Relaxation following large subduction earthquakes produces landward changes in surface velocities. Near-trench landward velocity changes in the vicinity of the rupture zone have been attributed to rapid viscoelastic relaxation in the sub-slab asthenosphere. Lateral landward velocity changes, hundreds of km along-trench from the rupture, have been variously explained invoking interplate coupling changes, transient slab acceleration, or overriding plate bending, with different implications for seismic hazard. We investigate whether the lateral landward velocity changes can result from postseismic relaxation with constant interplate coupling and convergence rate. We use a finite element model with periodic megathrust earthquakes resulting from unlocking of asperities on the megathrust, with instantaneous, dynamically driven coseismic slip and afterslip and with bulk visco-elastic relaxation. A mechanical contrast in the overriding plate is required to reproduce the observed near-trench focusing of interseismic deformation. A maximum depth limit to afterslip of around 100 km is consistent with inverted afterslip distributions and the hypothesized depth extent of the megathrust. The presence of both the contrast and depth limit causes viscous relaxation in the mantle wedge to result in lateral landward velocity changes. We discuss the responsible mechanism, which amounts to elastic deformation of the overriding plate and is due to the finite compressibility of the plate and its in-plane bending. The spatial pattern of landward velocity changes is consistent with observations for the Maule and Tohoku earthquakes. Velocity change magnitudes are comparable with observations, scale with viscosity and seismic moment, are only partly counteracted by the effect of primary afterslip, and are little affected by interplate coupling pattern. In the years following the largest earthquakes with rapidly decaying afterslip, this mechanism is expected to produce detectable landward velocity changes. The models also shows near-trench landward velocity changes close to the rupture, consistently with previous research. However, we find that the extent and timing of shallow interface (re)locking is critical for reproducing near-trench observations on the overriding plate.

We aim to better understand the overriding plate deformation during the megathrust earthquake cycle. We estimate the spatial patterns of interseismic GNSS velocities in South America, Southeast Asia, and northern Japan and the associated uncertainties due to data gaps and velocity uncertainties. The interseismic velocities with respect to the overriding plate generally decrease with distance from the trench with a steep gradient up to a “hurdle”, beyond which the gradient is distinctly lower and velocities are small. The hurdle is located 500–1000 km away from the trench, for the trench-perpendicular velocity component, and either at the same distance or closer for the trench-parallel component. Significant coseismic displacements were observed beyond these hurdles during the 2010 Maule, 2004 Sumatra-Andaman, and 2011 Tohoku earthquakes. We hypothesize that both the interseismic hurdle and the coseismic response result from a mechanical contrast in the overriding plate. We test our hypothesis using physically consistent, generic, three-dimensional finite element models of the earthquake cycle. Our models show a response similar to the interseismic and coseismic observations for a compliant near-trench overriding plate and an at least 5 times stiffer overriding plate beyond the contrast. The model results suggest that hurdles are more prominently expressed in observations near strongly locked megathrusts. Previous studies inferred major tectonic or geological boundaries and seismological contrasts located close to the observed hurdles in the studied overriding plates. The compliance contrast probably results from thermal, compositional and thickness contrasts and might cause the observed focusing of smaller-scale deformation like backthrusting.

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.

We aim to better understand the spatial distribution of interseismic overriding plate deformation at and near subduction zones. To this end, we analyze horizontal GNSS velocities in South America, southeast Asia, and northern Japan, computing and interpolating local trench-normal and -parallel velocity components. Velocities generally decrease with distance from the trench with a steep gradient up to a “hurdle”, beyond which the gradient is distinctly lower and velocities are near-zero. The hurdle is located 500–1000 km away from the trench for the trench-perpendicular component and either at the same distance or closer for the trench-parallel. In contrast, significant displacements during large megathrust earthquakes are generally observed beyond the hurdle. To test our hypothesis that the hurdle results from a lateral contrast in overriding plate compliance, we use cyclic three-dimensional finite element models . Our results are consistent with the observed interseismic velocity gradients and far-field coseismic displacement. The gradient in modeled trench-perpendicular velocities depends on the location of the contrast and on the plate compliance on both sides. Trench-parallel velocities have a progressively shallower gradient with distance from the trench and only depend on the near-trench modulus. The inferred contrast probably results from thermal, compositional and thickness contrasts. This interpretation is consistent with the presence, close to the observed hurdle, of major tectonic or geological boundaries separating the plate margin from a distinct, and likely less compliant, plate interior. Stress accumulation on the model’s locked megathrust patches is hardly affected by the distance to the contrast.