A TEST ON THE EARTHS CORE MANTLE BOUNDARY STRUCTURE WITH ANTIPODAL DATA - EXAMPLE OF FIJI-TONGA EARTHQUAKES RECORDED IN TAMANRASSET, ALGERIA

Conflicting models of the core-mantle boundary (CMB) structure ave e 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-degrees 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.