Seismic anisotropy in the lower mantle: A comparison of waveform splitting of SKS and SKKS

[ 1] We have carried out a global investigation on seismic anisotropy in the lower mantle by comparing the waveform splitting of SKS and SKKS found at the same seismogram. The two shear waves have very similar ray paths in the upper mantle but different ones in the lower mantle. We collected a total of 104 SKS+ SKKS waveform data recorded at 76 stations. The SKS+ SKKS data are first matched with a single anisotropic model ( 2 parameters: the fast polarization direction sigma and delay time deltat) and two independent anisotropic models ( 4 parameters). We then applied an F-test to examine whether the 4-parameter models are better than the 2-parameter ones based on error improvement. We found that the data from most of the stations can be explained by the simple 2-parameter models. While this observation provides compelling evidence that the vast part of the lower mantle below -1000 km ( including the D'' region) is transversely isotropic, it is still arguable that the uppermost lower mantle and the transition may partly contribute the SKS/SKKS splittings. We also found that the 4-parameter models provide a better fitting to the SKS and SKKS splitting at 8 stations, suggesting the existence of transverse anisotropy or anomalous dipping structures in some part of the lower mantle.