Pre-failure landslide deformation measured by satellite radar interferometry in the Northern Apennines of Italy

In this work we back-analyze two landslide failures that occurred in the Northern Apennines of Italy, in terms of surface movements and their relation with rainfalls. The two landslides have some similarities in that they are historical earthflows characterized by relatively long period of slow-moving stable sliding that can be interrupted by rapid acceleration and catastrophic failure. In both cases, the catastrophic failures were sudden and unexpected, causing problems to infrastructures (roads and railways) located at the toe of the landslides. We process interferograms of SAR images acquired by Sentinel 1 A/B with time spans ranging from 6 to 24 days, removing those having low coherence by manual inspection. The conventional 2-pass technique allowed us to obtain measurements of surface displacement despite the fact that no infrastructures nor major reflectors are present on landslide bodies. Our interferograms show that surface displacements are well visible since 2015. They display nearly continuous downslope motion with seasonal velocity changes. Time series between 2015 and 2018 shows that surface displacements can be appreciated throughout most part of the year with snow cover and summer peak of vegetation being the most notable exceptions. Distinct accelerations can be detected in space and time during the weeks and months preceding the catastrophic failures. We compare time-dependent deformations to precipitation patterns to explore interactions between precipitation and landslide kinematics and to document the transition from stable to unstable sliding. Our work indicate that InSAR interferometry can be successfully used to anticipate catastrophic failures.