HR: 16:15h
AN: T54A-02 INVITED     [Abstracts]
TI: What do Seismic Observations tell us About the Origin of Elastic Anisotropy in the Inner Core?
AU: * Buffett, B A
EM: buffett@geosci.uchicago.edu
AF: University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637 United States
AB: Three observations have guided the search for the origin of elastic anisotropy in the inner core. The most robust observation is that the orientation of anisotropy is aligned with the rotation axis of the Earth. This result suggests that the anisotropy is controlled by processes in the fluid outer core because rotation is expected to have a strong influence on fluid flow, heat flow and the structure of the magnetic field. A variety of mechanisms that depend on conditions in the outer core have been proposed to explain the elastic anisotropy. However, it is not presently clear whether any of these mechanisms are sufficient to produce the required alignment of crystals. Alternative explanations rely on the rotation of the inner core relative to the forcing that causes crystals to align. For example, periodic gravitational forcing from either the mantle or tides could conceivable produce a cylindrical fabric when the deformation is averaged over many revolutions. External control over the orientation of anisotropy raises serious problems when attempting to explain the hemispherical variations in travel-time anomalies. While it possible to freeze-in local variations in the degree of crystal alignment during solidification of the inner core, it is unlikely that these variations would persist in the same geographic locations over the age of the inner core. Fluctuations in the outer core and gradual rotation to the inner core should eliminate longitudinal variations in the source of anisotropy. The most plausible explanation for hemispherical variations involves processes internal to the inner core that provide a positive feedback on the development of anisotropy. The effects of crystal alignment on rheology or thermal conductivity in the inner core can alter the deformation or thermal structure, producing a feedback on the development of anisotropy. The final observation concerns the depth dependence of anisotropy in the inner core. A gradual increase in anisotropy with depth is expected because strain must accumulate before crystal alignment develops. The apparent lack of anisotropy from the top of the inner core suggests that the solidification texture is weak, and that crystals are buried by inner-core growth before substantial strain accumulates. Abrupt changes in the strength of anisotropy with depth are more difficult to explain, but provide important constraints if confirmed by observations. Recent suggestions of changes in the orientation of anisotropy with depth may indicate that the inner core is capable of true polar wander.
DE: 8115 Core processes (1507)
DE: 8125 Evolution of the Earth
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting