On steady rate coupling between an elastic upper crust and a viscous interior

Although it is commonly accepted that the upper (seismogenic) layer of the Earth's crust deforms elastically except in localized zones (faults) where deformation is discontinuous, the sub-seismogenic layer is not as well unde rstand. Possibilities for the rheology include viscous or viscoelastic beha vior, and localization of shear to relatively thin zones (that can sometime s be > 1 km in width). If the sub-seismogenic layer is viscous or viscoelas tic, then during interseismic periods a steady rate of deformation will be achieved after transients associated with viscoelastic relaxation have deca yed away (for sufficiently long earthquake cycles). I find that in this sit uation the motion in the elastic layer is decoupled from that deep within t he viscous layer, and they may be very different from each other. This impl ies that one can not use contemporary geodetic data to infer steady motion in a viscous sub-seismogenic layer (although it may be used to study viscoe lastic relaxation).