Long-term Postseismic Deformation and Its Implications for Rheological Properties From the 1976 M7.8 Tangshan Earthquake

It is well known that the viscoelastic relaxation of the upper mantle may last a long time after large earthquakes. About forty years after the 1976 M7.8 Tangshan earthquake, the postseismic surface deformation is still up to ~ 2 mm/yr. In this work, we have developed three-dimensional viscoelastic finite element models to study the rheological properties of the lower crust and upper mantle constrained from the postseismic deformation of the 1976 Tangshan earthquake. In our model, the viscoelastic relaxation is represented by the bi-viscous Burgers rheology. Transient Kelvin viscosity is assumed to be one order of magnitude lower than that of the steady Maxwell viscosity. Following previous studies, we simulate the afterslip of the fault through a 2-km weak shear zone attached to the fault. Afterslip plays an important role in controlling the early postseismic deformation, but the decadal postseismic deformation is mostly controlled by the viscoelastic relaxation of the lower crust and upper mantle. Preliminary model results have determined the viscosities of the lower curst and upper mantle to be at the order of 10¹⁹ Pa s and 8 x 10¹⁹ Pa s, respectively. Test models indicate that earthquake-induced stresses may last more than eighty years until the surface deformation is less than 1 mm/yr, that is, below the resolution of the modern geodetic method. We further study the stress interactions between the Tangshan fault and neighboring active crustal faults.