Olivine Fabrics in the Oceanic Lithosphere Constrained by Pn Azimuthal Anisotropy

We constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observations of Pn azimuthal anisotropy. We first analyze active-source data derived from a broadband ocean bottom seismometer array that was deployed in the Northwestern Pacific. We infer the azimuthal dependence of the Pn velocity, including the 2f and 4f terms of the sinusoidal functions, where f is the back azimuth. We observe a skewed azimuthal dependence of the Pn velocity, with a large peak-to-peak amplitude of about 12%. Similar features are observed using an independent data set of teleseismic Pn waves. We constrain the direction of the crystallographic axes of olivine to explain the observed azimuthal dependence and identify A-type olivine with a slightly dipping a-axis and slightly tilting b-axis being compatible with our observations. In contrast, we find that D-type olivine with any direction of crystallographic axes cannot explain our observations. Secondary deformation and recrystallization in the older Pacific may be responsible for this strong and complex seismic anisotropy in the study region.