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

Abstract:

In the post-seismic stage after a megathrust earthquake, the overriding plate moves predominantly oceanwards. We focus on regions hundreds of kilometers away from the rupture area that have instead been found to exhibit increased landward velocities. Various interpretations of these observations were proposed: changes in megathrust coupling, transient slab acceleration, or bending of the overriding plate. These have different implications for seismic hazard. Using 3D finite element models, we find that both viscoelastic relaxation of the mantle wedge and afterslip downdip of the rupture zone cause in-plane bending of the overriding plate and mantle wedge, enhancing the landward motion of regions along-strike from the rupture zone. Landward displacement accumulates at exponentially decaying rates until ~8 mantle wedge relaxation times. The location of the modeled enhanced landward motion is broadly consistent with observations after both the 2010 Maule and the 2011 Tohoku-oki earthquakes. The magnitude of the velocity changes is similar to that of published secular velocity changes following the Maule event. The spatial distribution of the modeled enhanced landward motion is rather insensitive to the locking pattern of the megathrust, although the magnitudes of the displacements and velocities are affected. For a particular megathrust locking pattern, the amplitude of the enhanced landward motion scales linearly with the seismic moment of the earthquake. In-plane bending is an intrinsic response of the overriding plate and mantle wedge to traction changes associated with megathrust earthquakes; it probably goes a long way in explaining increased landward motions.