Ground motion phenomena in Caltanissetta (Italy) investigated by InSAR and geological data integration

Urban areas are frequently affected by ground instabilities of various origins. The location of urban zones affected by ground instability phenomena is crucially important for hazard mitigation policies. Satellite-based Interferometric Synthetic Aperture Radar (InSAR) has demonstrated its remarkable capability to detect and quantify ground and building motion in urban areas, especially since the development of Advanced Differential Interferometric SAR techniques (A-DInSAR). In fact, the high density of reflectors like buildings and infrastructures in urban areas improves the quality of the InSAR signal, allowing sub-centimetric displacements to be reliably detected. The A-DInSAR techniques allow urban zones affected by ground deformation to be located and mapped, but clearly they are not able to point out the causes of the instability phenomena. These can only be highlighted by an integrated analysis of multidisciplinary data, like geological, geotechnical, SAR interferometric and historical data. The overlay of these data, which is possible within a Geographic Information System (GIS), is a useful tool to identify ground motion phenomena affecting urban zones. In this study we apply this kind of approach to Caltanissetta, a provincial capital in Sicily (Italy), where local damage has been detected. The reconstruction of the local near-surface geology shows the presence of zones affected by local natural hazard factors, essentially due to the local presence of soils with poor mechanical properties or swelling soils, high topographic gradients and steep slopes on loose soils. Processing 17 ASAR-ENVISAT SAR images covering the time interval October 2002–December 2005 by means of an A-DInSAR procedure, the Caltanissetta deformation map has been realized. It shows that most of the city is stable, with the exception of three zones, situated in the northwestern, northeastern and southern parts of the city, respectively. Two of them, characterized by high topographic gradients and steep slopes on sandy soils, are affected by subsidence ground motion. An uplift motion is recognized in the other zone, characterized by the local presence of expansible clays. Geotechnical swelling tests carried out on them have shown a swelling behavior. On site surveys have highlighted the presence of damage in the zones affected by ground motion.