Viscoelastic Relaxation Of The Upper Mantle And Afterslip Following The 2014 Mw8.1 Iquique Earthquake

Large earthquakes cause large coseismic deformation thousands of kilometers from the rupture zone.After the earthquake,the viscoelastic relaxation of stresses in the upper mantle due to the earthquake results in continuous postseismic deformation of the crust,that may last more than several decades.Coseismic and postseismic deformations can be recorded by geodetic stations at high spatial and temporal resolutions.The observed postseismic deformation help us better understand the rheological properties of the upper mantle.Several earthquakes larger than Mw8 have occurred at the South America subduction zone in recent years.With the help of dense and high-quality GPS observations in this area,we wish to improve the understanding of the main postseismic geodynamic processes and the rheological structure and properties of the upper mantle in subduction zones.Previous coseismic and postseismic studies on the 2014 MW8.1 Iquique earthquake have reported that most deformation of the earthquake take place in areas within the latitude range of 17°-23°S.Therefore,we select 23 continuous GPS stations within the same latitude range.First,we corrected the GPS time series for the interseismic and seasonal effects,fit the corrected postseismic time series,and then calculate the postseismic displacements at any given time windows due to the earthquake.We finally obtained 4-yr postseismic displacements of 19 stations.Based on Slab1.0 data,we compiled 30 latitude-parallel profiles of the slab geometry within the latitude range of 10°-30°S at the South America subduction zone.In order to eliminate possible numerical artefacts,we extended each profile to a depth of 2000 km based on the relocated seismicity data,the position of the trench,the location of the volcano arc,and the smoothness of the profile curvature.Based on previous seismology and geodynamics studies in this area,we used the mesh generation software Trelis to build the three-dimensional finite element mesh of the central South America subduction zone.We used the open-source finite element modeling software PyLith to simulate the postseismic deformation of the earthquake.We used the grid search method to determine the three parameters in the finite element model:the viscosity in the shear zone that is used to simulate the afterslip,the thickness and the viscosity of the oceanic asthenosphere.In the optimized model,the viscosity in the shear zone,the thickness and the viscosity of the asthenosphere are 1017 Pa s,110 km and 2×10 18 Pa s,respectively.Our results have shown that the postseismic seaward motion is mainly controlled by the afterslip and viscoelastic relaxation of the mantle wedge.The viscoelastic relaxation of the ocean upper mantle mainly controls the crustal landward motion.In the vertical direction,the viscoelastic relaxation of the mantle wedge causes uplift,while the relaxation of the oceanic upper mantle causes uplift near the trench but subsidence in the coastal area.The afterslip causes uplift updip of its slip region and subsidence downdip.Test models indicate that the afterslip decreases rapidly with time after the earthquake and takes place mostly within one year after the event.This is consistent with previous studies of other subduction zone earthquakes.