Variabilità spaziale della subsidenza attuale nell'area veneziana

A recent research has provided a high resolution map of the present land subsidence of the Venice coastland (Italy). The map, which refers to the 1992-2002 decade and covers an area of about 100x100 km2, has been obtained by the innovative "Subsidence Integrated Monitoring System" (SIMS) that efficiently merges the different displacement measurements obtained by spirit leveling, differential and continuous Global Positing System (GPS), Synthetic Aperture Radar (SAR)-based interferometry. The investigation points out a significant spatial variability of the ground vertical movements, both at regional and local scales, and displacement rates ranging from a slight (1-2 mm/yr) uplift to a serious subsidence of more than 10 mm/yr. Tectonics, differential consolidation of the Pleistocene and Holocene deposits, and human activities, such as groundwater withdrawals, land reclamation of marshes and swamp areas, and farmland conversion into urban areas, superimpose to produce the observed displacements. In this work we have distinguished the displacement components on the basis of the depth of their occurrence. Deep causes, acting at a depth generally greater than 400-600 m below m.s.l., refer to downward movements of the pre-Quaternary basement and land uplift (up to 2 mm/yr) most likely related to neo-tectonic activity connected with the Alpine thrust belts and a NW-SE fault system. The displacement factors located in the medium depth interval, i.e. between 400 and 50 m below m.s.l., are of both natural and anthropogenic origin. The former refers to the Medium-Late Pleistocene deposits that exhibit a larger cumulative thickness of clayey compressible layers at the lagoon extremities with respect to the central lagoon area where stiffer sandy formations prevail. Land subsidence due to aquifer exploitation mainly occurs in the north-eastern sector of the coastland where thousands of active wells are located. In a 10-15 km wide coastal strip thickness, texture, and sedimentation environment of the Holocene deposits play a significant role in controlling shallow land subsidence. Other factors that contribute in increasing land sinking at a smaller areal extent are the salinization of clay deposits due to saltwater intrusion and biochemical oxidation of outcropping peat soils enhanced by. Even the load of houses and structures after the conversion of farmland into urbanized areas cause local shallow compaction.