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Constraints on the anisotropic structure of D" by waveform inversion Mrinal K. Sen 1 ((512)471-0466; mrinal@utig.ig.utexas.edu) Jay Pulliam 1 1Institute for Geophysics, University of Texas at Austin, 4412 Spicewood Springs Road Bldg 600, Austin, Tx 78759 The phases S, ScS, Scd and SKS recorded in the distance range 80-100 degrees have been modeled with synthetic seismograms to reveal unusual features of the shear velocity structure in D". Recent observations reported time shifts between SH and SV arrivals, suggesting the possibility that D" is anisotropic. We have developed a fast waveform modeling and inversion method based on very fast simulated annealing (VFSA) to obtain best-fit earth models and their associated uncertainties. We designed a two-step modeling approach in which the reflectivity response of all the layers except those in D" is pre-computed. For each trial D" model, the reflectivity response is evaluated rapidly and combined with the pre-computed reflectivity response using invariant embedding. This allows for rapid evaluation of several hundred candidate D" models using exact 1D synthetic seismograms. VFSA searches for the following model parameters: thickness of the core-mantle transition zone, percent anisotropy, velocity gradients, and anisotropy parameter eta. The global search conducted by the VFSA algorithm allows us to estimate the posterior probability density (PPD) and posterior covariance in the model space. We obtain realistic error bars for our estimate of percent anisotropy and the thickness of the anisotropy zone. The trade-off between these two parameters is also clearly observed. We have applied the technique successfully to data that sample the D" zone beneath the Caribbean. |
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Last Modified: October 8, 1999
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