Core structure and heterogeneity: A seismological perspective

The aim of this paper is to give an overview of important historical results and review current knowledge of the Earth's core, as well as to discuss prospects for seismological studies of the core. Although the properties of the core of the Earth can only be determined indirectly, there has been considerable progress in elucidating its structure. The iron-rich core is dense but has lower P-wave speed than the mantle above; the solid inner core has fewer light constituents than the fluid outer core. The density contrast at the inner-core boundary is too large for just a phase transition. The fluid outer core is well stirred by the convective flows associated with the generation of the geodynamo and is expected to have a nearly adiabatic profile. Only inside the tangent cylinder defined by the presence of the inner core might there be some seismic heterogeneity in the bulk of the outer core. Some variability along the underside of the core - mantle boundary due to selective separation of lighter material is suggested by some observations. By comparison, the inner core is rather complex with heterogeneous and anisotropic structures that appear to have hemispherical differences. Significant attenuation occurs just below the inner-core boundary, probably due to a mushy zone associated with the growth of the inner core. A variety of seismic observations help to define inner-core structures, but it is important to take account of the influence of the complex structure at the base of the mantle. A slightly different zone has been suggested around the centre of the Earth, although it is difficult to get good control on this region.