Finite-frequency vectorial tomography: a new method for high-resolution imaging of upper mantle anisotropy |
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| Authors: | Sébastien Chevrot |
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| Institution: | Laboratoire de Dynamique Terrestre et Planétaire, CNRS, UMR 5562;, Observatoire Midi-Pyrénées, UniversitéPaul Sabatier, Toulouse III, France. E-mail: |
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| Abstract: | Amplitude measurements of the transverse component of SKS waves, the so-called splitting intensity, can be used to formulate a non-linear inverse problem to image the 3-D variations of upper mantle anisotropy. Assuming transverse isotropy (or hexagonal symmetry), one can parametrize anisotropy by two anisotropic parameters and two angles describing the orientation of the symmetry axis. These can also be written as two collinear pseudo-vectors. The tomographic process consists of retrieving the spatial distribution of these pseudo-vectors, and thus resembles surface wave vectorial tomography. Spatial resolution results from the sensitivity of low-frequency SKS waves to seismic anisotropy off the ray path. The expressions for the 3-D sensitivity kernels for splitting intensity are derived, including the near-field contributions, and validated by comparison with a full wave equation solution based upon the finite element method. These sensitivity kernels are valid for any orientation of the symmetry axis, and thus generalize previous results that were only valid for a horizontal symmetry axis. It is shown that both lateral and vertical subwavelength variations of anisotropy can be retrieved with a dense array of broad-band stations, even in the case of vertically propagating SKS waves. |
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| Keywords: | Fréchet derivatives scattering seismic anisotropy shear wave splitting tomography |
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