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 understand. Possibilities for the rheology include viscous or viscoelastic behavior, and localization of shear to relatively thin zones (that can sometimes be >1 km in width). If the sub-seismogenic layer is viscous or viscoelastic, then during interseismic periods a steady rate of deformation will be achieved after transients associated with viscoelastic relaxation have decayed away (for sufficiently long earthquake cycles). I find that in this situation the motion in the elastic layer is decoupled from that deep within the viscous layer, and they may be very different from each other. This implies 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 viscoelastic relaxation). ¿ 2000 American Geophysical Union