismic region. Antipodal waves have the peculiarity that they arrive from a wide range of azimuths so that observations at a single station allow us to sample a large volume in the Earth. In particular, PKP(AB) samples CMB structure along two circles at 48° distance from the station and from the epicentral region. We have collected short period and Geoscope seismograms from Fiji‐Tonga earthquakes recorded at the antipodal station Tamanrasset, Algeria. The differential traveltime residuals of PKP(AB)‐PKIKP are analysed as a function of back‐azimuth. Their mean value does not vary by more than ±0.4 s, whereas CMB models, including topography and in some cases heterogeneities above CMB, predict azimuthal variations of up to ±0.9 s. Consequently our data favour a small topography of the core‐mantle boundary or a compensating effect in traveltimes between CMB topography and heterogeneities in the lowermost mantle. Copyright © 1993, Wiley Blackwell. All rights reserved