Seismic velocity structure and anisotropy of the Alaska subduction zone based on surface wave tomography

Southcentral Alaska is a complex tectonic region that transitions from subduction of Pacific crust to flat slab subductionand collisionof overthickened Yakutat crust. Because much of the Yakutat crust has been subducted, seismic imaging is needed in order to understand the crustal and upper mantle structural framework for this active tectonic setting. Here we use teleseismic Rayleigh waves to image large-scale variations in shear wave structure. Our imaging technique employs a two-plane wave representation with finite frequency sensitivity kernels. Our 3-D isotropic model reveals several features: the subducting Pacific/Yakutat slab, slow wave speeds characterizing the onshore Yakutat collision zone, slow wave speeds of the Wrangell subduction zone, and a deep tomographic contrast at the eastern edge of the Pacific/Yakutat slab. We produce anisotropic phase velocity maps that exhibit variations in the fast direction of azimuthal anisotropy. These maps show the dominance of the Yakutat slab on the observed pattern of anisotropy. West of the Yakutat slab the fast directions are approximately aligned with the plate convergence direction. In the region of the Yakutat slab the pattern is more complicated. Along the margins of the slab the fast directions are roughly parallel to the margins. We identify notable differences and similarities with published SKS splitting measurements. Integrative modeling using 3-D anisotropy models and different seismic measurements will be needed in order to establish a detailed 3-D anisotropic velocity model for Alaska. This study provides a large-scale starting point for such an effort.