HR: 08:30h
AN: T21A-03    [PDF]
TI: Large Scale Anisotropy in D" from Global Waveform Inversion
AU: * Panning, M
EM: mpanning@seismo.berkeley.edu
AF: Berkeley Seismological Lab, 202 McCone Hall UC Berkeley, Berkeley, CA 94720 United States
AU: Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: Berkeley Seismological Lab, 202 McCone Hall UC Berkeley, Berkeley, CA 94720 United States
AB: The Earth's core-mantle boundary is both a thermal and chemical boundary layer between the silicate mantle and the fluid iron outer core. The mantle-side region (D") is also a mechanical boundary layer for the convection of the mantle, leading to intense deformation. This strain field can lead to detectable seismic anisotropy, either through the alignment of anisotropic crystals or of layers (or inclusions) of materials with strongly contrasting elastic properties (Karato, 1998; Kendall and Silver, 1996). Anisotropy in D" is observed in several regions using Sdiff or ScS (Vinnik et al., 1989; Kendall and Silver, 1996; Lay et al., 1998). However, these studies only sample limited areas of D". A global picture of long-wavelength anisotropic D" structure would clearly aid both dynamic flow modeling and mineral physics interpretations. Therefore, we have adapted a global tomography approach (Li and Romanowicz, 1996) to develop a 3D global model of mantle radial anisotropy using three component time-domain waveforms of surface and body waves. The model is parameterized by isotropic $V_S$ and $\xi$ ($\xi = V_{SH}^2 / V_{SV}^2$), a measurement of radial anisotropy in shear velocity. In our model, D" is characterized by a strong degree 0 signature of positive $\delta \ln{\xi}$ ($V_{SH}>V_{SV}$), similar to the uppermost mantle in previous anisotropic models such as PREM. The 3D isotropic velocity imaged in D" is consistent with earlier tomographic models of $V_S$ in this depth range (Masters et al., 1996; M\'egnin and Romanowicz, 2000), and is characterized by a strong degree 2 component representing a fast ring surrounding two low velocity features ("superplumes") beneath the central Pacific and Africa. For $\xi$ in D", the degree 0 component dominates, but the regions that deviate most from this structure correlate well with the locations of the superplumes. We see reduced $\delta \ln{\xi}$ under the central Pacific, Africa, and the south Atlantic, including patches with negative values ($V_{SV}>V_{SH}$). The observed anisotropy in D" is consistent with a boundary layer dominated by horizontal flow, while emphasizing the unique character of the two superplume regions. Our results also suggest similar relationships between anisotropic signature and flow prevail in the uppermost and lowermost mantle.
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2003 Fall Meeting