Hemispherical structure in inner core velocity anisotropy

We investigate the hemispherical pattern in inner core velocity anisotropy using a new and independent high-quality data set of PKPbc-PKPdf and PKPab-PKPdf body wave observations. Our data show no evidence for a tilted anisotropy axis with respect to the Earth's rotation axis and is significantly better fit by a model with 4.4% anisotropy in the western hemisphere and only 1.0% in the eastern hemisphere than by a model of uniform anisotropy. We carry out variance minimization and find the boundaries between the hemispheres at lines of constant longitude at 14 degrees E and 151 degrees W. Variance minimization enables us to extract the imprint of hemispherical structure from all of the data, and not just polar paths, resulting in boundaries which are nearly 30 degrees from the results of previous studies. The high quality of the data allows us to provide robust evidence that the isotropic velocity in the eastern hemisphere is 0.2% lower than in the western hemisphere in the top 660 km of the inner core. Our data set also suggests an increase in inner core anisotropy in the eastern hemisphere from 1% to around 6% at depths deeper than 660 km, indicating that the hemispherical pattern in anisotropy may disappear at greater depths. The presence of hemispherical structure rules out mechanisms for creating anisotropy which are unable to sustain longitudinal variations in the inner core. Furthermore, steady inner core superrotation of the order of 0.1 degrees/year would eradicate the hemispherical differences, though inner core oscillation would still be permissible.