## 1 Introduction  
Structural health monitoring (SHM) is a topic of major interest in different fields of applied engineering. In particular, in civil engineering SHM applications are becoming increasingly important since at global level a larger number of structures are subject to an advanced state of deterioration due to aging of materials or inadequate maintenance. In this regard, continuous monitoring systems can allow quick identification of structural defects and/or damage occurred, in order for effective maintenance programs for restoring optimal conditions and reducing direct and indirect costs to be planned efficiently. Furthermore, the maintenance program can be calibrated based on the real structural conditions, with a consequent reduction of intervention time and, at the same time, an increase in both safety and functionality of the building. Nowadays, the recent developments in dynamic identification techniques allow retrieval of the characteristics of a generic system, and then to suitably calibrate numerical models able to reproduce the observed behavior under certain operating conditions (i.e., weak- and strong-motion conditions). Over the past few decades, the application of such techniques in structural engineering has considerably increased due to the awareness that the dynamic properties of a building can provide a reliable overview of the state of integrity of the monitored structure. Considerable progress has been made in the field of structural monitoring and dynamic identification, using approaches based on the analysis of the variations of wave propagation [1; 2; 3; 4; 5; 6; 7] and on the techniques operating in the time-frequency domain [8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. The latter are particularly effective when the structural dynamic characteristics rapidly change over time. After an event generating strong motion, the rapid and reliable evaluation of damage on strategic structures and infrastructure in the epicentral area is of major importance for Civil Protection authorities. SHM may be used for rapid and first order assessment in order to provide, in near real-time, reliable information about a system's performance during and after critical events (such as earthquakes, explosions, etc.). The possibility to remotely monitor the state of health of an instrumented structure and detect potential damage during a critical event could also be used as an indicator to calibrate a damage scenario, also providing several benefits in reducing the potential loss of life and injuries and improving the effectiveness of a potential emergency response in case of a disaster. It is well known that the presence of damage on any type of structure is able to change the dynamic characteristics of the structural system. Several methods for damage detection are based on the evaluation of the variation of these characteristics, called modal parameters (frequency, mode shape, equivalent viscous damping) or non-modal parameters (e.g., operational deflection shapes) [18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. The presence of damage is detected by variations in these parameters in the damaged configuration compared to the reference state (undamaged) evaluated before the event (sometimes seconds before an event for continuously monitored structures). There are several approaches that allow such comparisons [30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42] to be made. It has been observed that methods based on the analysis of variations in the modal shapes and/or their derivatives, such as the mode curvature, are very effective [1] and can also be used as a diagnostic tool for structural and/or non-structural damage localization and quantification.