Decadal viscoelastic postseismic deformation of the 1964 Mw9.2 Alaska earthquake and its stress perturbations over crustal active faults

Giant subduction zone earthquakes provide important information on the dynamics of convergent margins. The postseismic deformation of the 1964 Mw9.2 Alaska earthquake has been well recorded by geodetic stations. We have developed a three-dimensional (3D) finite element model to study the viscoelastic postseismic deformation of the 1964 earthquake. Our model includes an elastic upper plate and subducting slab, and viscoelastic elements with different viscosities: mantle wedge, oceanic asthenosphere, and upper mantle. Viscoelastic relaxation of these rheological units is represented by the bi-viscous Burgers rheology. Afterslip on the megathrust is modeled by a 2-km thick weak shear zone attached to the fault. Present GPS velocities are not sensitive to the timing of afterslip, but vertical postseismic displacements in the first few decades after the earthquake constrain the shear zone properties. Most afterslip took place in the first 10-15 years after the earthquake, while the viscoelastic relaxation is predicted to last more than a century. We also explore stress perturbations of the 1964 event on the causative faults of recent large earthquakes, such as the 2002 Mw7.8 Denali earthquake and 2018 Mw7.1 Anchorage earthquake. We assume the fault frictional coefficient to be 0.4 to calculate the postseismic Coulomb stresses. The maximum shear stress (MSS) at 10 km depths on land decreases with the time and distance to the rupture area. For example, the present MSS rate is ~1.5 kPa/yr at the Denali fault and ~0.8 kPa/yr at the Kaltag fault (500 km away from the rupture zone).