Resonance frequency characteristics of buildings in Malta and Gozo using ambient vibrations

Abstract

The Maltese islands, lying in the Sicily Channel between Sicily and Tunisia, cover a total area of around 317 km2 and have a present population of around 452,000, which inflates in the summer season by the arrival of over 1 million tourists. The population density of 1,346 km-2 is one of the highest in Europe. A large proportion of residential buildings in the Maltese islands are traditionally constructed of loadbearing unreinforced masonry blocks, utilising local Globigerina limestone blocks, having roofs and diaphragms of reinforced concrete. Older buildings having stone slab roofs are still found in town and village cores, sometimes more than 5 floors in height. Other building stock, in particular taller buildings, are a mix of reinforced concrete frames and masonry construction. The last earthquake to produce some building damage on the islands occurred almost 100 years ago in 1923 (Galea, 2007), while in historical times, the earthquake known to have produced the maximum local intensity of VII – VIII (EMS98) was in 1693. This event was a magnitude 7.4 earthquake along the east coast of Sicily that caused more than 50,000 casualties on that island. Over the past 50 years, construction in the Maltese islands has mushroomed, with most towns and villages growing steadily outwards along their peripheries. The Northern and Southern Harbour districts, around the Valletta Grand Harbour, have developed into one large conurbation housing almost half the whole population of the islands. The western half of Malta and the smaller island of Gozo are more rural and still relatively sparsely populated, although pockets of urbanisation, especially in coastal areas, are growing rapidly to cater for tourism. The exposure of the islands to earthquake risk has thus increased significantly, while the public perception of earthquake hazard is still low. The islands are composed entirely of an Oligo-Miocene layer-cake sequence of marine carbonates and clays, heavily disrupted by mainly normal faulting, with a patchy cover of Quaternary deposits and soils of terrestrial origin. The main formations in order of deposition are the Lower Coralline Limestone (LCL), the Globigerina Limestone (GL), the Blue Clay (BC) and the Upper Coralline Limestone (UCL). In the eastern half of Malta, which is the more populated and industrialised, the two youngest layers of the sequence are absent, and the GL outcrops over most of the area. In the western half of Malta, and on most of Gozo, however, the full sequence is present, so that a number of urban centres are constructed on the UCL which is underlain by the soft, and easily weathered BC layer, which can reach thicknesses of up to more than 50 m. The behaviour of local constructions under dynamic earthquake loading is a problem that has begun to draw attention in recent years. Moreover the effect of the thick buried layer of clay on the dynamic seismic response is also being investigated. In this paper, we report on preliminary measurements of ambient vibrations in a set of buildings to yield information on fundamental resonance frequencies and period-height relationships. Although it is expected that building resonance frequencies would depend on a number of factors, such as construction typology, connectedness with other buildings, plan shape, etc. this study does not attempt to investigate such factors, but only the effect of building height, and the transfer of the site response to the building in the case of underlying clay. Nonetheless, most buildings studied are either of unreinforced masonry, or mixed masonry/reinforced concrete structures.