GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.4 221 GNSS DATA PROVIDE UNEXPECTED INSIGHTS IN HYDROGEOLOGIC PROCESSES R. Devoti, F. Riguzzi Istituto Nazionale di Geofisica e Vulcanologia, sez. ONT, Roma, Italy The slow tectonic processes are not the only ones responsible for the deformation detected of the Earth’s crust. Since the end of 1800s, a pioneering study investigated the connection between water mass redistribution due to ocean tide loading and land deformation (Darwin, 1882); more recently, Terzaghi introduced the principle on the poroelastic effect of soils which states that all measurable effects of a change of stress, such as compression, distortion and a change of shearing resistance are due exclusively to changes in effective stress. The effective stress σ’ is related to total stress and pore pressure by σ’ = σ - u (Terzaghi, 1943). Therefore, when a rock is subjected to a stress, it is opposed by the fluid pressure of pores in the rock. The Earth’s crust reacts to water circulation basically because of its weight and pressure variations. Water content variations cause time varying loadings on the Earth surface and cause time-varying deformations of the solid Earth. In certain circumstances the Earth surface deforms elastically but in most cases the surface deformation is determined by the poroelastic property of the ground. The first hydrologic-driven crustal deformations observed by space geodesy techniques have been the ocean tide loading (by VLBI, Schuh and Moehlmann, 1989), the poroelastic effect (by GPS and InSAR, Ikehara and Phillips, 1994), the land water mass loading (by GPS, van Dam et al. , 2001) and the non-tidal ocean loading (by GPS and gravity, Zerbini et al. , 2004). The recent development of several permanent GNSS networks has significantly improved the spatial and temporal resolution of deformation monitoring in the Italian area. At present, at least 30 GNSS networks maintained by different institutions constitute the grid of monitoring sites that includes over 1000 permanent stations, mainly devoted to real-time positioning services but that has proven to be suitable also to detect slow deformations (Devoti et al. , 2017). Recent studies have shown that heavy rainfall or water level variations in confined aquifers, are able to induce significant sporadic or long-term periodical deformations that affect the Fig. 1 - Ground deformation is correlated with rainfall excess in the L’Aquila area: long-term maximum elongation of the GPS horizontal displacements (colors in legend) compared with rainfall excess (i.e. with respect to the average rain, blue line). GRACE long-term gravity change (cyan) in Central Apennines is synchronous with GPS displacements.