Use of InSAR-derived ground displacements to characterize coastal instability: the Ciro' Marina case (Italy)

Here we present the results of a geological investigation carried out following a sudden ground settlement occurred along a 2 km long linear discontinuity on July 28th 2004 in the Ciro' Marina village (Southern Italy). The destructive movement which caused severe damage to the built-up area, was initially ascribed by the civil protection authorities to different possible phenomena, including land subsidence induced by fluid extraction (water and/or gas), a large landslide, partially submarine and an aseismic creep along a tectonic structure. An InSAR analysis aimed at identifying the spatial distribution of the movements connected to the 2004 event and their temporal evolution, i.e. the presence of precursory movements before the event and residual displacement after the failure, was carried out through the Permanent Scatterers technique. All the available SAR images acquired by ERS1/ERS2 and Radarsat satellites were processed. Such an analysis detected the presence since 1992 of slow movements (2-3 mm/y) located along a narrow sector of the village, between the 2004 discontinuity and the shoreline. Temporal series of displacements recorded a sudden acceleration of those movements between the two SAR acquisitions covering the event, reporting average displacements of 7- 10 mm in 24 days, not preceded by any precursory sign. Furthermore, after the failure PS measured residual displacements with velocity values higher than the pre-event ones. The availability of both ascending and descending SAR datasets allowed, through the combination of the two l.o.s. geometries, the retrieval of the vertical and horizontal components of the displacement vectors over the whole unstable area. Vertical movements were identified along the linear discontinuity, whilst moving towards the coastline vectors with a strong horizontal component were retrieved. Then, an inversion of the superficial displacement vectors obtained from the combination of ascending and descending PS was used to estimate the movement geometry (depth and shape of the sliding surface). A graphical method developed for reconstructing the slip surface of landslides was applied along several cross sections by using the displacement vectors and the location of the main scarp provided by the InSAR analysis. Such a method, based on the assumption of a rigid deformation between each available measurement, allowed the estimation of a three-dimensional slip surface. Currently, the combination of the results obtained from the InSAR analysis with conventional methods, such as geotechnical and geophysical data, and more recent technologies, such as bathymetric surveys carried out with multi-beam techniques, is helping the civil protection authorities to understand the origin of the movement and consequently to properly define the connected risk scenarios.