Site effects variability in Palermo induced by local geological variability
by
Maria Stella Giammarinaro
Department of Geology - University of Palermo
Coauthors: Mario Leta (Department of Geology and Geodesy - University of Palermo), Paola Vallone (Department of Geology and Geodesy- University of Palermo), Raimondo Catalano (Department of Geology and Geodesy- University of Palermo

A large number of data concerning the geological and geotechnical properties of subsoil is available inside urban areas. After a careful processing, reliable dataset can be elaborated from these data, in particular by encoding, digitising and implementing them in a GIS framework. In fact the possibility to display data in the mutual spatial relationships makes easier data consistency control and allows to work out reliable dataset, basic starting point for a realistic 3D model of the subsoil geology and physical- mechanical properties. The elaboration of a detailed 3D geological and geotechnical model of city subsoil is essential for natural hazard assessments, for urban planning, to set going an utilization of underground space. The underground urbanization may provide sustainable solutions for the development of a city.

Although Palermo is characterized by a quite complex geology, a detailed reconstruction of the geometry of the surface geology (within 40 meters in depth) is well constrained by a reliable and detailed stratigraphical and geotechnical dataset elaborated at the Department of Geology and Geodesy of Palermo University. This dataset, containing 2600 geo-referred boreholes, has been stored in a GIS application, called CityGIS.

Palermo is characterized by Pleistocenic-Holocenic deposits overlying the Numidian Flysch (Oligo-Miocene age). In the Quaternary deposits two main lithofacies are recognised: silty-clayey sands, Late Emilian-Sicilian in age, belonging to the “Argille Azzurre” Unit and Calcarenitic deposits, Pleistocenic in age, consisting in two facies, different for geomechanical behaviour: white calcarenites more cohesive than yellow calcarenites. The stratigraphical dataset, managed in CityGIS, allowed to draw the separation line between the areas respectively interested by white and yellow calcarenites.

The several boreholes intercepting the Numidian Flysch allowed to reconstruct the trend of its top depth. In Palermo urban area the Numidian Flysch top depth shows a very large (0-100m) variability. Consequently the Pleistocenic deposits thickness can change from few to several tens of meters. Two typical geologic successions can be distinguished in the area. Calcarenitic deposits directly overlay the Numidian Flysch where it is shallow. On the contrary a succession of silty-clayey sands, passing upward to calcarenites, occurs where the Oligo-Miocene substratum is deeper. The areas respectively interested by lacking and occurrence of silty-clayey sands have been also singled out using CityGIS.

Therefore many wide areas (some tens of km2) in Palermo are characterized by different stratigraphical models that may give rise to a diversified seismic response.

Moreover CityGIS has made easier to reconstruct the Palermo’s hydrographic system, most of which is presently covered and to define, with high resolution, the zones interested by alluvial deposits, characterized by a limited extension.

So the variability of the stratigraphical model in Palermo may induce variations in the seismic response characterized by different amplitude and length scales (from a few hundred m to a few tens of km).

These reasons, together with the availability of reliable and detailed geological and geotechnical dataset stored and managed in a GIS framework, as well as damage data of historical and recent earthquakes, make Palermo an optimal site test for carrying out analyses aimed to define the relationships linking the parameters defining the local stratigraphical model (lithotypes, thickness, mechanical properties) with those characterizing the local seismic response (amplitude and peak frequency).

On account of it Palermo has been selected as test site in SESAME ( Seismic Effects assessment using Ambient Excitations, funded by the European Union) project.

Date received: September 8, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caon-36.