## 4 Correlation between the Structural Monitoring and the Seismic Activity  
A recently published research article analyzed different seismic sequences and proposed a methodology to establish if a series of earthquake events is related to a succession (aftershocks), or if it is belonging to events of a preliminary phase before a main event (foreshock) [21]. The parameter used for this real-time discrimination between foreshock and aftershock phases, reported in the original paper, is the behavior of the average value calculated on the \(b\)-value of the seismic time series [21]. The study of the seismic sequences was applied principally to the Amatrice and the Norcia earthquake activity in 2016 [21]. It was possible to observe how the average value undergoes a growth, approximately equal to 20%, following a \"main event.\" On the contrary, in the time window between events of similar intensity, an evident reduction in the average value of the \(b\)-value is observed (foreshock), which therefore contributes to find a forecast parameter with respect to a very probable new main event [21]. Based on the approach described above, similar results were obtained in the \(b\)-values trends computed by the seismic events recorded in the area near to the monitoring site (Turin) in a circular portion of territory with radius = 100 km around the tower. In addition, a correlation between the AE and the seismic data were found based on the theoretical and experimental observations  
Figure 11: AE localization results. In figure (**a**) and (**b**), the portion of the wooden frame where the AE sensors were positioned is showed, (**c**) The localized points have been indicated with black points, for a total of 24 AE sources. recently reported in some works dedicated to AE monitoring of monumental structures in seismic active regions [18]. First of all, since the monitored structure falls in an area characterized by less seismic activity, the trend of the \(b\)-value of the seismic events was studied in order to verify that, even on sequences characterized by lower magnitudes, the behavior observed by [PERSON] and [PERSON] (2019) [21] can be used also in this case to distinguish between the \"foreshocks\" and \"aftershocks\" phases. Therefore, the earthquakes occurred in an area surrounding the bell tower of the Cathedral of Turin were considered in a six-month time window. The events selected are those that occurred between 15 March and 14 September 2018; the monitoring performed by the AE technique on the structures of the Cathedral is located in the final part of the time window here considered. The reference events evaluated in the period have the following characteristics \(m_{L}\) (local magnitude): 2.2 occurred on 12 April 2018 with the epicenter in Massello, a region far from Turin about 86 km, and \(m_{L}\): 3.1 occurred on 5 June 2018, with the epicenter localized between Italy and France (about 90 km from Turin). The results confirmed (for time series with a lower range in the magnitude levels) the evidence reported by [PERSON] and [PERSON] (2019) [21]. In particular, in Figure 12, the trend of the \(b\)-value is reported together with the local magnitude of the seismic events considered. The average value of the \(b\)-values before the event considered as the \"anticipator\" is equal to 0.95, following the seismic event with \(m_{L}\): 2.2, there is an evident reduction in the average value. The average \(b\)-value number decreased of about 15% down to 0.82. Following the main event (\(m_{L}\): 3.1), which occurred about 50 days after the reduction of the average \(b\)-value, an increase of 30% was recorded, and the average \(b\)-value was equal to 1.12. The evidence reported in Figure 12 represented a good confirmation that the procedure recently proposed for regions characterized by an intense earthquake activity (high magnitude levels) can be successfully used also for region where the seismic level is lower (Piedmont, northwest of Italy).