On the orientation of the fast and slow directions of anisotropy in the deep inner core

Abstract

Several studies of inner core anisotropy suggest the presence of an innermost inner core (IMIC) of about 300–600 km radius, with distinct anisotropic geometry. There is significant variability between different proposed models for cylindrical anisotropy in the IMIC, but studies based on ballistic travel time measurements of inner-core sensitive PKP waves generally favor an axis of symmetry that is also the fast-axis and is quasi-parallel to the Earth's rotation axis. This debate has been recently reopened following the results of a study based on coda wave interferometry – rather than earthquake data – which suggest that the fast, symmetry axis of anisotropy in the IMIC is oriented in a direction close to the equatorial plane. We have augmented the existing database of ballistic PKP travel times by sampling the deep inner core along equatorial directions. We find that, while the slowest velocity is observed in a direction that changes with depth, the fast, symmetry axis remains quasi-parallel to the Earth's rotation axis at all depths in the inner core, as previously found in most other ballistic studies. The direction of slowest velocity is close to the equator in the shallow inner core but changes to a steeper direction about 300–500 km below the inner core boundary, and is then accompanied by a gradual increase in the amplitude of the velocity minimum with increasing depth, which may indicate increased compaction of iron towards the centre of the inner core. We find that a sharply defined boundary between outermost and innermost inner cores is not required by the data, but that if an IMIC does exist, its preferred radius is ∼750 km.