T. ISSE
Institute for Frontier Research on Earth Evolution (IFREE), Japan Marine Science and Technology Center (JAMSTEC), YOKOSUKA, JAPAN
One of the most exciting topics about the inner core is the velocity anisotropy. Since Morelli et al. (1986) and Woodhouse et al. (1986) have suggested that the inner core is anisotropic with a cylindrical symmetry axis aligned parallel to the Earth's rotation axis and that the P-wave velocity in the inner core parallel to the symmetry axis is faster than that parallel to the equatorial plane, many researchers have studied the inner core anisotropy. Recent studies have revealed the complicated structure of the inner core anisotropy, such as degree one heterogeneity, small scale heterogeneities and depth dependence. However, in most of previous studies on the inner core anisotropy, it was difficult to distinguish the anisotropy from the isotropic heterogeneity in the inner core because the rays of PKP(DF) and PKP(BC) nearly parallel to the Earth's rotation axis were studied. The regional difference of the inner core anisotropy is not studied yet and the inclination of the anisotropy symmetry axis from the Earth's rotation axis is now under discussion. We have studied the regional difference of the inner core anisotropy using differential travel times of PKP(BC) minus PKP(DF). We have collected rays nearly parallel to not only the Earth's rotation axis but also the equatorial plane to distinguish the anisotropy from the isotropic heterogeneity. We have studied the anisotropy in the inner core beneath Australia, Africa and the Southern Pacific Ocean, where the rays of PKP(DF) pass through in various directions, assuming the locally uniform anisotropy in the inner core. Beneath Australia, it is suggested that the size of the inner core anisotropy is less than 1\%. However, our study shows that the size of the anisotropy is 3.1\% with symmetry axis crossing the Earth's surface at 80\degr N and 103\degr E. We suggest that this discrepancy may be caused by the inclination of the symmetry axis and the existence of an isotropic layer in the upper 190 km of the inner core, which were not considered in previous studies. Beneath Africa and the Southern Pacific Ocean, the size of the anisotropy is 2.5\% and 2.7\%, respectively. It is difficult to determine the location of the anisotropy symmetry axis because of the small number of data whose rays are nearly parallel to the Earth's rotation axis. The variations of the velocity anomalies caused by the inner core anisotropy with respect to the angles of the rays from the Earth's rotation axis are different among those three regions. It suggests the complicated structure of the inner core and that the preferred orientation of elastically anisotropic iron crystals is not aligned uniformly. The determination of regional difference of the anisotropy in the inner core provides us with important information for studying physical and chemical state of the inner core. Another exciting topic about the inner core is that the inner core rotates faster than the crust and the mantle by about 1\degr per year. Glatzmaier \& Roberts' (1995) numerical modeling of the geodynamo suggests that the inner core can be driven to rotate eastward at a few degrees a year with respect to the mantle. Song \& Richards (1996) have inferred that the inner core rotates eastward faster by about 1\degr per year than the mantle and crust from the analysis of the temporal variation of the differential travel times of PKP(BC) minus PKP(DF). The existence of this differential rotation has been examined by many seismologists, but a conclusive result is not obtained. Temporal variation of the velocity anomalies in the inner core may suggest the differential rotation of the inner core. We have studied the regional structure of the inner core anisotropy beneath Australia considering the possibility of the differential rotation of the inner core. Seismograms recorded at Syowa station in Antarctica for 27 years are suited to detect the differential rotation of the inner core. We have found no evidence for significant differential rotation of the inner core though we cannot rule out small rate of the differential rotation of about 0.2\degr per year or less. Even if the inner core rotates faster than the crust and mantle, the rate must be rather small and we need not consider the influence of the differential rotation to study the anisotropy in the inner core.