Relative Importance of Poroelastic Effects and Viscoelastic Relaxation for Co- and Postseismic Coulomb Stress Changes on Normal and Thrust faults: Insights from 3D Finite-Element Modeling

Earthquakes on faults in the brittle upper crust cause sudden changes in pore fluid pressure as well as postseismic viscoelastic flow in the lower crust. Such transient processes change the velocity field in the crust and cause significant Coulomb stress changes on receiver faults in the vicinity of the source fault, which may trigger or delay next earthquakes. As previous studies focused on natural earthquakes and/or considered poroelastic and viscoelastic processes separately, the combined influence of poroelastic effects and viscoelastic relaxation on postseismic velocity and stress fields has not been systematically studied so far. In a previous study with 2D finite-element models, we showed that postseismic velocity fields contain signals from overlapping poroelastic and viscoelastic effects (Peikert et al., Tectonophysics, 2022). Here, we use 3D finite-element models with arrays of normal and thrust faults, respectively, to analyze the Coulomb stress changes resulting from the interaction between poroelastic effects and viscoelastic relaxation. In different experiments, we vary the permeability of the crust and the viscosity of the lower crust or lithospheric mantle, while keeping the other parameters constant. We also performed experiments with and without pore fluid flow and viscoelastic relaxation, to isolate the effects of fluid flow and viscoelastic relaxation from each other. Our results show that the coseismic (= static) Coulomb stress changes are immediately altered by the signal from poroelastic effects during the first month after the earthquake. In the first postseismic year, Coulomb stress changes arising from poroelastic effects are one order of magnitude stronger than Coulomb stress changes arising from viscoelastic relaxation. In models considering fluid flow, poroelastic effects dominate the stress field in the first two years. Viscoelastic relaxation already occurs in the early postseismic phase, but is overlapped by the strong signal from poroelastic effects and dominates the Coulomb stress change pattern from about the fifth postseismic year onward for several decades.  The Coulomb stress change patterns show a combined signal from poroelastic and viscoelastic effects already during the first postseismic year, if the viscosity is sufficiently low. For sufficiently low permeabilities, Coulomb stress changes induced by poroelastic effects overlap with the signals from viscoelastic relaxation and interseismic stress accumulation for decades. Finally, poroelastic and viscoelastic effects have a strong impact on the magnitudes and patterns of Coulomb stress changes and should therefore be considered together when analyzing Coulomb stress transfer between faults.