Isotropy overlying anisotropy at the top of the inner core

We investigate the existence of anisotropy near the top of the inner core tinder the western hemisphere using differential travel times of pole-parallel 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 uppermost 50-150 kin of the inner. core in this region is isotropic. This implies 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 layered structure of anisotropy.