Three-dimensional Numerical Model Of The Viscoelastic Postseismic Deformation Following The 2010 Mw8.8 Maule Earthquake

In a subduction zone,a large amount of energy due to the coupling between the oceanic and continental plates is accumulated as elastic potential energy.The accumulated strain is released through earthquakes when yielding stress over the megathrust is reached.Due to the persistent subduction,the subduction zone earthquake occurs repeatedly.The Earth material will respond to the stress perturbation of earthquakes.The recorded postseismic deformation after an earthquake,however,is the superimposed responses of stress responses in different rheological materials,such as the viscoelastic relaxation in the upper mantle,afterslip of the megathrust,poroelastic rebound,etc.On the other hand,the rheological properties of the upper mantle may be constrained by studying the viscoelastic postseismic deformation of large earthquakes.In 2010,the Mw8.8 Maule earthquake occurred in the South America subduction zone where the Nazca plate subducts beneath the South America plate.It was the largest earthquake in this region since the 1960 Mw9.5 earthquake.This study aims to establish a more complete three-dimensional finite element model to study the postseismic deformation of the Maule earthquake,to better understand the postseismic deformation processes,to determine the first-order rheological parameters,and to constrain the distribution and evolution of the afterslip of the megathrust.In recent decades,interseismic,coseismic and postseismic deformation of earthquake have been well recorded due to the advances in the geodesy technology such as GPS.And the main impact area of the Maule earthquake has dense GPS network which provides good constraints on the crustal postseismic deformation.In this study we processed the GPS time series data in this area:first,we calculate the pre-earthquake velocity and seasonal effects;then,we fit the postseismic time series after removing the calculated pre-earthquake trends;finally,we calculate the postseismic displacements at any given time window through the fitted curve.In the six-year postseismic displacement field,most GPS stations are moving in the same seaward direction.The maximum cumulative seaward horizontal displacement is about 68 cm.The maximum uplift is about 15 cm in the middle field with subsidence at near-coast field and far field.Based on the Slab 1.0,tomography studies,relocated seismicities,location of the arc,etc,we compiled dozens of latitude-parallel profiles of the slab geometry.Then we developed the three-dimensional finite element mesh.The model includes a viscoelastic mantle wedge,a viscoelastic oceanic asthenosphere and the oceanic upper mantle.The viscoelastic relaxation of the upper mantle is simulated through the bi-viscous Burgers rheology.The afterslip of the megathrust is simulated through a 2 km weak shear zone attached to the fault.We carried out the grid-search tests to obtain the preferred model,with the minimum misfit between the postseismic GPS displacements and model predictions.The preferred model indicates that a 120 km thick asthenosphere with a viscosity of 1019 Pa s at the top of the oceanic upper mantle is required to fit the data.The afterslip is up to 2 m within the first 2 year,equivalent to a modeled earthquake with a magnitude Meq8.2.The afterslip decays rapidly with time.This study provides a reference model for further studies on the earthquake cycle deforamtion,lithospheric stress evolution and secondary-order processes such as afterslip anisotropy and poroelastic rebound etc.