HR: 16:30h
AN: U34A-03    [Abstracts]
TI: A Whole-Mantle Three Dimensional Radially Anisotropic S Velocity Model
AU: * Panning, M P
EM: mpanning@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, 202 McCone Hall Univ. Calif., Berkeley, CA 94720 United States
AU: Romanowicz, B A
EM: barbara@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, 202 McCone Hall Univ. Calif., Berkeley, CA 94720 United States
AB: We present a 3D radially anisotropic model of the whole mantle obtained using a large three component surface and body waveform dataset and an iterative inversion for structure and source parameters based on Nonlinear Asymptotic Coupling Theory (NACT) (Li and Romanowicz, 1995). The model is parameterized by isotropic $V_S$ up to spherical harmonic degree 24 and $\xi$ ($\xi = V_{SH}^2 / V_{SV}^2$), a measurement of radial anisotropy in shear velocity, up to degree 16. While the isotropic portion of the model is consistent with previous shear velocity tomographic models, the anisotropic portion suggests relationships between flow and anisotropy in a vairety of depth ranges. In the uppermost mantle, we confirm observations of regions with $V_{SH}>V_{SV}$ starting at $\sim$80 km under oceanic regions and $\sim$250 km under old continental lithosphere, suggesting horizontal flow beneath the lithosphere (Gung et al., 2003). We also observe a $V_{SV}>V_{SH}$ signature at $\sim$200-300 km depth beneath major ridge systems with amplitude significantly correlated with spreading rate for fast-spreading segments. In the transition zone (400-700 km depth), regions of subducted slab material are associated with $V_{SV}>V_{SH}$. We also confirm the observation of strong radially symmetric $V_{SH}>V_{SV}$ in the lowermost 300 km (Panning and Romanowicz, 2004). The 3D deviations from this degree 0 signature are associated with the transition to the large-scale low-velocity superplumes under the central Pacific and Africa, suggesting that $V_{SH}>V_{SV}$ is generated in the predominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes. We also solve for source perturbations in an interative procedure. Source perturbations are generally small compared to published Harvard CMT solutions, but significantly improve the fit to the data. The sources in the circum-Pacific subduction zones show small but clearly systematic shifts in location due to an improved structural model.
DE: 7260 Theory and modeling
DE: 7207 Core and mantle
SC: Union [U]
MN: 2004 AGU Fall Meeting