Regional variations of velocity and attenuation anisotropy at the top 300 km of the inner core

The attenuation structure is an important parameter of the Earth's core, which can be combined with the velocity structure to provide more comprehensive information on the formation and evolution of the inner core. In this study, we systematically collect PKPDF and PKPBC data from global, regional and temporary seismic networks from year 1991 to 2014, and investigate both the velocity and attenuation anisotropy structures at the top 300 km of inner core. Based on data availability, we select three regions in the inner core, which are beneath Australia, Africa and Central Pacific regions. Our results show regional variations in both velocity and attenuation anisotropies in these regions. There is no obvious velocity anisotropy beneath Australian, but there exist strong velocity anisotropies beneath Africa and Central Pacific regions. Meanwhile, the average velocity is 0. 5% faster underneath Australia than AK135 model, but it is similar to that of the reference model underneath Africa and Central Pacific. For the inner core attenuation structure, we have the following results: 1) In equatorial direction, at the top 200 km of the inner core, Australia has the strongest attenuation, with Q value around 400. Africa and the Central Pacific have Q values at around 600 and 500, respectively. 2) Attenuation shows no/weak anisotropy in Australia, but shows obvious anisotropy in Africa and Central Pacific regions. 3) Finally, we find that the velocity and attenuation in all three areas have good correlation, with fast/slow velocity corresponds to high/low attenuation. Considering the locations of three regions, we conclude that inner core attenuation exhibit regional variations rather than simply hemispherical pattern. The core-mantle boundary thermal heterogeneities may couple with the inner core heat flow, generate different deformation and growth process at the inner core boundary, and create different iron crystal alignment in different regions, which causes different anisotropy in the inner core.