The inner core hemispheric boundary near 180 °W

The inner core (IC) east-west hemispheric dichotomy is widely recognized, but the reported position of the hemispheric boundary varies among studies due to uneven sampling coverage and the data analyzed. This study investigates the sharpness of the western hemispheric boundary (WHB) near 180 °W by analyzing differential time residuals of PKiKP-PKPdf and PKP(bc-df) for PKPdf phases that sample 155 °E-130 °W in various azimuthal directions. Using PKiKP-PKPdf observations, the WHB is located at 175 °E-180 °W in the southern hemisphere, based mainly on the lateral isotropy-anisotropy transition. However, based on the lateral isotropic velocity contrast and this isotropy-anisotropy transition between the two hemispheres, its location is 170-160 °W in the northern hemisphere. These findings indicate that the WHB is sharp and exhibits a latitudinal dependence with a 10°-20° kink, as well as 1.75% anisotropy in the uppermost IC across the 180-155 °W range of the western hemisphere. As suggested by PKP(bc-df), the WHB could remain at 160 °W at depth. The isotropic velocity contrast near the WHB (160 °W) between the eastern and western hemispheres is lower than previous estimates using PKPdf phases sampling the bulk part of each hemisphere.