Isotropy overlying anisotropy at the top of the inner core

We investigate the existence of anisotropy near the top of the inner core t inder the western hemisphere using differential travel times of pole-parall el PKPcd-PKPdf phase pairs and comparisons to equatorial data. We invert a large dataset of PKP differential travel-time residuals in the same region for a layered model of inner core anisotropy. Our analysis shows that the u ppermost 50-150 kin of the inner. core in this region is isotropic. This im plies that anisotropy at greater depths is higher than previous estimates, perhaps as high as 5%.. The transition front isotropy to strong anisotropy is modeled as a discontinuity, but a broad transition zone is not excluded by the data. We offer several possible mechanisms for the existence of a la yered structure of anisotropy.