Finding Simplicity in the Complexity of Postseismic Coastal Uplift and Subsidence Following Great Subduction Earthquakes

Abstract

AbstractFollowing great subduction earthquakes, postseismic deformation of coastal areas shows consistent seaward motion but complex vertical deformation. Understanding both the horizontal and vertical components in the same geodynamic framework presents challenges. Here, by modeling short‐term (a few years) postseismic viscoelastic relaxation (VER) and afterslip following synthetic and real subduction earthquakes, we demonstrate that the complexity of the vertical deformation can be explained in simple terms. Along a margin‐normal profile, VER results in an up‐down‐up trisegment, long‐wavelength pattern common to most megathrust earthquakes, including near‐trench uplift, midway subsidence, and near‐arc uplift, with locations controlled by coseismic fault slip. The magnitude of the first two segments is controlled mainly by oceanic mantle viscosity, and the third by mantle wedge viscosity. In contrast with VER, afterslip results in an up‐down bimodal pattern of variable wavelengths specific to individual earthquakes. Its site‐specific and heterogeneous nature is primarily responsible for the complexity in vertical deformation, but its effect can be adequately modeled using a simple elastic model. If the coast is near the megathrust rupture zone, variable combinations of the VER and afterslip effects lead to either uplift or subsidence. If the coast is in the near‐arc segment of VER deformation, uplift usually occurs. Modeling the common VER process enables the identification of site‐specific afterslip, which helps to understand the mechanism of afterslip in the context of the broad spectrum of fault slip behavior. Our results also have important implications to deciphering coastal paleoseismic records to constrain coseismic versus postseismic deformation of ancient earthquakes.