Preliminary analysis of potential daily cyclic movements on the surface of Brenva rockslide scar based on the GB-InSAR monitoring (Mont-Blanc massif, Aosta Valley, Italy)

With global warming, geological hazards such as rockfalls, rockslides and rock avalanches have increased in alpine areas recently. In many studies, this increase has been attributed to the redistribution of the slope stress field, fluctuations in the temperature field (surface layer thaws during summer), and changes in the seepage field (infiltration of snow and ice melting water), which are led by permafrost degradation and glacier retreat. On the other hand, it is necessary to assess the long-term effects of these changes on rock mass fatigue, which could lead to rock instability. The GB-InSAR technique can detect deformation in the mm range. It is ideal for monitoring small deformations caused by daily physical weathering or other factors in high mountains.

A GB-InSAR campaign was performed from 12 August 2020 to 19 October 2020 in the Brenva glacier basin to assess the displacement of the Brenva rockslide scar. We found a daily cycle of expansion and shrinkage on the scar surface during the summer after examining the movement of different control points along the line-of-sight (LOS). Consequently, we explored possible causes behind such displacement. In this case, we realized that the crest and trough of the displacement curve occurred at a certain period of each day. For instance, in the cases of control points 2, 7, and 8, most crests in the displacement curve occurred in the early morning of each day and the troughs in the late afternoon or evening of each day during 06 September and 13 September, with amplitudes of displacement around 0.15mm, 0.25mm, and 0.4mm, respectively. The preliminary correlation between air temperature and daily deformation shows that point 7 moves towards SAR as the air temperature increases, and away from SAR as the temperature decreases. This phenomenon means that such displacement could be caused by the daily changes in temperature (leading to thermal expansion and contraction of materials, and movement of ice in micro-macro cracks) in the rock mass and air.

However, a comprehensive analysis of the LOS displacement that consists of checking the raw data of GB-InSAR (i.e., radar signal comparison), setting more specific control points at locations with various dip directions, and clear correlation between meteorological data and displacement is undergoing to verify and explain such kind of displacement.

In conclusion, continuous daily physical weathering (behaving as cyclic movement) that led to rock mass fatigue probably exists on the surface of alpine slopes, and GB-InSAR could be an effective technique to detect such movement. Despite only slight daily displacement fluctuation on the surface, it could play a crucial role in the initiation of geo-disasters.