Abstract

A detailed knowledge of the local geological-geotechnical setting is a basic starting point to establish the hazard exposure of an area (in particular, urban areas), characterized by high geological variability (Giammarinaro et alii, 2000). In fact the local stratigraphical geometries and the trend of the most relevant quantities, controlling soil strength and deformability, affect the trend of a number of meaningful hazard factors. To build a realistic geological-geotechnical model with a high spatial resolution, it is necessary to use reliable datasets, possibly managed in a GIS environment, able both to display data in their mutual spatial relationships and to check more easily the consistency of the data. In this work a detailed analysis of soil properties in the southern sector of Palermo has been carried out, with the help of City-GIS (Giammarinaro & Maiorana, 2001b; Giammarinaro & Maiorana, 2001c), a GIS application developed at the Department of Geology and Geodesy of Palermo University, dedicated to natural hazard evaluation in urban areas. City-GIS is equipped with tools and research keys to easily carry out specific analysis aimed at the process of natural hazard assessment. The great quantity of data in this sector and their management through City-GIS have made it possible to outline, in detail and with a high resolution: a) the geometry and the thickness of Quaternary deposits, overlying the bedrock; b) the geology of the first 50 metres of subsoil, that mainly control hazard processes such as site effects. Among the reasons that led us to study this sector of the city, there are the very high damage level and human response recorded in this area during the September 2002 earthquake (Giammarinaro et alii, 2003a, Giammarinaro et alii, 2005). The Palermo southern sector is a recent urbanization area where reinforced concrete structures predominate. This area is characterized by a great vulnerability because of the presence of strategic structures of the city such as a regional hospital, industries, some monuments, a meaningful service network and relevant transport structures such as railways and the motorways. The study has been carried out along different lines: a) ground survey, aimed at identifying the vulnerable elements in the area; b) geological survey aimed at producing a surface geological map; c) modelling of geometry and thickness of the first 50 metres of subsoil; d) geotechnical characterization of the lithotypes of the recognized successions. For each lithotype the variability fields of thickness and S-wave velocity have also been evaluated also from the relationships linking the geotechnical parameters obtained from direct measurements on soil samples. The analysis we carried out allows us to state that the study area (excluding that affected by the Oreto river alluvial deposits) can be described with a single geological succession model. However the lithotypes of the recognized succession exhibit great variability in thickness in the area. In detail: in the north-eastern sector of the investigated area, small thickness (<15 m) of calcarenitic deposits overlie major thicknesses (up to 80 m) of silty clayey sands, while in the south-western sector calcarenitic deposits are at least 30 meters thick. The locally expected values of the acoustic impedance contrast and of the V <sub>s30</sub> (S wave velocity in the first 30 meters) have been evaluated and two portions of the area, potentially affected by site effects have been recognized. In one of them, site effects are to be ascribed to acoustic impedance contrast, while in the other one, to a low value (<180 m/s) of V <sub>s30</sub> . The process of recognition of these two areas has been made possible by an extensive use of the reliable stratigraphical-geotechnical dataset built up for Palermo urban area and by the tools equipped by City-GIS.

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