HR: 16:15h
AN: S34C-02    [Abstracts]
TI: Testing Inner Core Alignment Processes With Seismic Travel Times
AU: * Creager, K
EM: kcc@ess.washington.edu
AF: University of Washington, Dept. Earth and Space Sciences Box 351310, Seattle, WA 98195-1310 United States
AU: DeRosier, S
EM: sderosie@u.washington.edu
AF: University of Washington, Dept. Earth and Space Sciences Box 351310, Seattle, WA 98195-1310 United States
AU: Buffett, B
EM: buffett@geosci.uchicago.edu
AF: University of Chicago, Dept of Geological Sciences 5734 S. Ellis Ave. HGS 449, Chicago, IL 60637 United States
AU: Brown, J M
EM: brown@ess.washington.edu
AF: University of Washington, Dept. Earth and Space Sciences Box 351310, Seattle, WA 98195-1310 United States
AB: Our current understanding of Earth's inner core is based primarily on disciplinary studies: a model is constructed from seismic data, physical properties of core constituents are independently estimated, and processes within the core are invoked to link the mineral physics properties with the seismic models. We undertake an integrated approach in which a general physical process is examined. The critical parameters are analyzed such that models of seismic anisotropy can be determined for any plausible set of parameter values Finally, the raw seismic observations (differential travel time in this case) are inverted to determine the range of parameter values that are consistent with a particular physical process. We analyze the model presented by Buffett and Wenk (Nature, 2001) in which the alignment of the crystal axes is produced by a magnetic shear stress that is strongest at the top of the inner core and which drives a horizontal flow along lines of latitude. Crystals preferentially rotate into orientations that facilitate deformation by basal slip. During the accumulation of strain the inner core continues to grow by solidification, burying the crystal fabric in the interior so the pattern of alignment depends on competition between growth rate and depth dependence of shear strain rate. Parameters required to link the physical model to the spatial distribution of seismic anisotropy include: 1) the spatial structure of the magnetic field which is represented by low-degree spherical harmonics, 2) inner core viscosity, 3) relationship between total strain and degree of crystal alignment assuming hcp crystals preferentially rotate into orientations that facilitate deformation by basal slip, 4) inner core growth rate, 5) elastic constants for iron. To a first approximation the crystal axes will be oriented with the c-axis perpendicular to the rotation axis. The strength of the anisotropy should increase as the square of depth because the radius of the inner core is expected to increase as the square root of time. Constraints on the values and trade-offs among these parameters, inferred from differential travel times by linear and non-linear search methods will be presented.
DE: 1507 Core processes (1213, 8115)
DE: 3902 Creep and deformation
DE: 3909 Elasticity and anelasticity
DE: 7203 Body waves
DE: 7207 Core (1212, 1213, 8124)
SC: Seismology [S]
MN: Fall Meeting 2005