Models of postseismic deformation after megaearthquakes: the role of various rheological and geometrical parameters of the subduction zone

Abstract. The postseismic deformations following subduction megaearthquakes are characterized by a horizontal velocity which, once non-dimensionalized by the coseismic displacement, increases with distance to the trench then presents an almost constant value for distances between 500 and 1500 km. The vertical velocity features a strong narrow peak on the trenchward side of the volcanic arc. Subsidence is observed in the far-field.

In order to understand better the implications of these observations, the influence of the geometry of low viscosity regions in subduction zones on the postseismic deformations is analyzed using a 2-D finite element model with viscoelastic rheologies. The slab dip in the top 80 km Θ_top, and deeper Θ_bottom and the locking depth all have a limited impact on the ratio of horizontal postseismic velocity over coseismic displacement. The smaller Θ_bottom, the smaller the amplitude of the predicted vertical velocity in the middle-field (200–500 km from the trench). The presence of the slab at asthenospheric depths affects very significantly both the horizontal and vertical velocities. Models with an 80 km thick lithosphere, where the relaxation occurs only in the asthenosphere, are characterized by a trenchward horizontal velocity decreasing very moderately in the middle-field and an uplift maximum on the continental side of the volcanic arc, at odds with the observations. A low viscosity channel (LVCh) over the deep parts of the subduction interface or a low viscosity wedge (LVW) have a considerable impact on the middle-field horizontal and vertical velocities: the trenchward horizontal velocities are very significantly increased while the vertical velocities are characterized by strong uplift over the deep parts of the subduction interface. In the case of a low viscosity wedge, a marked subsidence further away from the trench, on the continent side of the volcanic arc is predicted. While the low-viscosity wedge affects little the far-field horizontal velocities, the LVCh increases them significantly. The thicknesses of the lithosphere and the asthenosphere also have a strong impact on both the middle-field and the far-field velocities. The larger they are, the further from the trench are the maxima of the ratio of the postseismic over coseismic horizontal displacement and of the far-field subsidence. 3-D modeling with a geometry as precise as possible of the various zones with postseismic creep associated with each megaearthquake is necessary to derive more precise conclusions. However, the 2-D modeling results obtained here, compared with postseismic data, point towards lithospheres and asthenospheres surprisingly similar in various areas of the world, with thicknesses around 70 and 200 km respectively and towards the presence of a LVW and/or a LVCh. The systematic description of the role of each parameter presented here will facilitate the choice of the parameters to vary in 3-D models.