A stress diffusion model is adapted for application to subducted lithosphere by including a gravitational body force parallel to the direction of diffusion. In the model a sequence of step function boundary displacements represents a history of interplate thrust earthquakes. The response of subducted lithosphere to these displacements is approximated by considering the transient response of the slab to be purely elastic and the response of the surrounding mantle to be purely viscous. The model indicates that following even the largest interplate thrusts, the tension due to the weight of the slab is only partially relaxed and the preseismic stress orientation is maintained outside of the immediate vicinity of the interplate thrust. In contrast, others have previously found that the focal mechanisms of intermediate depth earthquakes sometimes change from T downdip to P downdip orientation following great interplate thrusts. Thus some focal mechanisms in subducted lithosphere may have an orientation in conflict with the mean stress in the slab. This apparent conflict could result from seismic failure in zones of heterogeneous stress or strength. The stress diffusion model shows that the orientations of mean stress and mean strain rate in subducted lithosphere can differ, at least transiently. If strength is heterogeneous, then focal mechanisms will be consistent with mean strain rate rather than mean stress.