## 2 Dynamic Properties' Changes Due to the Seismic Action
In comparison to natural frequencies, the estimation of mode shapes is generally more difficult and requires both a high signal to noise ratio and a large number of optimally located sensors. In addition to natural frequencies, [PERSON] et al. [23] in their study analyze the changes of mode shapes in form of the MAC factor due to the earthquake. Through their research, they showed that higher mode shapes are not suitable for direct comparison of seismic properties. During testing, the first transverse mode shown on the average MAC is equal to 0.95. Other modes showed lower MAC (lower than 0.80). This means that although the shape of the mode is similar, the mode is not exactly the same due to the damage caused. [PERSON] et al. [27] in their work shows that the small variation in the natural frequency's values (in their case for the first and third modes) corresponds to large MAC factor variation. Thus, this confirms that the combination of the natural frequencies and mode shapes can avoid misleading conclusions for outcomes of dynamic identification of complex structural system such as masonry structures. In addition to the MAC, several other mode shapes comparison criteria [28] can be used for damage detection of masonry buildings such as Modal Displacement Index (MDI), Modal Strain Energy (MSE), Modal Curvature Changes, etc. Third, dynamic parameters which change can also serve for damage detection is the modal damping. These incorporate the contribution of hysteretic damping arising from non-linear phenomena and yielding of structural members. Therefore, they serve as a highly sensitive indicator of the magnitude of inelastic deformation occurring during intense vibrations. The understanding of damping mechanisms in structural dynamics and modal testing problems is currently limited. In general, assume viscous damping does not accurately represent the true damping characteristics of structures. Modal damping ratios tend to increase gradually as oscillation levels rise [29]. [PERSON] et al. [30] studied the effect of the damping ratio on the seismic behavior of historical structures. Based on the analysis performed on the historical masonry structure they concluded that the displacement, stress level, and strain values increase, and the damping ratio decrease. It is also highlighted that the damping ratio is a valuable source of information for structures that are weak to external effects, especially earthquakes, and that before the numerical evaluation of seismic behavior, the damping ratio should be experimentally determined. Other relevant studies [31; 32; 33] also discuss changes in the damping coefficient with respect to seismic action, where only estimates of this value are given without comparing the initially undamaged and damaged state. In addition, the mentioned studies point out that with the cracking of the walls, significant increases occur in the period values and damping ratios. Consequently, accurately determining modal damping ratios poses a significant challenge due to considerable variability associated with estimates of natural frequency and mode shape. In practical scenarios, conducting a free vibration test can be an effective means of identifying modal damping ratios. Based on the review, the following can be highlighted. The phenomenon of decreasing the natural frequencies can be attributed primarily to the deterioration of structural stiffness due to the deterioration of masonry elements. The presence of cracks, displacements, and the loss of material strength all contribute to a reduction in structural stiffness and thus lead to a decrease in natural frequencies. In conjunction with the decrease in natural frequencies, the mode shapes of masonry buildings may change after an earthquake. The appearance of new cracks or changes in the stiffness distribution can trigger shifts in mode shapes that manifest as changes in the dominant vibration patterns of the building.