Shear wave splitting measurements across the North Anatolian Fault (NAF) in Northern Turkey are calculated from teleseismic SKS arrivals recorded by the dense temporary DANA seismological array to investigate the lithospheric anisotropy of an active shear zone. Our results show fast polarisation direction (φ) changes from NNE-SSW to NE-SW from south to north and a peak in splitting magnitude (δt) immediately north of the northern branch of the NAF. We show the first evidence for varying lithosphere-scale anisotropy across the NAF and suggest the presence of a strongly anisotropic region in the lower crust and/or upper mantle adjacent to the surface expression of the northern branch of the NAF (NNAF). We interpret multiple sources of anisotropy contributing to the overall splitting patterns, most likely dominated by distributed strain in the lithospheric mantle that is localised north of the NNAF.

Seismic radial anisotropy is a crucial tool to help constrain flow in the Earth’s mantle. However, Earth structure beneath the oceans imaged by current 3-D radially anisotropic mantle models shows large discrepancies. In this study, we provide constraints on the radially anisotropic upper mantle structure beneath the Pacific by waveform modelling. Specifically, we objectively evaluate three 3-D tomography mantle models which exhibit varying distributions of radial anisotropy through comparisons of independent real datasets with synthetic seismograms computed with the spectral-element method. The data require an asymmetry at the East Pacific Rise with stronger positive radial anisotropy ξ=V/V=1.13-1.16 at ~100km depth to the west of the East Pacific Rise than to the east (ξ=1.09-1.12). This suggests that the anisotropy in this region is due to the lattice preferred orientation of anisotropic mantle minerals produced by shear-driven asthenospheric flow beneath the South Pacific Superswell. Radial anisotropy reduces to ξ=1.09-1.12 beneath the central Pacific and to a minimum of ξ<1.05 in the west, beneath the oldest part of the oceanic lithosphere at ~100km depth. This reduction in the magnitude of radial anisotropy estimated beneath the west Pacific possibly reflects a deviation from horizontal flow as the mantle is entrained with subducting slabs, a change in temperature or water content that could alter the anisotropic olivine fabric or the shape-preferred orientation of melt. In addition to a lateral age-dependence of anisotropy, our results also suggest that a depth-age trend in radial anisotropy may prevail from the East Pacific Rise to Hawaii (~90Ma).