Offshore landward motion shortly after a subduction earthquake implies rapid relocking of the shallow megathrust

Abstract

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.