## 5 Discussion
The plots of Figure 13 clearly summarize this differentiation between MS event distributions, showing that if excluding the winter storm of February 2018 from the analysis, more than 70% of all detected events occurred during the cooling phases of the rock mass (Figure 13). These behaviors can be interpreted as the consequence of different driving mechanisms at the base of local failure. For what concerns the transient and extreme meteorological perturbation (i.e., the burn atom) that caused the rock mass to experience freezing conditions, the combined effect exerted by ice formation inside fractures, freezing-thawing cycles, and high positive thermal gradients may have significantly contributed to the rapid acceleration of incipient fracturing processes (Figure 14a), as witnessed by the evident increase in the MS activity observed during the heating phases of a four-day time window. Conversely, in the absence of brief and violent meteorological events, the occurrence of MS events could be interpreted as the response of a complex interaction between the intensely jointed rock block and the continuous fluctuations in near-surface thermal fields which may eventually cause cyclic expansion and the contraction of fractures and microcracks. This mechanism could act as a thermal fatigue process able to drive a slow yet continuous accumulation of unrecovered deformations that, if exceeding yield thresholds of plastic deformations, could cause localized failures followed by the release of energy in the form of MS signals (Figure 14b). Figure 13: Percentage distribution of MS events during heating (in red) and cooling phases (in blue) for the whole monitoring period (**a**), the only winter storm of February 2018 (**b**), and for the entire monitoring period excluding the winter storm (**c**). However, a clear cause-and-effect relationship between MS events and near-surface temperature fluctuations is not easy to discern, since several environmental factors actively exert their influence on the monitored rock block. For this reason, longer monitoring intervals comprising a higher number of MS detections are required to better isolate the contribution of continuous and transient factors to the destabilization of the investigated rock block. Nevertheless, the limited number of MS events recognized during the entire monitoring period might be related to the general stable conditions of the monitored rock block, as also witnessed by the absence of irreversible deformation trends of major fractures and microcracks (Figure 15). Figure 14: Conceptual model of the different thermally driven mechanisms interpreted as primary causes of the observed microseismicity at the Acuto field laboratory. Ice expansion pressure on crack surfaces generated by ice formation (I) and freezing–thawing cycles (II–III) can trigger dynamic and very rapid crack opening or propagation, with the release of energy in the form of MS events, as also described by the plot of crack velocity (v\({}_{\rm c}\)) versus crack driving force expressed as the stress intensity factor (K\({}_{\rm I}\)) (**a**). Conversely, the continuous variation in thermally induced expansion and contraction cycles of the crack (I–III) due to near-surface temperatures can determine the slow opening and propagation of the crack, acting as a thermal fatigue process (**b**).