Modeling of lower mantle seismic anisotropy beneath subduction zones

Abstract

Recent observations of up to 6 secs of shear-wave splitting in deep focus earthquakes from the TongaKermadec subduction zone provide insights into mantle dynamics near the 660 km discontinuity. Modeling of subduction body force stresses predicts large deviatoric stress (∼40 MPa) in the topmost lower mantle below a viscosity increase at the 660 km discontinuity. Finite strain calculations produce significant amounts of natural strains (∼7) in the lower mantle, and coherently aligned strain ellipses. For a viscosity model with a viscosity increase at 660 km, 5-10 secs of shear-wave splitting is predicted, compared to only 0-5 secs for uniform mantle viscosity. For a viscosity increase model, we predict that shear-wave splitting reduces from 7-10 secs to 4-6 secs when deformation above 410 km is ignored, and further reduces to 2-3 secs when deformation above 660 km is zeroed. Various conversions from finite strain to anisotropy have been explored. Predicted shear-wave splitting magnitudes are comparable to those observed from the Tonga-Kermadec subduction zone. Copyright 2004 by the American Geophysical Union.