Land subsidence and Late Glacial environmental evolution of the Comourban area (Northern Italy)

摘要

The Late Glacial and Holocene geological and environmental evolution of the Como lacustrine-palustrine basin has been broadly reconstructed based on (a) the collection and analysis of more than 100 borehole stratigraphies, (b) the drilling of three new shallow boreholes and the analysis of a new geological section and (c) several in situ and laboratory analyses (mineralogical, organic and pollen content,~(14) C dating). The long-term, natural tendency of the ground surface to sink where the Como urban area has developed has been verified and assessed. This tendency has resulted clearly controlled by the compaction of young unconsolidated sediments accumulated at the hydrologically closed SW tip of Lake Como. The definition of a minimum age for the deglaciation of the basin and for a correlated stratigraphic marker allowed broad quantification of the average velocity of land subsidence during the last ca. 17 000calBP, with a gradient of values close to 2.5 mm/yr along the axial zone of the basin toward the present-day lake. Burial depth of archaeological remains, related to Roman Age, and levelling monitoring since 1928 are in agreement with this estimate. During 1950-1975, because of deep water withdrawal, the Como area was affected by a human-induced, accelerated subsidence, with velocity (10-20 mm/yr) one order of magnitude higher than the natural one. After a period of deceleration (between the end of the 1970s and the end of the 1990s), during which some benchmarks even reversed their trend, persistent scatterer interferometry (PSI) data provided by Tele-Rilevamento Europa (TRE) for the period 1992-2003, and levelling data obtained by APAT in 2004, show that during the last decade the subsidence rate has returned to its physiologic long-term trend.