Earthquakes and ancient leaning towers: Geodetic monitoring of the bell tower of San Benedetto Church in Ferrara (Italy)

In May–June 2012, several seismic events took place in the Po River Plane in northern Italy, with a maximum magnitude of MW 5.86 (ML 5.9)] and the epicentre located about 32 km from the centre of Ferrara. Many historical buildings were seriously damaged and others showed marked deformations or differential settlements. Therefore, it was necessary to place many of them under monitoring, using fast and safe measurement techniques to quickly obtain accurate information on ongoing structural deformation. This paper presents the case study of the bell tower of San Benedetto Church in Ferrara (17th century), which was already under monitoring at the time of the earthquake because of its remarkable leaning angle. Immediately after the seismic events, monitoring of the bell tower was repeated. Digital levelling indicated a marked differential settlement of the foundations with regard to the trend of secular movement, while terrestrial laser scanning (TLS) revealed a significant increase of the inclination of the tower's axis, confirming the results from levelling and showing good integration of both monitoring techniques.