Horizontal surface velocity and strain patterns near thrust and normal faults during the earthquake cycle: The importance of viscoelastic relaxation in the lower crust and implications for interpreting geodetic data

Abstract

In recent years, more and more space-geodetic data on the surface deformation associated with earthquakes on intracontinental normal and thrust faults have become available. However, numerical models investigating the coseismic and postseismic deformation near such faults in a general way, i.e., not focused on a particular earthquake, are still sparse. Here we use three-dimensional finite element models that account for gravity, far-field (“regional”) extension/shortening and postseismic relaxation in a viscoelastic lower crust to quantify the surface deformation caused by an Mw ~7 earthquake on a dip-slip fault. The coseismic deformation is characterized by horizontal shortening in the footwall of the normal fault and extension in the hanging wall of the thrust fault—consistent with elastic dislocation models, geological field observations, and GPS data from earthquakes in Italy and Taiwan. During the postseismic phase, domains of extensional and contractional strain exist next to each other near both fault types. The spatiotemporal evolution of these domains as well as the postseismic velocities and strain rates strongly depend on the viscosity of the lower crust. For viscosities of 1018–1020 Pa s, the signal from postseismic relaxation is detectible for 20–50 years after the earthquake. If GPS data containing a postseismic relaxation signal are used to derive regional rates, the stations may show rates that are too high or too low or even an apparently wrong tectonic regime. By quantifying the postseismic deformation through space and time, our models help to interpret GPS data and to identify the most suitable locations for GPS stations.