Crustal versus asthenospheric relaxation and post-seismic deformation for shallow normal faulting earthquakes:the Umbria-Marche (central Italy) case

Following a normal mode approach for a stratified viscoelastic earth, we investigate the effects induced by shallow normal faulting earthquakes, on surface post-seismic vertical displacement and velocity at the surface, when stress relaxation occurs in the crust or in the asthenosphere. The modelled earthquake is a moderate one characteristic of some slowly deforming plate boundaries in the central Mediterranean region. We focus on the Umbria–Marche (central Italy) region where deep seismic reflection studies (CROP03) and the 1997 earthquake sequence clearly show a seismogenic layer decoupled from the lower crust by a sizeable transition zone. Accordingly, the crust is subdivided into three layers: an elastic upper crust, a transition zone and a low-viscosity lower crust. The fault is embedded in the upper crust. The layered viscoelastic structure of the crust and mantle imposes a pattern and scale on the modelled coseismic and post-seismic deformation with a major contribution from the transition crustal zone and low-viscosity lower crust, stress relaxation in the mantle being negligible. We show that significant vertical deformation rates of the order of 1 mm yr− 1 could be expected for a shallow and moderate event such as the recent Umbria–Marche earthquake for viscosity values of 1019 and 1018 Pa s in the crustal transition zone and lower crust, respectively.