ANISOTROPY OF THE INNER CORE FROM DIFFERENTIAL TRAVEL-TIMES OF THE PHASES PKP AND PKIKP

The difference between the travel times of compressional waves that turn in the Earth's liquid outer core (PKP-BC) and those that run in the solid inner core (PKIKP-DF) is primarily sensitive to structure near the inner-core boundary. Such differential travel times measured from short-period waveforms are consistent with small-amplitude deviations from radial symmetry except when ray paths are nearly parallel to the Earth's spin axis. These near-axial paths consistently produce large travel-time anomalies, providing compelling evidence for large-amplitude, axisymmetric anisotropy in the outermost portions of the inner core.