Why is interseismic shortening of the overriding plate largely
restricted to the near-trench region?
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.