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Using PKIKP and PKiKP travel times and amplitude ratios, we examine the velocity and attenuation anisotropy structures in the uppermost 100 km of the inner core beneath the central Pacific region. Our results indicate a clear change from approximately 180 degrees E to 190 degrees E longitude. Below the western Pacific, velocity anisotropy is weak and attenuation is high (Qp similar to 200), whereas below the eastern Pacific, velocity anisotropy is strong (3.3%) and attenuation is low (Qp ranges from 400 to 150). Furthermore, the velocity and attenuation anisotropies are correlated with each other. Our results indicate that the growth of the inner core may be coupled with core mantle boundary thermal heterogeneities. The heterogeneities may affect directional heat flow or deformational processes in the uppermost inner core, cause different iron crystal alignments, and result in lateral variations in anisotropy structures.

Plain Language Summary The inner core solidified from the liquid outer core, so the structure at the top inner core is closely linked to its formation process. In this study, based on seismic data availability, we choose the uppermost 100 km of the inner core beneath the Pacific region and investigate in detail its velocity and attenuation differences in different directions, or anisotropy structures. Our results not only depict a transition boundary between the eastern and western Pacific but also show different anisotropy characteristics of these two regions, in which the western/eastern Pacific region shows weak/strong anisotropy in both velocity and attenuation. Moreover, the velocity and attenuation anisotropies are correlated. These results indicate that the inner core growth may be coupled with the core mantle boundary heterogeneity, which may cause different texture alignment through different deformation or heat flow, and result in different physical properties in the inner core.