A study of the uppermost inner core from PKKP and P′ P′ differential traveltimes

We have assembled a data set of PKKP_df-PKKP_bc (PKKP_df-bc) and PKPPKP_df-PKKP_bc differential traveltimes. This data set is used to study the Earth's uppermost inner core in the western hemisphere. Data from the eastern Pacific are recorded at the Canadian Yellowknife array and travel through the inner core twice, once perpendicular to the Earth's rotation axis and once parallel to the rotation axis. Our polar paths sample a similar region to paths from the South Sandwich Islands to Alaska, an important path geometry for inner core anisotropy studies. Our data set has the potential to be an independent source of information on the structure of the inner core, and to be a good supplement to past PKP studies. Differential time residuals (relative to isotropic inner core models) of up to several seconds are found. The differential traveltimes show significant scatter (up to 6 s) making it difficult to attribute them to inner core anisotropy alone. Alternatively, strong heterogeneity in the inner core and along the mantle paths may contribute to observations.