Aimin Cao

Short wavelength topography on the inner-core boundary

Constraining the topography of the inner-core boundary is important for studies of core–mantle coupling and the generation of the geodynamo. We present evidence for significant temporal variability in the amplitude of the inner core reflected phase PKiKP for an exceptionally high-quality earthquake doublet, observed postcritically at the short-period Yellowknife seismic array (YK), which occurred in the South Sandwich Islands within a 10-year interval (1993/2003). This observation, complemented by data from several other doublets, indicates the presence of topography at the inner-core boundary, with a horizontal wavelength on the order of 10 km. Such topography could be sustained by small-scale convection at the top of the inner core and is compatible with a rate of super rotation of the inner core of ≈0.1–0.15° per year. In the absence of inner-core rotation, decadal scale temporal changes in the inner-core boundary topography would provide an upper bound on the viscosity at the top of the inner core.

Seismic anisotropy in the Earth's innermost inner core: Testing structural models against mineral physics predictions

Using an updated data set of ballistic PKIKP travel time data at antipodal distances, we test different models of anisotropy in the Earths innermost inner core (IMIC) and obtain significantly better fits for a fast axis aligned with Earths rotation axis, rather than a quasi-equatorial direction, as proposed recently. Reviewing recent results on the single crystal structure and elasticity of iron at core conditions, we find that an hcp structure with the fast c axis parallel to Earths rotation is more likely but a body-centered cubic structure with the [111] axis aligned in that direction results in very similar predictions for seismic anisotropy. These models are therefore not distinguishable based on current seismological data. In addition, to match the seismological observations, the inferred strength of anisotropy in the IMIC (6–7%) implies almost perfect alignment of iron crystals, an intriguing, albeit unlikely situation, especially in the presence of heterogeneity, which calls for further studies.