Investigating the dynamic behavior of historical watch towers in Malta and Gozo

Seventeenth century coastal watch towers, which are part of the Maltese cultural heritage, are at risk due to natural processes such as coastal weathering, erosion and land movement. In some cases, this makes them even more vulnerable to earthquake ground shaking and thus subjected to a possible considerable seismic risk. In this study, we present the results related to seismic vulnerability of these significant cultural heritage sites. In order to do so, the dynamic characteristics of these towers as well as soil-structure interaction were investigated. The three chosen towers were Isopu Tower and Dwejra Tower (located in Gozo) and the Ghajn Tuffieha Tower (located in Malta). Measurements of ambient noise were recorded inside each tower at each level. The towers’ fundamental frequencies were determined from the Horizontal-to-Vertical Spectral Ratio and the Horizontal-to-Horizontal Spectral Ratio techniques. In addition to this, “free-field” measurements were also taken around the towers in order to study the ground response and also the soil-structure interaction. Based on a geometric survey, 3D CAD models of each watch tower were built that were subsequently employed to build the finite element models. To this aim the commercial code ANSYS was employed. To properly investigate the effect of seismic loading on these historical towers, synthetic seismograms (for different magnitude and distances) were computed, taking into account also the local site conditions, and applied as input to the base of the towers.