Seismic Anisotropy beneath the Global Mid-Ocean Ridge System from Source-Side Shear Wave Splitting

Shear wave splitting is our most direct method for assessing seismic anisotropy and inferring mantle flow. Observations over the oceanic realm, and therefore most of the Earth's surface, however, are limited by a severe lack of station coverage. Alternatively we can utilize the distribution of seismicity, instead of receivers, with the source-side shear-wave technique to target anisotropic signatures beneath seismically active oceanic spreading centers. Following a strict station selection quality control we identified 60 "null" stations situated across the world that were ideal for such an analysis. From these we made over 700 source-side splitting measurements along the mid-ocean ridge system of which 60% where classified as null, and 40% as splits with a mean delay time of 1.7 seconds. We find no obvious relationship with the seafloor age, spreading direction or spreading rate as might have been expected. We do find however a strong and coherent azimuthal dependence, which varies between different ridge segments and suggests that regionally localized processes dominate. We consider the range of geometries of both olivine LPO and transverse isotropy (which can be considered as a proxy for melt alignment) that can explain the observations at each well sampled locality.