Liquid core structure and PKP station anomalies derived from PKP(BC) propagation times

Abstract

PKP(BC) travel time residuals in the distance range 148.5–155.5° have been extracted from the catalogues of the International Seismological Centre and inverted to retrieve possible heterogeneities or anisotropy at the base of the liquid core, and PKP(BC) station anomalies. A binning has first been applied to the data to minimize the biases that result from the uneven sampling of the Earth by the PKP(BC) rays and from the heterogeneities in the source regions. For the liquid core structure, the data reveal weak heterogeneities, with travel time delays slightly higher near the South Pole and lower beneath southeast Asia, but most of these variations stay below the error bar level. The search for anisotropy at the base of the liquid core also leads to inconclusive results, with a weak tendency for lower velocities in the direction of the Earth's rotation axis. Station anomalies appear to be by far the major contribution to the PKP(BC) residuals. Their values are compared with the P station corrections, and with the values predicted by lower-mantle tomographic models. They show that the lower mantle makes an important contribution to the PKP residuals at medium wavelengths; these delays are generally underpredicted by tomographic models.