## 5 Discussion
Consequently, these monitoring observations were interpreted as rest phases of the landslide that may depend on other control factors that were neither monitored nor investigated in this study. Differently, the RM pattern showed an episodic nature linked to multi-day rainy periods characterized by both intense and moderate precipitations which led to pore water pressure peaks that in turn induced sharp displacements (Figure 18). Interestingly, time lags were observed between the occurrence of rainfall peaks and the corresponding peaks in both water pressure and recorded movements. Particularly, it was found that the development of water pressure peaks required a preparatory period of several days after the most severe rainstroms, indicating a delayed pore water pressure response along the failure surface. Other studies on reactivated and slow-moving landslides reported similar evidence [51]. In this research, the pore water pressure response to rainfall at the failure depth was found to be complex and probably related to the combined role of rainfall magnitude, duration, and frequency rather than to isolated and very intense precipitations. This would also be consistent with the largest acceleration events occurring in historical times [55]. However, reliable thresholds of cumulative rainfall levels antecedent to pore water pressure peaks were not identified, probably because of the insufficient number of significant rainfall events that occurred during the period of measurement, along with the complex hydrogeological setting of the landslide body. In fact, several factors such as the heterogeneity and anisotropy of the landslide materials, as well as their initial hydrologic conditions, can influence the rainfall infiltration processes and thus groundwater recharge and pore water pressure accumulation [25; 27]. The RM phases were responsible for the largest magnitude of deformations and were driven by pore water pressure growth along the failure surface (Figure 18). This pointed out a connection between the pore water pressure-induced reduction in effective stresses and residual shear strength decay, resulting in accelerated movement along existing sliding surfaces [10; 35; 72]. However, no obvious relations between landslide velocity and pore water pressure were found after the onset motion, making it difficult to reconstruct the possible deformation mechanisms involved. Like in other documented cases [10; 23; 38; 75], similar values of pore water pressure were found to correspond to different values of velocity, suggesting hysteretic behaviour. Therefore, the relationship between changes in pore water pressure and residual shear strength appeared to be far more complex than might be expected. In the literature, these field observations have often been interpreted and modelled assuming viscous behaviours [26; 33; 48]. For the landslide under investigation, phases of increasing pore water pressure tended to be associated with higher displacement rates than phases of decreasing water pressure (Figure 19). Moreover, during one rapid movement event, displacements tended to accelerate at a constant value of pore water pressure, probably due to an increased gradient of pore water pressure variation (Figure 19). These outcomes indicate that the resistance against sliding changes after the initiation of the movement [40; 43; 46]. Some authors interpreted these styles of motion behaviour assuming rate- and state-dependent frictional models [39]. Accordingly, for the landslide in question, it seems reasonable to assume that during RM stages, the motion is likely initiated as rigid-plastic frictional sliding mechanisms which subsequently may evolve as deformation processes regulated by rate- and state-dependent frictional factors. Eventually, it is worth noting that since this interpretation relied on a single measurement point, further measurements in other sectors of the landslide along with analyses of future monitoring data will contribute to improve knowledge.