GNGTS 2014 - Atti del 33° Convegno Nazionale

ambient noise was measured using MicromedTromino® 3-component portable seismographs located at the top and lowest floors of a building, and intermediate levels in some cases. Maltese geology. Malta is located in the southern Mediterranean and is composed mainly of marinesedimentary rocks (Fig. 1). The oldest sedimentary formation of the Maltese Islands, of Triassicage, is not outcropping and the main exposed rocks were deposited from the Oligocene- MiocenetoQuaternary periods. The litho-stratigraphic sequence is relatively simple, consistingof fivemajor layers namely the Lower Coralline Limestone (LCL), theGlobigerina Limestone(GL), the Blue Clay (BC), the Greensand Formation and the Upper Coralline Limestone (UCL). UCL, GL and LCL strata are essentially considered as being stiff rocks, whereas the BC and theGreensands are soft sediments (Pedley et al. , 1978, 2002).The LCL is the oldest exposed rock in the Maltese Islands, outcropping to a height of 140 min the vertical cliffs near Xlendi (Gozo). GL is the second oldest rock and outcrops overapproximately 70% of the area of the islands. On weathering and erosion, it assumes a broadgently rolling landscape. The thickness of this formation changes considerably, ranging from 23 m near Fort Chambray (Gozo) to 207 m around Marsaxlokk (Malta). The BC formation overliesthe GL. It erodes easily when wet and it forms taluses which flow out over the underlying rock.Variations in thickness are considerable, ranging from 75 m at Xaghra (Gozo) to nil in easternMalta. Greensand consists of bioclasticlimestones rich in glauconite that were deposited in awarm sea. The deposit attains a maximum thickness of 11 m in localised depressions at Il-GelmusinGozo, but elsewhere is less than 1 m thick. UCL is the youngest Tertiary formation in theislands, reaching a thickness of approximately 160 m in the Bingemma area (Malta). Localtectonic activity appears to have resulted in the brief emergence of the formation above sea level.The UCL is very similar to the lowest stratum (LCL) in the Maltese Islands. These rocks aresporadically overlain by terrestrial, aeolian and alluvial deposits laid down following theemergence of the Maltese Islands above sea level. Much of the central and south-eastern portion of Malta comprises outcrops ofGL while the northern and north-western regions are characterized by highlands on which UCLis the dominant outcrop. The geology of Gozo is more varied than that of Malta, with morefrequent outcrops of BC being a characteristic feature. Methodology and data set. The building response survey was carried out by recording 10 – 30 minutes of ambient vibrations at a sampling frequency of 128 Hz, on 21 different buildings (Fig. 1) around Malta with difference in shape, different construction material (reinforced concrete or masonry) and with height in the range of 1 – 23 floors. Considering the respective interstorey height, the range of building height is between 5 and 98 meters.The instruments were aligned with their North-South direction parallel to the long axis of the building. Studies have shown that the nature and thickness of surface sediments are two important local geological factors that can significantly affect the level of earthquake shaking (Navarro et al ., 2004). Site response studies using ambient noise over the Maltese islands (Vella et al ., 2013) have shown that sites consisting of Lower Coralline and Globigerina Limestone layers, underlying most of the urban and industrial areas, exhibit mostly flat H/V response curves above 0.5 Hz, which is the essential range for engineering purposes in the construction and design of typical local structures. On the other hand, the presence of the Blue Clay layer, whether outcropping at the surface or as a layer lying under the hard Upper Coralline Limestone layer, was found to induce site amplification. For this reason, all the studied buildings were chosen to lie on hard rock in order to avoid possible effect of ground amplification due to the clay layer. This is an interesting and important effect which will be investigated in a separate study. Where possible, buildings located near construction work or industries were avoided. Since this study was aimed at investigating the fundamental period – height relationship, we tried to analyse mostly buildings of regular and isolated configuration. This was not always possible as Maltese building density is high and it is difficult to find completely isolated buildings. It is also known that the period-height relationship may differ for unreinforced masonry buildings and reinforced concrete ones ( e.g. Panzera et al. , 2013). In this first study no GNGTS 2014 S essione 2.3 389