Core–mantle boundary structure investigated using SKS and SKKS polarization anomalies

SUMMARY Occasionally, SKS and SKKS waveforms in the same seismogram are affected differently by anisotropy. One source of this discrepancy may be structures in D″. In this study, we examine the discrepancy in order to determine where it arises in the propagation path and what it is due to. We find that D″ is the most likely source of the signal. The relatively minor differences in differential shear‐wave splitting in SKS and SKKS limit large‐scale azimuthal anisotropy in D″ to less than 2 per cent, though it may be locally stronger. The most effective way to develop the splitting differences is through polarization differences between SKS and SKKS imposed at the CMB. We examine how effective relief on and lateral gradients in CMB structure are at generating polarization anomalies and conclude that topography generates them best. Ramps and ridges are more effective generators of polarization anomalies than hills. Laterally extensive (500–1000 km) sloping CMB topography greater than 15° can develop the observed polarization anomalies in the data. The topography required exceeds constraints from other sources, so is unlikely to be the major factor explaining the anomalies. We produce a global map of SKS / SKKS exit points from the core where anomalous polarization behaviour is found, in relation to the velocity structure known to exist in D″: the anomalies appear to be restricted to faster than average areas of the CMB, suggesting a contribution from anisotropy related to the post‐perovskite phase transition.