2D finite-element modelling of the interaction between poroelastic effects and viscoelastic relaxation during the seismic cycle

The analysis of Coulomb stress changes has become an important tool for seismic hazard evaluation because such stress changes may trigger or delay next earthquakes. Processes that can cause significant Coulomb stress changes include coseismic slip, earthquake-induced poroelastic effects and transient postseismic processes such as viscoelastic relaxation. In this study, we use 2D finite-element models for intracontinental normal and thrust faults to investigate the spatial and temporal evolution and the interaction of pore fluid pressure changes and postseismic viscoelastic relaxation. In different experiments, we vary (1) the permeability of the upper or lower crust and (2) the viscosity of the lower crust or lithospheric mantle, while keeping the other parameters constant. The results show that the highest pore pressure changes occur within a distance of ~ 1 km around the lower fault tip. In the postseismic phase, the pore pressure relaxes depending on the permeability in the upper crust until the pore pressure reaches the initial pressure of the preseismic phase. For high permeabilities in the upper crust, postseismic velocities within a few kilometers around the fault reach around 120 mm/a and decrease rapidly with time, whereas for low permeabilities velocities remain lower over the years after the earthquake. Models with low viscosity of the lower crust show that postseismic viscoelastic relaxation and poroelastic effects overlap in the early postseismic phase and decrease gradually within a few years after the earthquake. Higher viscosities lead to lower velocities, that last for decades on scales of several tens of kilometers.