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[1] Deltas are highly dynamic coastal systems that over the last few decades have generally experienced a substantial area loss caused by trapping of river sediments in upland drainage basins as well as land subsidence due to natural and anthropogenic causes. A major example is the Po Delta in the Mediterranean in northeastern Italy. This area has experienced as much as 3 m of land subsidence from the 1930s to the 1970s primarily because of the extraction of gas-bearing waters. However, present subsidence rates are largely unknown and the ground settlement is supposedly controlled by natural long-term deep processes. We have combined radar Interferometric Point Target Analysis (IPTA) with previous geomorphological investigations on aerial/satellite images and seismic surveys, and geochronological data from core samples and geomechanical in situ tests, to assess the current sinking of the delta and to understand the processes controlling the vertical movement. The high density of the measurable point targets (more than 15,000) allows characterization of the spatial variation in the vertical land motions (VLM), ranging from −1 to −15 mm/yr. We find that subsidence rates are significantly correlated with the age of highly compressible Holocene deposits that compose the shallowest 30-40 m of the sedimentary sequence. A typical log-type consolidation equation applicable at the scale of the entire delta has been obtained. We conclude that the consolidation of late Holocene sediments is the major cause of the present land subsidence in the Po River delta. This finding has significant impact on the understanding of many other modern deltas that were formed in the lower Holocene epoch.Citation: Teatini, P., L. Tosi, and T. Strozzi (2011), Quantitative evidence that compaction of Holocene sediments drives the present land subsidence of the Po Delta, Italy,

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We detected land displacements of Venice by Persistent Scatterer Interferometry using ERS and ENVISAT C-band and TerraSAR-X and COSMO-SkyMed X-band acquisitions over the periods 1992–2010 and 2008–2011, respectively. By reason of the larger observation period, the C-band sensors was used to quantify the long-term movements, i.e. the subsidence component primarily ascribed to natural processes. The high resolution X-band satellites reveal a high effectiveness to monitor short-time movements as those induced by human activities. Interpolation of the two datasets and removal of the C-band from the X-band map allows discriminating between the natural and anthropogenic components of the subsidence. A certain variability characterizes the natural subsidence (0.9 ± 0.7 mm/yr), mainly because of the heterogeneous nature and age of the lagoon subsoil. The 2008 displacements show that man interventions are responsible for movements ranging from −10 to 2 mm/yr. These displacements are generally local and distributed along the margins of the city islands.

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Nineteen percent of the global population may face a high probability of subsidence

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