Inner-core anisotropy beneath Australia and differential rotation

Abstract

Summary The inner-core anisotropy of P-wave velocity has been suggested since 1986. The detail of the anisotropic structure is now under discussion. We have studied a regional structure in the inner core beneath Australia to reveal the detail of the anisotropy using the residuals of the differential traveltimes of PKP (BC) minus PKP (DF). We have collected the rays not only nearly parallel to the Earth's rotation axis but also nearly parallel to the equatorial plane to distinguish the anisotropy from the isotropic heterogeneity in the inner core. The collected rays cross each other with different angles in the inner core beneath Australia. We have studied inner-core anisotropy assuming locally uniform transverse isotropy. The velocity anomalies decrease with the angles between the ray directions in the inner core and the Earth's rotation axis, which supports inner-core anisotropy beneath this region. The top 190 km of the inner core is isotropic with a velocity anomaly of 0.11 per cent and the deeper part shows the anisotropy with an amplitude of 3.1 per cent. The symmetry axis of the anisotropy crosses the Earth's surface at 80°N and 103°E. This result suggests that previous studies underestimated the size of the anisotropy in the inner core beneath this region. Because the symmetry axis is inclined from the Earth's rotation axis, we can observe the temporal variation of the velocity anomalies caused by the anisotropy in the inner core if it rotates at a different rate from the crust and the mantle. The possibility of differential rotation is studied by considering the regional structure of the inner-core anisotropy beneath Australia. We use seismograms recorded at Syowa station in Antarctica over the past 27 yr. We have found no evidence for significant differential rotation though we cannot rule out a small rate of about 0.2 deg yr−1 or less. Even if the inner core rotates faster than the crust and mantle, the difference is fairly small and we need not consider the influence of the differential rotation on the study of anisotropy in the inner core.