The role of remote sensing to enlarge knowledge on health state of a historical building hit by earthquake: the case of Garisenda leaning tower (Bologna)

An evaluation of the health state of a fragile historical building like a leaning tower, especially if it is located in an environment characterized by significant seismic risk, requires the integration of data obtained through various types of observation techniques. This is the case of the Garisenda Tower in Bologna, which is a 48 m high leaning tower, whose tilt angle reaches 4°, and for which some concerns about its stability recently grown. Historical sources, past and current monitoring of both structure and subsoil with several instruments, geological, geophysical and geotechnical surveying (in particular, continuous borehole coring) and experimental modal analysis provide a large amount of information on the tower and its environment. This study is aimed at pointing out how remote sensing can contribute to the achievement of a reliable picture of the current health state of the tower, considering that this picture could be useful for planning future reinforcement/restoration strategies. In particular, the results of the multitemporal morphological analysis of the tower façades, based on remote sensing data acquired in 2010, during the 2012 Emilia Romagna earthquake and in 2023, are shown and discussed on the basis of some available information. The morphological analysis can provide real and interesting information on geometric deformations, also highlighting their possible partial or total recovery when the stress ceases, in particular when a seismic sequence ends, although an almost total recovery of such deformations does not necessarily imply that the structure is not affected by permanent damage.