GNGTS 2013 - Atti del 32° Convegno Nazionale

Fundamental period of buildings, seismic site response and implications on earthquake seismic action definition in the Siracusa area, Italy F. Panzera 1,3 , S. D’Amico 2 , G. Lombardo 3 , E. Longo 3 1 Istituto Nazionale di Geofisica e Vulcanologia - Osservatorio Etneo, Catania, Italy 2 Physics Department - University of Malta, Msida, Malta 3 Dipartimento di Scienze Biologiche, Geologiche e Ambientali - Università di Catania, Italy Introduction. The town of Siracusa is located in the Hyblean plateau, which represents the outcropping foreland forming the south eastern portion of Sicily. The area is affected by a moderate seismicity, however, the potentially most hazardous event, witnessed by the seismic history of the area, is represented by an earthquake that occurred in 1693 that reached a M W of 7.4 (Rovida et al. , 2011). The seismic source for this event is particularly doubtful and debated in literature. Some authors locate it inland (e.g. Tinti and Armigliato, 2003; Basili et al. , 2008) whereas others locate it offshore (e.g. Argnani and Bonazzi, 2005; Gutscher et al. , 2006). Such uncertainty appears linked to the lack of clear evidence of surface faulting and to the sparsely located epicentres of recoded seismic activity in the last twenty years. The most important moderate size instrumental seismic event occurred in south eastern Sicily on December 13 th 1990. This earthquake and its aftershocks were located along a 5 km long transverse segment striking EW separating two sub-parallel segments of the Ibleo-Maltese fault scarp (Laurenzano and Priolo, 2005). Although the magnitude of this event is moderate (M W =5.68; Rovida et al. , 2011), it caused the collapse of a few buildings. It was felt throughout Sicily with a maximum seismic intensity of VII-VIII (Locati et al. , 2011). It is well known that the grade of building damage and its distribution during an earthquake is due to the combined effect of local site response, based on subsurface ground conditions, and the dynamic features of the structures. The evaluation of the expected ground motion caused by an earthquake is an important problem in earthquake seismology. A seismic hazard assessment (SHA) can be performed by using either a deterministic (DSHA) or a probabilistic (PSHA) approach. Recently, the SHA in major urban areas of south eastern Sicily has been performed by several authors (Faccioli and Pessina, 2000; Barbano et al. , 2001; Panzera et al. , 2011a, 2011b) who used either the DSHA or the PSHA approaches, taking into account different source areas of the major historical earthquakes that struck the area. As previously stated, the seismic action on buildings is strongly dependent on their dynamic characteristics and on the fundamental vibration period, in particular. The engineering practice usually derives the dynamic behaviour of buildings through numerical or experimental methods. The results obtained for different typologies of buildings are often processed, through statistical regression analysis, to achieve empirical relationships that let the estimate of building resonant period as a function of either the height or the number of floors (Goel and Chopra, 1997; Hong and Hwang, 2000; Messele and Tadese, 2002; Crowley and Pinho, 2006; Panzera et al. , 2013). In the present study the dynamic response of buildings is estimated through experimental measurements based on microtremor recordings. Moreover, a deterministic study aiming to estimate the seismic ground motion expected in the Siracusa urban area, is performed. The 1990 earthquake source (Fig. 1a) is used since it is quite close to the study area (about 20 km north) and represent the only seismic source for which both macroseismic and instrumental information are available, so that they can be used for validation purposes. The intent of the present research therefore is to highlight the areas in which major seismic site effects can occur as a function of the outcropping lithology. In this way it would be possible to point out amplification effects occurring at frequencies comparable with those observed for manmade structures, which may cause a pronounced increase of damage. Moreover this kind of study may be useful to governmental agencies tasked with emergency response and rescue. 103 GNGTS 2013 S essione 2.1