HR: 13:55h
AN: S32C-02    [PDF]
TI: Lateral Variations in Radial Anisotropy Down to 1200 km Depth
AU: * Beghein, C
EM: beghein@geo.uu.nl
AF: Utrecht University, Budapestlaan 4 PO Box 80021, Utrecht, 3508 TA Netherlands
AU: Trampert, J
EM: jeannot@geo.uu.nl
AF: Utrecht University, Budapestlaan 4 PO Box 80021, Utrecht, 3508 TA Netherlands
AB: The presence of radial anisotropy in the upper mantle, transition zone and top of the lower mantle is investigated by applying the neighbourhood algorithm to degree 8 Rayleigh and Love wave phase velocity models constructed from fundamental mode and overtone measurements. Probability density functions are obtained for S-wave anisotropy ($\xi$), P-wave anisotropy ($\phi$), intermediate parameter $\eta$, equivalent isotropic $V_p$ and $V_s$ anomalies and density perturbations. Shear wave anisotropy with $V_{SH}>V_{SV}$ is found down to the transition zone. The signal within continents is clearly age-related in the top 220~km, with platforms and tectonically active regions characterized by a rapid decrease in $\xi$ with depth while cratons display a more constant signal, extending slightly deeper than the 220-discontinuity. A similar age dependence of S-wave anisotropy is also observed beneath oceans, with a strong and rapidly decreasing anisotropy for young oceans and a more constant anisotropy for older oceans. $V_{SH}>V_{SV}$ is also found in the transition zone, most likely occurring beneath oceans (and stronger for young oceanic areas than for old ones), but not beneath continents. This could indicate a possible deep origin for ocean ridges. In the uppermost mantle, $\eta$ presents a similar depth dependence as $\xi$ in various age-related regions, and perturbations in P-wave anisotropy are small and limited to the shallowest part of the continents. Both P-wave anisotropy and $\eta$ change sign below 220~km depth beneath the oldest oceans and beneath continental regions, which might reflect a change in the mechanical behaviour of the materials in these regions. Below 400~km depth, no strong deviation from PREM for $\phi$ or $\eta$ is required by the data. Likelihoods were also obtained for the ratios $d\phi /d\xi$ and $d\eta /d\xi$ in different regions. We find strong lateral variations in $d\eta /d\xi$ beneath continents in the top 100~km, with a clear change of sign between old cratons and younger continental areas. This could indicate age-related chemical variations in the continents or changes in the strain field that aligns crystals in the uppermost mantle. At larger depths the distributions are wide and they show that the data do not favour any particular relation between perturbations in the anisotropic parameters. Similar observations can be made for $d\phi /d\xi$, but the change of sign in the uppermost mantle is not as clear as the one found for $d\eta /d\xi$.
DE: 7218 Lithosphere and upper mantle
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting