Effects of the 6th September 2002 earthquake: damage amplification in the south-eastern sector of Palermo explained by GIS technology

During the 6th September 2002 earthquake the highest damage level in Palermo was observed in the SE sector. This is a recent urbanization area where reinforced concrete structures predominate. A detailed analysis of soil properties in Palermo was carried out by City-GIS to investigate a possible role of nearsurface geology on earthquake effects. City-GIS is a tool dedicated to natural hazard evaluation in urban areas. The availability of high density of well log data (stratigraphic and geotechnical) allowed a realistic modeling of surface geology and physical-mechanical properties that control the seismic response. In wide zones of the above mentioned sector of Palermo, outcropping terrains are composed of thin calcarenite layers, lying above remarkably thick siltyclayey sands that overlay the Numidian Flysch, commonly considered the bedrock of Quaternary sediments. Since silty-clayey sands feature greater deformability properties (Young's modulus) and smaller resistance properties (undrained cohesion and shear resistance angle) than Numidian Flysch, these zones of the SE sector exhibit high values of the acoustic impedance contrast. Moreover, a quite wide portion of the study area, crossed by the Oreto River, is characterized by very thick alluvial deposits. Here, the significant lateral variations of the lithostratigraphic geometry may be an additional cause of strong site effects.