Identification of amplified damage zones in Palermo, Sicily (Italy), during the earthquakes of the last three centuries

The damage scenario for the city of Palermo has been reconstructed for the three most damaging earthquakes that have occurred since the eighteenth century (1 September 1726; 5 March 1823; 15 January 1940). Research has been carried out on the original written documents, mostly kept in the archives of administrative and clerical institutions in Sicily. The chance to collate and compare the information coming from the different coeval sources provides an assurance as to the reliability of the collected data and optimizes the information framework. All the damage episodes have been pinpointed in the ancient urban pattern of the city. The historic urban area has been divided up using a grid with square-module geometry, each grid element being 100 x 100 m in size. The resulting damage distribution shows a systematic amplification of effects in some of the grid elements. In order to formulate an interpretation of the damage anomalies in terms of near-surface geology, a geographic information system (GIS) has been used to handle a geological database of about 2000 geo-referenced soil profiles in Palermo. This approach has provided us with a quick 2D and 3D visualization of the upper-layer structure in the study area. As expected, a close correlation was found between the zones of major damage and the thickness of recent deposits characterized by weak cohesiveness and high compressibility: the damage level increases at sites where thickness exceeds a 12-m threshold. The strongest effects occur on water-saturated deposits thicker than 22 m. In terms of the macroseismic effects, a transition from the hardest (Oligo-Miocene Numidian flysch and calcarenites) to the softest ground (recent alluvial deposits and man-made fill) present in the historic center of Palermo implies a difference in Mercalli Cancani Sieberg (MCS) intensity by one to two degrees during the same earthquake. An M 5.6 earthquake occurred off the Palermo coast on 6 September 2002 subsequent to our analysis of the macroseismic data. The instrumental data from this most recent shock provide a posteriori experimental evidence of significant ground-shaking variations between the different geological formations. This validates the macroseismic results and indicates that research and applications based on crossing different datasets can be successfully performed for urban areas using GIS technology.