The giant Seymareh Landslide (Zagros Mts., Iran): a lesson for multi-temporal hazard scenario evaluation

The Seymareh Landslide, located along the border between Lorestan and Ilam provinces in the south-west of Iran, is recognized as one of the largest rock mass movements occurred on the Earth surface (the volume of its debris is about 44 Gm3). The main goal of this research is to define the multi-temporal hazard and the consequent time-dependent risk related to the multi-stage evolution of the Seymareh landslide including: i) the Mass Rock Creep (MRC) process responsible for the evident slope deformations of the Kabir-Kuh fold; ii) the landslide dam which generated three lakes (Seymareh, Jaidar and Balmak); iii) the natural cut of the dam which caused the lakes emptying (except for Balmak); iv) the intense erosion due to a complex drainage system which originated after the cut of the landslide dam. Such a multi-stage scenario can be regarded as an extreme reference for future events in similar geological contexts. A multi-modelling approach, incorporating contribution from landscape evolution and engineering-geological modelling allowed to point out the existence of geomorphic markers, among which several levels of fluvial terraces formed before and after the landslide damming, as well as lacustrine deposits emplaced after the landslide damming. The plano-altimetric distribution of these markers allowed to provide time constrains to different stages of evolution of the Kabir-Kuh slopes. The engineering-geological model supports the reconstruction of the landslide mechanism for evaluating slope stability and landslide evolution toward a generalized failure which caused the damming of the Seymareh River. Through remote and field surveying on landforms associated to the landslide deposit, the gravity-induced movement was classified as a complex rock block slide evolved in debris avalanche leaded to failure by Mass Rock Creep (MRC) processes. The study showed also how the different geomechanical properties of the outcropping geological formations played a key role in the failure mechanism. The complex succession of events responsible for the Seymareh landslide failure justify a multi-hazard evaluation approach, including the slope proneness to MRC processes, the risk related to landslide run-out, the residual risk associated to the scar area as well as to the landslide dam.