## 2 Study Area and Past Monitoring Analysis
### Unstable Slope Behavior  
The monitoring system installed by Regione Piemonte and ARPA since 1990 has been composed of several instruments like inclinometers, laser distance-meters, and topographic benchmarks network [19]. Most of these monitoring activities have been performed by limited periods. Wire-strain gauges collected data only for a short period and were used to understand the thermal behavior of rock mass and fractures systems (Figure 3). In the Colombero study [22], the geo-structural study was combined with borehole seismic and micro-seismic (MS) monitoring campaigns. The authors also used a detailed analysis of topographic monitoring data of ARPA Piemonte to understand the rock mass evolution and fracture system behavior. The most important results of Colombero studies are:  
1. The observation of a seasonal cycle of thermal expansion of the rock mass. This interesting behavior was detected by combining MS investigation, topographic monitoring, and wire-strain gauge measurements. The MS rate was coherently recognized to be mainly driven by temperature. The maximum frequency of events occurred during summer, with thermal expansion, while a minimum of the MS events was recorded in winter months when the fractures are opening due to contraction on the bedrock.
2. The MS investigation pointed out the presence of many MS events, which are related to rapid air temperature variations (temporal gradient), or marked temperature differences between the cliff's  
Figure 3: (**A**) detail of the fracture systems that limits the unstable blocks (**A**) and (**B**), based on results reported in [18; 21]. The map also reports former and current monitoring systems installed by ARPA and Regione Piemonte on Madonna del Sasso site, together with seismic stations used by [27]; (**B**) shows the stereonet of the fracture system.  
faces (spatial gradient). These events could cause differential thermal dilation and induce thermal stresses leading to microcracking processes within the cliff, as already demonstrated by [28].
3. The temperature control was found to be the cause not only of fracture opening/closing with a seasonal frequency, but also for a general change in the vibration directions measured during MS investigations of the two unstable blocks. In addition, the daily cycle of thermal variation influences unstable blocks behavior as observed on seismic noise resonance [22].
4. The observation of a long-term trend in which block \"A\" tends to slide towards NE of 2/3 mm per year. This trend is probably related to progressive weakness and degradation of rock bridges in fractures caused by the expansion-contraction cycle.  
In addition, past displacement monitoring campaigns (2007-2008) at the wire extensometers across fracture K4 highlighted a partially reversible seasonal fluctuation of fracture opening, driven by the air temperature (Figure 4A). This trend is visible in Figure 4B, where the change of air temperature is compared with the changes in fracture opening from October 2007 to July 2008. Maximum fracture opening was recorded during winter months (up to 1 mm/month), likely due to the rock mass thermal contraction; while a slight closure (about 0.2 mm/month) was registered in summer as a result of the rock mass thermal expansion.  
[PERSON] et al. [27] also suggested to install a continuous topographic monitoring system, joined with MS noise measurement on the site, in order to better define the seasonal cycle of thermal dilation and the long-term cycle of degradation and slide of the blocks. According to this suggestion, this site has been chosen for the development of the presented low-cost GNSS monitoring network.  
Figure 4: (**A**) shows the monthly averaged temperature [\({}^{\circ}\)C] and fracture opening [mm] from October 2007 to July 2008; (**B**) reports a plot of the variation of the fracture opening [mm] as a function of the temperature variation [\({}^{\circ}\)C], calculated on a monthly temporal scale for the period October 2007–June 2008; data source: wire extensometers of ARPA Piemonte.