Development of mantle seismic anisotropy during subduction-induced 3-D flow

The dynamics of subduction can be indirectly constrained by studying the induced mantle flow. However, inferring the circulation of the mantle around subducting plates from the interpretation of shear wave splitting patterns remains elusive. We calculated the strain-induced lattice preferred orientation (LPO) developed in 3-D models of subduction where retreat motions are maximized and found that in the mantle layer entrained with the downgoing slab the seismic anisotropy is trench-perpendicular, and becomes trench-parallel deeper, where the toroidal flow accommodates slab retreat. Synthetic SKS splitting shows that in the fore-arc slab rollback favors trench-parallel polarization of the fast shear wave component, while plate advance enhances trench-perpendicular seismic anisotropy. The interference between these two competing mechanisms yields subslab delay times of 0.5 to 1.3 sec, comparable with those observed at most natural subduction zones. The magnitude of the subslab trench-parallel splitting is independent of the rate at which the slab migrates, instead it is proportional to the amount of retreat. Citation: Faccenda, M., and F. A. Capitanio (2012), Development of mantle seismic anisotropy during subduction-induced 3-D flow, Geophys. Res. Lett., 39, L11305, doi:10.1029/2012GL051988.