Il terremoto di Palermo del 6 settembre 2002: effetti macrosismici

Abstract

On September 6, 2002, at 01:21 GMT, the western part of the Tyrrhenian coast of Sicily was hit by an earthquake of magnitude ML = 5.6, representing the mainshock of a seismic sequence of roughly 500 earthquakes recorded in about one month. The event, although characterized by a moderate magnitude and located some forty kilometres offshore, produced relevant macroseismic effects in the Palermo area. A macroseismic survey was carried out soon after the earthquake with the aim of defining, for Civil Protection purposes, the damage scenario over a territory which is densely urbanised. In all, 134 localities were investigated. Damage generally depended on the high vulnerability of the buildings both for problems related to the old age - as is the case of the Palermo historical centre - and for peculiar unfavourable conditions due to site effects, particularly in some quarters of the city and in the nearby locality of Ficarazzi. Damage to masonry consisted of falling plaster, failure of corner walls, diagonal cracks in walls and widening of previous fissures. Some degraded buildings, almost abandoned and with evident lack of maintenance, suffered partial structural failure of roofs and floors. As regards reinforced concrete (RC) frame buildings, several edifices with 5-10 storeys suffered cracks in partitions and infill walls, sometimes with failure of these elements, and falling plaster. Fine cracks in beams of frames have also been observed, but without determining significant structural problems. In general, damage affects the lowest storeys of the RC frame buildings. According to the European Macroseismic Scale 1998, the maximum value of intensity, 6, was assigned to Palermo and Ficarazzi. The earthquake has also provoked effects on natural surroundings. Two types of phenomena have been detected: (i) hydrological effects and (ii) slope instability processes. The most relevant was the landslide which occurred near the village of Cerda, 60 km from the epicentre.