A Classification of Palermo Historical Centre Heritage based on the local seismic response

A very high density of monumental heritage falls within Palermo Historical Centre, an area characterized by high variability of near surface geology and soil physical-mechanical properties on a very short distance scale, even few meters. So, neighboring heritage elements can have a totally different hazard exposition. Within a project regarding Palermo area (Project 7T7, Cluster 29, funded by Ministero dell’Università e della Ricerca, Italy) a very reliable high density stratigraphic-geotechnical dataset has been assembled, by means of a careful data reviewing process. This dataset has been stored in a Geographic Information System (GIS) application, named City-GIS, having all GIS functions and tools specifically dedicated to the assessment of natural hazards in urban areas. City-GIS manages the whole dataset developed for Palermo, including also the Historical Centre heritage database. The available dataset and the utilities of City-GIS have allowed reconstructing the geological structure characterizing Palermo Historical Centre. After the 6th September 2002, Mw 5.9, earthquake, instrumental and macroseismic surveys have been performed in Palermo, by the INGV and Geology and Geodesy Department of Palermo University the first, by the Regional Department of Civil Protection the latter. Instrumental survey - Eight temporary digital seismic stations were installed to record aftershocks on sites sampling the main different stratigraphic-geotechnical models recognized inside Palermo Historical Centre (Giammarinaro et al., 2000). The variability of ground shaking in Palermo has been evaluated by applying different spectral ratio techniques to seismograms of about 30 aftershocks, with magnitude ≥ 2.8. Relevant spectral variations have been estimated among the different stratigraphic-geotechnical models, in frequency bands varying from 1 to 3.5 Hz. Macroseismic survey – A high-density damage census has been performed, using the “AeDES” census damage card, which codifies the damage suffered by structural elements in few features (damage class, typology and area of the damaged structural element). The “AeDES” database has been stored in the City-GIS archive and easily related to the whole dataset concerning Palermo area. Sectors of Palermo Historical Centre, described by stratigraphic-geotechnical models having a different value of VS30 quantity, are also characterized by different values of the damage indicators used in the “AeDES” system. The obtained values of the damage indicators, used in the “AeDES” system, show a remarkable correlation with the local stratigraphic-geotechnical model, according to the instrumental data. The performed reconstruction of the near-surface geology allowed defining for each heritage element the corresponding stratigraphic-geotechnical model, obtaining a classification of Palermo Historical Centre heritage based on the vulnerability induced by this one.