The potential of two-pass DInSAR to investigate the spatial and temporal evolution of a large landslide in the Northern Apennines of Italy

Landslides in mountainous regions are key processes shaping the landscape and pose significant challenges to human activities, particularly due to their potential impact on infrastructures. Even dormant deep-seated landslides remain a persistent threat, as heavy rainfall events can often trigger catastrophic reactivations. The Cà di Sotto landslide in San Benedetto Val di Sambro (BO), Italy is a well-documented large phenomenon (> 45 hectares) that in 1994 destroyed some buildings and occluded the stream below, necessitating extensive drainage systems to mitigate flood risks. This landslide is classified as a complex movement, originating as a rotational slide and evolving into an approximately 2 km-long earthflow. The affected material, the Monte Venere Formation, consists of tectonized calcareous-marly turbidites interbedded with arenaceous-pelitic strata. After 30 years of dormancy in October 2024, following a heavy rainfall event, the entire body underwent a new catastrophic failure reaching peak velocities of several meters per day and disrupting previously established mitigation measures. Multi-temporal InSAR techniques (PS and DS-InSAR) are widely used to monitor slow-moving landslides, but the targeted phenomena strongly exceeded their maximum detectable velocity (Vmax ~ 100 mm/yr). All analysis were consequently performed through the two-pass DInSAR technique using Sentinel-1 A/B C-band SAR images, acquired with a minimum acquisition interval of six days, from 2015 to early 2025. This method grants higher territorial coverage in mountainous areas and increases the maximum detectable velocities (Vmax ~ 20 mm/week). Our results show signs of activity in the crown area in the period preceding the catastrophic failure while no clear deformation signals were detected in the landslide body. During the failure event, the quality of InSAR data varied depending on the perpendicular baseline, atmospheric disturbances and vegetation cover. Peak deformation (V > 10 m/day) exceeded the detection capabilities of InSAR, requiring ground-based monitoring techniques for effective tracking. However, low-noise interferograms clearly delineated the spatial distribution of the active area with frequent phase jumps and decorrelation soon after the failure. During the later post-failure stage interferograms have high enough coherence to map the deformation field. The comparison between InSAR data and on-site ground measurement (including subsequent UAV surveys and topographical data) helped to understand and interpret the remotely sensed information and highlights the potential and the limits of standard interferometry to identify and monitor active landslides in mountainous regions.