## 1 Introduction
Modal parameters are identified using the frequency response function of the signal [10; 11]. Unlike input-output techniques, output-only techniques rely on the excitation that can be considered white noise (often unknown and treated probabilistically [12]). Known as Operational Modal Analysis (OMA), this approach is particularly applicable when dynamically exciting massive structures (e.g., bridges and buildings) is challenging and costly, as well as in many typical civil engineering cases [12; 13; 14]. Both techniques lead to the determination of the structural response, with the primary difference being that OMA cannot identify modal participation factors due to unknown excitation input. Additionally, differences exist in instrumentation requirements; the output-only technique requires highly sensitive equipment due to reduced excitation levels and particular attention to data processing, given that only output data are available. However, the advantages of this technique are numerous, such as the speed of execution and low costs. Allows acquisitions under conditions similar to those in actual operation, as it can be performed without compromising the structure's intended use. This technique can identify closely spaced modes and can be applied for structural monitoring and damage localization. Methods using ambient noise vibrations are widely used for the definition of dynamic properties of heritage buildings such as old towers and churches, making it possible to obtain information on the resonance frequency, damping, and propensity of the structure to undergo torsional effects [15; 16; 17; 18; 19; 20; 21]. In order to address the need for efficient and cost-effective monitoring, this study demonstrates that a simplified experimental dynamic characterization approach, implemented through passive single-station seismic surveys, can effectively capture the key modal parameters of a structure. Specifically, we apply this methodology to the Messina Bell Tower, an important UNESCO heritage site located in Messina, in the northeastern part of Sicily (Figure 1a). It hosts one of the largest astronomical clocks in the world (Figure 1b). It was first built in the XII century, but catastrophic seismic events such as the \"Val di Noto\" earthquake in 1693 and the \"Messina earthquake\" in 1908 seriously damaged it. The current bell tower dates back to 1933; the structure is characterized in the highest part by a quadrangular spire on either side of which are four lower spires. It is 65 m high and is divided into eight floors plus the ground floor characterized by a square base. The bell tower hosts the astronomic clock, which evokes, with states and mechanical combinations, the most interesting episodes of Messina's religion, astronomy, and history (Figure 1b). The subsoil beneath the bell tower is characterized by sedimentary deposits with varying grain sizes, including silt, clay, sand, gravel, and conglomerates (Figure 1c). These sediments date from the Upper Miocene, characterized by evaporitic series, to the Upper Holocene and present day, featuring recent alluvial and coastal plain deposits of considerable thickness [22; 23]. Figure 1: (**a**) Location of the Messina Bell Tower (the insert in the bottom left-hand corner shows the location of the Messina city); (**b**) states and details of astronomical clock; (**c**) geological sketch map of Messina area (modified from [23]). Research has demonstrated that seismic wave amplification is particularly emphasized in areas with thick alluvial or coastal plain deposits, posing heightened risks to structures located in these regions [24]. The properties of underlying materials significantly influence the seismic response of structures built on sedimentary deposits. Variations in grain size, stiffness, density, and layering can amplify or attenuate seismic waves, resulting in localized ground motion effects. Soft or loose deposits, such as clay or silt, often amplify seismic waves more due to their lower stiffness and higher deformability compared to bedrock.