Anisotropy heterogeneity of the top inner core in the Pacific region

Top 100 km of the inner core shows complex structure than hemispherical pattern. Regional variations of isotropic velocity, as well as equatorial attenuation have been reported before. However, velocity and attenuation anisotropies, as well as their relations in the inner core are not well explored. Here we examine both the velocity and attenuation anisotropy of the Central Pacific region. We carefully selected PKiKP and PKIKP data from global and regional networks, and measured the residuals and amplitude ratios of the two phases. We inverted velocity anisotropy structure of this region iteratively, then we calculated attenuation of each data by applying inverted velocity models to different ray paths. Our preliminary results show that isotropic velocity in Pacific is roughly divided by 180 degree longitude, with fast and slow velocities in the west (eastern hemisphere) and east side (western hemisphere) of it. However, the velocities in polar direction show the contrary. The average anisotropy of this region may exceed 2%. Attenuation has obvious anisotropy too, with stronger attenuation in near north-south direction. Moreover, the attenuation changes with longitude in both equatorial and polar direction, although the change is smooth. Finally, the velocity and attenuation show good correlation, which may provide additional information to the nature of inner core anisotropy.