## 2 Materials and Methods
### Bell Tower Experimental Dynamic Characterization  
Proper planning of experimental tests is essential for determining data length, sampling frequency, sensor positioning, and the number of instruments required. Ideally, devices (e.g., velocimeters or accelerometers) should be placed at all locations critical for assessing structural behavior [33]. However, in practice, the number of sensors is often constrained by financial resources. Based on these limitations, a minimum number of instruments are deployed to ensure the collection of necessary data. Avoid placing instruments at nodal points, as these can complicate mode interpretation. Additionally, while a specific sensor arrangement may optimize the detection of one mode, it could be less effective for others. A classic example is a supported beam: placing a sensor at midspan highlights the first mode values but renders the second mode values null. To mitigate this, sensors should generally not be placed along axes of symmetry to prevent the suppression of asymmetric modes. Passive single-station seismic surveys were carried out at different levels of the bell tower to carry out its expeditious dynamic characterization. Five portable three-component digital seismometers equipped with three velocimeter sensors were used for the acquisitions. At each measurement point, environmental seismic noise was recorded with a sampling frequency of 256 Hz. All acquisitions were performed on the same day. The structure vibration frequencies were investigated by performing passive seismic single-station acquisitions in two configurations. Initially, non-synchronous measurements were carried out on each floor of the tower, in this phase No. 18 ambient noise recordings were performed; the measurements were carried out by positioning the three-component digital seismographs along two verticals, A and B, as shown in Figure 3a, keeping instrumental north oriented in the same direction for all measuring points. In addition, a measurement was made in a room below the ground floor, indicated in Figure 3a as RIF, with an acquisition time equal to the entire acquisition time of all the measurements made in the structure. This approach enabled the capture of the complete dynamic behavior of the structure, including accurate determination of natural frequencies and higher-order mode shapes, characterized by the presence of multiple nodal points, including at intermediate locations along the height of the structure. The second configuration carried out to obtain information on the vibration modes of the structure provided for the acquisition of the environmental seismic noise at four positions, in a partially overlapping time interval, located at different structure levels but aligned vertically, as shown in the diagram in Figure 4a. Even in this case, the same orientation of instrumental north was maintained for all the measurement stations. Since it was not possible to synchronize all the available devices either digitally or via radio, because not all of them were equipped with such synchronization instruments, the environmental seismic noise was acquired for a longer time, equal to 20 min, including a bell toll in the initial parts of this interval. This strong stress, clearly distinguishable from ambient noise in all recordings, was used for the subsequent traces synchronization, through a Matlab (R2023b) script appositely implemented. Figure 3: (**a**) Location of the passive seismic single-station non-synchronous surveys performed inside the Messina Bell Tower; spectral ratio plots obtained from ambient seismic noise records performed at the A (**b**) and B (**c**) verticals.