HR: 16:45h
AN: T54A-04    [Abstracts]
TI: Transition From Isotropy to Anisotropy in the Upper Inner Core: the Role of Anisotropic Thermal Conductivity and Horizontal Convection
AU: Ouzounis, A
EM: ares@ess.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Seattle, WA 98195-1310 United States
AU: * Creager, K C
EM: creager@ess.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Seattle, WA 98195-1310 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. 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. Because of the strong velocity gradients, anisotropic ray tracing through the inner core is required to infer accurate models. We find that models with an isotropic layer ranging from 150 to 300 km thick all adequately fit the travel-time data. 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. 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. The elastic anisotropy is most likely caused by the systematic alignment of crystals. If the thermal conductivity of iron is anisotropic then within regions of well-aligned crystals, conductive heat flux will be direction dependent. Simple models show that 10 percent anisotropy in thermal conductivity in the lower inner core, below a 200-km thick isotropic upper inner core, would produce latitudinal temperature differences at the base of the isotropic layer of about 2 degrees K. Scaling inner core parameters to the analogous problem of horizontal convection(flow driven by variations in heat flux along a horizontal boundary), which has been studied in the laboratory and numerically, suggest that this mechanism may play an important role in aligning iron crystals. For example, scaling laws suggest the overturn time is of the order 500 Ma.
DE: 8115 Core processes (1507)
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 5134 Thermal properties
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting