The reactivation of the Niscemi landslide (southern Italy) after the Cyclone Harry

Abstract On January 25, 2026, a large compound slide affected the southern and western slopes of the Niscemi mesa (SE Sicily), causing severe damage to settlements and the evacuation of approximately 1500 people. The event occurred approximately one week after Cyclone Harry, which brought intense rainfall over southern Sicily, and represents the latest reactivation within a historically unstable slope system previously affected by major failures in 1790 and 1997. This study integrates geomorphological mapping, DInSAR and feature-tracking analysis, slope stability modelling, and meteorological data analysis to gain initial insights into the failure mechanisms, precursory and ongoing kinematics, and triggering conditions of the last reactivation of the Niscemi landslide. Results highlight a deep-seated compound mechanism controlled by weak layers and pre-existing slip surfaces within the clayey Mt. San Giorgio Formation. Pre-event interferometric data show limited accelerating deformation at the mesa edge, suggesting rapid failure onset. Numerical simulations suggest the key role of weak layers in governing large-scale instability. The findings emphasize the importance of combining geomorphological memory and satellite data for an improved hazard assessment of dormant landslides that frequently involve slopes of the Apennine Mountains of Italy.