Francesco Pintori, Enrico Serpelloni, Laurent Longuevergne, Alexander Garcia-Aristizabal, Maria Elina Belardinelli, Licia Faenza , Lucio D’Alberto, Adriano Gualandi

Changes in continental water storage generate vertical surface deformation, induce crustal stress perturbations and modulate seismicity rates. However, the degree to which regional changes in terrestrial water content influence crustal stresses and the occurrence of earthquakes remains an open problem. We show how changes in groundwater storage, computed for a ~1000 km^2 basin, focus deformation in a narrow zone, causing large horizontal, non-seasonal displacements. We present results from a karstic mountain range located at the edge of the Adria-Eurasia plate boundary system in northern Italy, where shortening is accommodated across an active fold-and-thrust belt. The presence of geological structures with high permeabilities and of deeply rooted hydrologically-active fractures focus groundwater fluxes and pressure changes, generating transient surface horizontal displacements up to 5 mm and perturbations of crustal stress up to 25 kPa at seismogenic depths. The background seismicity rates appear correlated, without evident temporal delay, with groundwater storage changes in the hydrological basin. With no evidence of pore pressure propagation from the hydrologically-active fractures, seismicity modulation is likely affected by direct stress changes on faults planes.