A latitudinal pattern in the structure of the outermost liquid core, revealed by the travel times of SKKS‐SKS seismic phases

Abstract

Differential travel times of SKS and SKKS seismic phases are analysed in order to specify the properties of the outermost liquid core. The data have been selected in order to obtain a good sampling of the Earth, they are mainly intermediate period GDSN and broadband GEOSCOPE and NARS records of events at epicentral distances 83–130°. The SKKS‐SKS travel times are referred to the mean Earth model PREM (Dziewonski and Anderson, 1981) and are corrected for ellipticity. A general baseline shift and slope with respect to PREM is also removed from the data. The SKKS‐SKS residuals indicate a departure from spherically symmetric structure in the liquid core. In particular, they exhibit a clear latitude dependence with lower values at the poles and higher values at the equator, indicating that heterogeneities with cylindrical symmetry are present in the most external liquid core. This latitudinal pattern exhibits some similarities with that of the zonal flow in the outermost liquid core deduced from the secular variations of the magnetic field. This suggests that the distribution of the seismic velocities in the outermost liquid core may be influenced by the circulation of the fluid beneath the core‐mantle boundary, or that both are similarly influenced by the Earth rotation.