The Giant Seymareh Landslide (Zagros Mts., Iran): A Lesson for Evaluating Multi-temporal Hazard Scenarios

Abstract

The giant Seymareh Landslide (hereafter referred to as SL), located close to the border between Lorestan and Ilam provinces in the southwest of Iran, is one of the largest rock mass movements occurred on the Earth surface with an estimated volume of 44 Gm3. This work focuses on geological and geomorphological constraints for the analysis of multi-temporal hazard related to the sequence of events and processes linked to the landslide occurrence, i.e., from the initial stage of deforming rock mass to the landslide debris cut by the Seymareh River. With this aim, the following four stages have been distinguished: (1) the mass rock creep (MRC) process evolving toward the paroxysmal slope failure; (2) the massive rock failure occurrence that caused the Seymareh rock avalanche and originated the natural dam responsible for the formation of a three-lake system (Seymareh, Jaidar, and Balmak lakes); (3) the lake infilling until the overflow of the landslide dam; (4) the cutting by the present Seymareh River of the natural dam, which caused the emptying of the lake system (except for the Balmak Lake) and the intense erosion due to the complex drainage system that originated after the cutting. A quantitative morphometric evaluation was also performed through the Tu index to predict the catchment-scale suspended sediment yield on the scar area and derive the erosion rate affecting the landslide after the cut of the landslide dam by the Seymareh River. Due to the very huge dimensions of the SL, the here depicted multistage scenario can be regarded as an “end-member” in view of future events that can occur in similar geological contexts.