Upper and mid-mantle anisotropy beneath the Tonga slab

Measurements of source-side splitting in S waves from events within the Tonga slab reveal anisotropy in the upper and mid-mantle beneath the slab. We observed splitting for events originating at both upper mantle and transition zone depths. Anisotropic fast directions (phi) are trench parallel or sub-parallel for both upper mantle and transition zone events. Delay times (delta t) decrease with depth for upper mantle events. The source of anisotropy for the upper mantle events is likely in the sub-slab mantle, and is likely indicative of trench parallel flow due to slab rollback. The source of anisotropy for the deeper earthquakes is more difficult to constrain, but the pattern of splitting measurements argues for an uppermost lower mantle anisotropic source, and slab induced deformation may be responsible for this anisotropy as well. Additional constraints from mineral physics studies are necessary to interpret the mid-mantle anisotropy signal in terms of geodynamical processes. Citation: Foley, B. J., and M. D. Long (2011), Upper and mid-mantle anisotropy beneath the Tonga slab, Geophys. Res. Lett., 38, L02303, doi: 10.1029/2010GL046021.