Understanding Postseismic Uplift and Subsidence Following Great Subduction Earthquakes

Abstract:

Postseismic deformation after great subduction earthquakes contains important information on Earth rheology and subduction zone dynamics. The horizontal component of observed postseismic deformation shows a consistent pattern of opposing motion of the trench and inland areas that has been well explained by viscoelastic relaxation of the upper mantle, with contributions from afterslip of the megathrust. But the vertical component exhibits a more complex pattern that has been difficult to explain. The main reason for the complexity is that the vertical deformation is more sensitive to details of the rheological structure and coseismic slip distribution, as well as to the location of the observation point relative to the rupture zone. Here we use a viscoelastic model to explain the vertical postseismic signals in a simple and systematic way. We demonstrate that postseismic subsidence occurs around the pivot line of coseismic vertical deformation, due to the relaxation of the sub-slab oceanic mantle, and that uplift occurs just seaward of volcanic arc, caused by a deflection of seaward postseismic mantle flow by the cold forearc mantle wedge. This general viscoelastic deformation pattern can be modified by afterslip. The vertical deformation at the coast is controlled by the location of the pivot line of coseismic vertical deformation and the distance between the coast and the trench. We apply 3-D finite element deformation models to subduction earthquakes for which vertical measurements are available, such as the 1960 Chile, 2004 and 2005 Sumatra, 2011 Tohoku-oki, 2010 Maule, and 2007 Bengkulu earthquakes. The results also help to understand potential contribution of postseismic deformation to coastal subsidence data that are used to constrain coseismic deformation in paleoseismic research.