HR: 08:55h
AN: T31D-04    [Abstracts]
TI: Upper Mantle Anisotropy and Normal Mode Coupling
AU: * Beghein, C
EM: beghein@mit.edu
AF: MIT, 77 Massachusetts avenue room 54-526, Cambridge, MA 02139 United States
AU: van der Hilst, R
EM: hilst@mit.edu
AF: MIT, 77 Massachusetts avenue room 54-526, Cambridge, MA 02139 United States
AU: Trampert, J
EM: jeannot@geo.uu.nl
AF: Utrecht University, Seismology 4 Budapestlaan, Utrecht, 3508 TA Netherlands
AB: We show that upper mantle models that account for lateral variations in radial anisotropy offer a better explanation for the coupling of normal mode multiplets of the type ${}_0S_l-{}_{0}T_{l+1}$ than isotropic models. These modes are sensitive to the upper mantle only and, although their coupling is known to be mostly due to the Coriolis force, a large part of the degree 2 signal measured by Resovsky and Ritzwoller [1998] remains to be explained. Here, we compare the effect of isotropy and radial anisotropy on the coupling of these pairs of modes. We test several isotropic mantle models filtered at degree 2, and anisotropic models of the upper mantle previously obtained by Beghein and Trampert [2004] with surface wave phase velocity maps and a model space search approach. We find that most of the signal cannot be explained by Coriolis coupling and isotropic upper mantle structure. On the contrary, degree 2 models including shear-wave radial anisotropy in the upper mantle predict structure coefficients that are significantly closer to the data than any existing isotropic models. We also show that the correlation between predictions and data is much higher when anisotropy is included, especially for multiplets whose sensitivity to elastic parameter $N=\rho V_{SH}^2$ increases in the uppermost mantle and transition zone. Interestingly, coupled mode multiplets that are sensitive to the entire mantle (e.g. ${}_3S_1-{}_{1}S_{3}$ or ${}_3S_7-{}_{5}S_{5}$) can be relatively well explained by isotropic degree 2 structure. However, it should be noted that these modes are sensitive to both shear-wave and P-wave related elastic parameters, as opposed to modes such as ${}_0S_l-{}_{0}T_{l+1}$ which can only see shear-wave anomalies. ${}_nS_l-{}_{n'}S_{l'}$ coupled mode structure coefficients could, therefore, bring some constraints on upper mantle P-wave anisotropy or on anisotropy at larger depths.
DE: 8180 Tomography
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting