GNGTS 2014 - Atti del 33° Convegno Nazionale

caused. Different buildings respond differently to the same ground motion and will resonate at different frequencies. The building’s vibrations tend to centre around one specific frequency, namely the building’s natural or fundamental frequency, which in general decreases with height (Nelson, 1996). Seismic building codes are designed to protect people inside buildings by preventing collapse and enable secure evacuation. A structure should withstand minor earthquakes undamaged, withstand moderate earthquakes without large structural damage and withstand major earthquakes without collapse. After any seismic action, the structure needs to be ensured to retain a residual strength and stiffness against horizontal ground motion and retain its entire load bearing capacity against vertical loads.However, even if all new buildings are built properly under more advanced codes, older buildings are still very vulnerable to earthquakes and therefore remain hazardous (MCEER, 2007). Most of the residential buildings around Malta consist of load-bearing unreinforced masonry walls and reinforced concrete roofs (Borg et al., 2008). It is known that this type of construction is very vulnerable under even moderate levels of ground shaking. This is a first study aimed at investigating the fundamental dynamic behavior of Maltese buildings utilizing the information contained in ambient noise vibrations. In particular it was intended to provide a preliminary idea of which kind of period-height relationship applies to local buildings, compared to similar relationships elsewhere. It was decided to limit this initial study to buildings constructed on bedrock, and, where possible, to isolated buildings. The Fig. 1 – a) The geological stratification of the Maltese Islands including the members, sublayers, geological era, texture and outcrop occurrence of all the layers. The same colour code isfollowed in Fig. 1 (b) [modified from The Geological Map of the Maltese Islands (Oil Exploration Directorate, Office of the Prime Minister, Malta, 1993)]; and (b) outcropgeology of Malta and Gozo (Oil Exploration Directorate, Office of the Prime Minister, Malta, 1993) with topography and major fault patterns (Mapping Unit, Malta Environment andPlanning Authority, MEPA, 2010). Location of recording sites are also indicated. 388 GNGTS 2014 S essione 2.3