Lateral Viscosity Heterogeneity of Mantle Wedge and Back-Arc Mantle: Implications from the 2010 Mw 8.8 Maule and 2011 Mw 9.0 Tohoku-Oki Postseismic Deformation

Dehydration during slab subduction plays an important role in controlling the rheological properties of the continental mantle. Fluid release from slab dehydration induces variations in rheological strength within the upper mantle, leading to a weak mantle wedge (MW) and a relatively strong back-arc mantle (BM). We quantitatively analyzed the effects of a weak MW and a strong BM on back-arc subsidence and parameterized a laterally heterogeneous viscous structure with the MW–BM rheological interface and corresponding viscosity contrast. A generalized model with key elements of the subduction zone (subducting slab, continental crust, lithospheric mantle, asthenosphere, deep mantle, and cold-nose) shows that the larger the viscosity contrast between the MW and BM, the larger the amplitude of subsidence around the MW–BM interface. The different viscoelastic relaxation times of MW and BM cause relative horizontal viscoelastic deformation expressed as surface subsidence. The postseismic quasi-static stress changes manifest as enhanced extensional stress in the crust above the weak MW. The rheological contrast between the MW and BM induces lateral variations in the postseismic stress distribution within the overriding plate. We simulated the back-arc subsidence following the 2010 Mw 8.8 Maure earthquake and the 2011 Mw 9.0 Tohoku-Oki earthquake, using the MW–BM structure. The inferred MW–BM interface in the Chile and Japan subduction zones was located at ∼550 and 325 km from the trench, respectively. The spatial correlation between the location of the back-arc subsidence and the extent of the low-viscosity MW suggests localized dehydration of the subduction zone.