*3.1. Correlation between AE and Earthquakes*

The plot of the AE signals count rate over the monitoring time (Figure 2) suggests that the masonry wall is undergoing a damage process. The clusters of AE hits, especially around the time 500 h, can be regarded as signature of high crack growth rate. During the monitoring period, frequent seismic events occurred in the region [30], but only those that might have affected the stability of the tower have been considered. Basing on their magnitude and epicentral distance, we have selected the two strongest regional earthquakes (the magnitude 4.1 event that hit the Rimini area on 13 October 2010 at 11:43 p.m. with epicenter about 100 km far from Bologna, as shown in Figure 3, and the magnitude 3.4 event recorded in the Modena Apennines on 21 November 2010 at 4:10 p.m.) and the nearby earthquakes with magnitude ≥0.5 and epicentral distance ≤20 km from the monitoring site.

**Figure 2.** (**a**) Time series of the AE signals count rate and nearby earthquakes (extracted from http: //www.ingv.it/it/, see Supplementary Materials) printed from [28]; (**b**) Expansion of the dashed frame: the red line marks the occurrence time t*EQ* = 486 h of 13 October; earthquake (EQ).

Figure 2a shows the AE signals count rate and the earthquake time series, referring to the entire monitoring period, where a correlation in time between AE clusters and seismic events can be observed. A significant example is given by the seismic and the AE sequences that occurred between 1500 and 1800 h. Even more remarkably, the densest AE cluster (formed by 4000 hits and highlighted by the dashed frame in Figure 2) and the magnitude 4.1 earthquake that occurred on 13 October 2010 at 11:43 p.m. appear to be closely correlated.

Unless uncontrolled factors affecting the measurements, the main seismic shock apparently triggered temporary intensification of the AE activity, revealing local instability until the recovery of equilibrium. The comparatively longer duration of this AE cluster with respect the earthquake duration could be explained by a viscoelastic behavior, assumed for masonry structures [39], which would produce stress and strain over time in response to an impulsive load [40].

**Figure 3.** Map showing the epicenter of the 4.1-magnitude earthquake (yellow point) occurred on 13 October 2010 with epicenter 100 km far from the monitoring site in Bologna (red point).
