Importance of post-seismic viscous relaxation in southern Iceland

Large earthquakes change the distribution of stress in the crust, leading to aftershocks and triggered earthquakes. Several time-dependent processes, such as the flow of pore fluid, post-seismic slip (also termed afterslip) and viscous relaxation of the lower crust and upper mantle may further alter the state of stress in the crust and thereby influence the occurrence of future earthquakes. However, distinguishing between these processes on the basis of field observations has prove to be difficult because models of the different processes can predict similar ground displacements and multiple processes may be acting concurrently. Here, I compare results of time-series analysis of satellite radar interferograms and modelling calculations to show that multi-year ground movements following two magnitude 6.5 earthquakes in southwest Iceland were most likely driven by viscous relaxation. Viscoelastic models of a strong lower crust and a weak upper mantle can explain the magnitude as well as the pattern of the deformation, whereas afterslip models are by themselves not compatible with the observations. These results suggest that afterslip which is an important process on mature faultsmay not play as significant a role in young and immature fault zones.