A Comparison of Sub-Slab Anisotropy Beneath the Scotia and Caribbean Subduction Zones

Subduction systems are vitally important to plate tectonics and mantle convection but questions remain about many aspects of subduction dynamics. In particular, the mantle flow field beneath subducting slabs remains poorly understood. Seismic anisotropy can shed light on the pattern of mantle flow in subduction zones, but most studies that have examined subduction zone anisotropy focus primarily on the anisotropy of the system as a whole, or on the mantle wedge. Here we examine shear wave splitting due to anisotropy in the sub-slab mantle beneath the Caribbean and Scotia subduction zones. These subduction systems share many morphological similarities, with high arc curvature and short arc length, and yet are substantially different morphologically from other subduction zones. We examine and compare the anisotropy beneath the Scotia and Caribbean slabs to determine if similarities are also found in the flow fields beneath the slabs. We use the source-side splitting technique for direct teleseismic S phases for earthquakes originating in the down going slabs. We restrict our analysis to seismic stations where we have examined the receiver side splitting in detail; for stations with complex upper mantle anisotropy on the receiver side, it is difficult to make accurate corrections for anisotropy near the station. Since we are only using stations which we examined and have simple single layer anisotropy, we can be very confident in the corrections made to the wave forms to account for the anisotropy along the receiver-side portion of the ray path. Once the receiver-side anisotropy has been removed from the incoming wave, the only anisotropic signal left is that of the sub-slab mantle as the rest of the wave path is through the isotropic lower mantle. The Scotia subduction zone shows weak toroidal flow around the down going slab, possibly due to the curved nature of the subducting slab coupled with trench roll back. We are currently working on characterizing the Caribbean subduction zone sub-slab anisotropy, and while the seismicity of the Caribbean subduction system is substantially less than that of Scotia, we hope to see similar toroidal flow.