HR: 0800h
AN: S41C-1014    [Abstracts]
TI: Inversion of anisotropic inner core structure from three dimensional ray tracing
AU: * Sun, X
EM: xsun@uiuc.edu
AF: Geology Department, Univ. of Illinois, 245 NHB, 1301 W. green st, Urbana, IL 61801 United States
AU: Song, X
EM: xsong@uiuc.edu
AF: Geology Department, Univ. of Illinois, 245 NHB, 1301 W. green st, Urbana, IL 61801 United States
AB: Seismological studies have generally suggest that the Earth's inner core is anisotropic and the anisotropic structure change significantly both laterally and with depth. Previous body-wave studies of the inner core have relied on 1-D ray tracing or waveform modeling, which do not account fully the 3D anisotropic structure. Here we adopt a pseudo-bending ray tracing (PBR) method in spherical coordinates (Koketsu and Sekine, 1998) for seismic rays that traverse the inner core (PKP-DF phase). The method iteratively perturbs each discontinuity points and continuous segment of the ray through 3D (but isotropic) earth structure so that its travel time is minimum. Our implementation also includes a flexible scheme in calculating the velocity gradient needed to perturb the ray. A large volume is included in calculating the velocity gradient initially to find the global minimum, but a small volume surrounding the ray is used eventually to obtain the precise local velocity gradient that is sampled by the ray. Tests show that our implementation is very stable, reliable, and fast. We have traced the rays for over 3000 event-station pairs that we have differential PKP travel-time measurements using both the PBR method and a shooting method for a 1D model (AK135). The travel-time difference from the two methods is generally within 0.05 s with a few up to 0.07 s and the largest path difference is within 24 km; Even with a model of strong velocity gradient, the travel time difference is still less than 0.08s and the largest path difference is within 40km. Because the ray direction in the inner core does not change much (within 10 degrees even with a strong velocity gradient in the inner core), the 3D anisotropic structure of the inner core can be approximated to the first order as 3D heterogeneous (but isotropic) structure for a given ray, assuming the inner core anisotropy is axisymmetric. We are implementing the PBR method and B-spline interpolation to invert for 3D anisotropic structure of the inner core using differential PKP travel-time measurements.
DE: 7203 Body waves
DE: 7207 Core (1212, 1213, 8124)
DE: 7290 Computational seismology
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Seismology [S]
MN: Fall Meeting 2005