HR: 0800h
AN: DI31A-0249    [Abstracts]
TI: Three-Dimensional Anisotropy and Texturing of Iron Crystals of Earth's Inner Core
AU: Sun, X
EM: xsun@uiuc.edu
AF: Geology Dept., Univ. of Illinois at Urbana-Champaign, 245 NHB, 1301 W. Green St., Urbana, IL 61801, United States
AU: * Song, X
EM: xsong@uiuc.edu
AF: Geology Dept., Univ. of Illinois at Urbana-Champaign, 245 NHB, 1301 W. Green St., Urbana, IL 61801, United States
AB: Seismological studies have generally suggested that the Earth's inner core is anisotropic and the anisotropic structure changes significantly both laterally and with depth. Previous body-wave studies of the inner core have relied on ray tracing or waveform modeling using 1-D models. Here we present non-linear tomographic inversions of the inner core anisotropy using 3-D ray tracing, spline parameterization, and a large collection of PKP differential travel times. We adapt a pseudo-bending ray tracing (PBR) method in spherical coordinates for seismic rays that traverse the inner core. The 3-D anisotropic structure of the inner core is approximated to the first order as 3-D heterogeneous (but isotropic) structure for a given ray, making it possible to apply the PBR method. The inner core anisotropy model obtained has the following major features. (1) The model has strong hemispherical and depth variation. The isotropic velocity in the topmost inner core is greater in quasi-eastern hemisphere (QEH) (40 to 160^oE) than in quasi-western hemisphere (QWH) (other longitudes). The anisotropy is weak in QEH to the depth of 600-700 km below the inner core boundary (ICB), while in QWH, the anisotropy increases at much shallower depth (about 100-200 km below the ICB) to about 3- 4%, then remains at about 2-4% throughout the rest of the inner core. (2) The anisotropy form changes abruptly (over a depth range of about 150 km) at the radius of about 600 km, slightly less than half of the inner core radius, forming a distinct inner inner core (IIC). The velocity in the IIC has maximums at equatorial and polar directions and minimum at an angle of about 40^o from the equatorial plane. The velocity in the outer inner core (OIC), however, changes little for ray directions 0 to 40^o from the equatorial plane. (3) Despite large variation of the anisotropy, the Voigt average velocity throughout the inner core is nearly uniform. The results suggest that the OIC is likely composed of iron crystals of a single phase with different degrees of preferred alignment along the spin axis of the Earth. The IIC may be composed of a different type of crystal alignment or a different iron phase. Our tests on model parameterization, mantle correction, and linear and non-linear inversions suggest the main features of our model are very robust. However, fine scale structures are likely to differ, particularly in the major transition zones, in the topmost QWH, between OIC and IIC, and between QEH and QWH in OIC. Searches for possible waveform complications from these boundaries need to be aware of the directional dependence and geographical variation to be successful.
DE: 1507 Core processes (1213, 8115)
DE: 7203 Body waves
DE: 7207 Core (1212, 1213, 8124)
DE: 7270 Tomography (6982, 8180)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting