Mid-mantle anisotropy beneath Japan and South America from source-side shear wave splitting

Abstract:

Measurements of seismic anisotropy such as shear wave splitting are commonly used to constrain deformation in the upper mantle; however, observations of anisotropy at mid-mantle depths are relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) in the northwest Pacific (Japan and Izu-Bonin) and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from seismic sources deeper than 350km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, splitting as large as 1 sec is consistently observed, indicative of seismic anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as 600km argues for a contribution from anisotropy in the uppermost lower mantle, as well as the transition zone. Beneath Japan, fast splitting directions are perpendicular or oblique to the slab strike (opposite of what is observed in the shallower sub-slab mantle) and do not appear to depend on the propagation direction of the waves. Beneath South America, splitting directions vary from trench-parallel to trench-perpendicular and depend on propagation direction (that is, whether the shear waves travel in an up-dip or down-dip direction). This difference may reflect the different deformation styles of each slab as it interacts with the transition zone discontinuities; the subducting Pacific plate beneath Japan stagnates atop the 660km discontinuity, while the subducting Nazca plate beneath South America penetrates into the lower mantle. While the elasticity and fabric development for phases that are present in the transition zone and uppermost lower mantle remain imperfectly known, our results provide unequivocal evidence for the presence of anisotropy at mid-mantle depths in the vicinity of subducting slabs.