Groundwater changes affect crustal deformation, elastic properties and seismicity rates in the Southern Alps (Italy) (Invited)
Résumé
We study the effects of an hydrologically-induced deformation signal in the Southern Alps (Northern Italy) integrating geodetic, seismological and hydrological observations. The study region is part of the Adria-Eurasia boundary, where N-S shortening is accommodated across a S-verging fold-and-thrust belt. GNSS time-series show the occurrence of non-seasonal horizontal transient displacements, characterized by a sequence of extensional and contractional deformation episodes oriented along the direction of the tectonic shortening. The temporal evolution of this signal well correlates with changes in the total water storage content estimated using a lumped hydrological model based on precipitation, temperature, potential evapotranspiration and river flow measurements. We identify, through a 2D mechanical model, the damage zone of a back-thrust of an active south-verging thrust fault as the geological structure responsible for the measured transient displacements. We model this structure as a sub-vertical, deeply rooted hydrologically-active fracture, which focus groundwater fluxes generating ground displacements and pressure changes. Accordingly, seismic velocity changes computed from the analysis of ambient seismic noise cross-correlation show a temporal (anti) correlation with the evolution of water storage changes, suggesting that fluid increase in the aquifer perturb the Earth crust at depth by decreasing the seismic velocity (and vice-versa, during water storage decrease phases). Finally, by analyzing the seismicity recorded between 2012 and 2017 by a local, high-resolution, seismic network using a covariate model, we found that seismicity rates from a cluster of background seismicity correlate with changes in water storage. Although a spatial correlation between these seismic events and Coulomb stress changes associated with transient deformation episodes is not clear, it is worth noting that our model suggests stress perturbations of the order of 5-10 kPa down to 5-10 km of depth.