Geodetic observations after large subduction earthquakes reflect
multiple postseismic processes, including megathrust relocking. What the
timing of relocking is, and how well observations constrain it, is
unclear. It has been inferred to explain some observed landward motion
that occurs within months. It has also been considered unable to explain
other, greater landward motion, including off the coast of Japan
beginning weeks after the 2011 Tohoku earthquake, which is attributed to
postseismic relaxation. We use generic, 3D numerical models to show that
relocking, particularly of the shallow interface, is needed for
postseismic relaxation to produce landward motion on the tip of the
overriding plate. We argue that this finding is consistent with previous
simulations that implicitly relock the megathrust where afterslip is not
included, that the Tohoku megathrust thus relocked within less than two
months of the earthquake, and that the shallow megathrust probably
behaves as a true, unstably sliding asperity.