Anisotropy in the Pacific upper mantle from inversion of a surface-wave dispersion dataset

Abstract:

We present work towards a three-dimensional model of the anisotropic velocity structure of the Pacific upper mantle. Models of seismic anisotropy in oceanic regions provide important constraints on the geometry of strain in the mantle, the nature of the lithosphere-asthenosphere transition, and the possible presence of partial melt in the asthenosphere. The goal of this work is to produce a three-dimensional model of isotropic and anisotropic velocities in the Pacific, which will improve constraints on olivine fabrics and strain geometries in the oceanic upper mantle. Measurements of fundamental-mode dispersion for Rayleigh and Love waves traversing oceanic paths are drawn from the waveform dataset used to construct the global dispersion model GDM52. We develop anisotropic phase-velocity maps of the Pacific basin for Rayleigh and Love waves between 25 s and 250 s and invert the phase-velocity maps for anisotropic velocity structure at depth. The resulting models are radially anisotropic and include the G parameters that are related to the azimuthal anisotropy of v_SV. We compare results of these two-step inversions with direct inversions of fundamental-mode phase anomalies for three-dimensional anisotropic structure. In much of the central and western Pacific, vertical gradients in both v_S and anisotropy are consistent with the transition from rigid lithosphere to viscously deforming asthenosphere. In future work we plan to incorporate waveform data providing constraints on higher-mode dispersion in the modeling of the three-dimensional anisotropic structure.