On the cause of enhanced landward motion of the overriding plate after a
major subduction earthquake
Abstract
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.