HR: 08:15h
AN: S31B-02    [PDF]
TI: Transition From Isotropy to Anisotropy in the Upper Inner Core
AU: * Creager, K C
EM: creager@ess.washington.edu
AF: University of Washington, Earth and Space Sciences Box 351310, Seattle, WA 98195 United States
AU: Ouzounis, A
EM: ares@ess.washington.edu
AF: University of Washington, Earth and Space Sciences Box 351310, Seattle, WA 98195 United States
AB: We investigate models of the radial structure of the western hemisphere of Earth's inner core. Differential travel times of PKiKP $-$ PKIKP indicate that the outermost inner core is isotropic and slightly slower than global models such as PREM. On the other hand, observed travel-time residuals of PKP$_{BC}$ $-$ PKIKP and PKP$_{AB}$ $-$ PKIKP increase systematically from 1 to 6 s as a function of increasing ray turning depths for ray paths that are parallel to Earth's spin axis. Rays perpendicular to the spin axis typically have slightly negative residuals. These observations suggest the outermost inner core is nearly isotropic and that strong anisotropy exists deeper in the inner core. We invert these times for models characterized by an outer isotropic layer and a deeper anisotropic layer separated by a transition zone with thickness varying from 0 to 150 km. Models determined by linear inversions using ray paths calculated from isotropic models can only adequately fit the observations if the isotropic layer is between 50 and 150 km thick. However, the strong gradients imposed by the anisotropic model force large deviations in ray paths, so a non-linear scheme with appropriate ray tracing is needed. Using anisotropic ray tracing we find that models with an isotropic layer ranging from 150 to 300 km thick all adequately fit the travel-time data. However, thick isotropic layers require correspondingly stronger anisotropy below. Finally, models with discontinuities and linear gradients up to 150 km thick cannot be distinguished by travel times alone. On the other hand, synthetic seismograms demonstrate that amplitudes of direct and reflected arrivals are very sensitive to the width of the transition zone. While there is some evidence for secondary arrivals in the seismograms, robust stacks of waveforms across regional arrays do not exhibit coherent arrivals, suggesting either that the transition is broad, or that sharp transitions are not coherent over large spatial scales.
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 7260 Theory and modeling
DE: 8115 Core processes (1507)
SC: Seismology [S]
MN: 2003 Fall Meeting