Introduction

Tectonic forces originate from within the earth and cause deformation of the earth's surface. Tectonics is important in the science of geomorphology, especially from the perspective of the formation of surfaces, and the formation of mountain ranges is the function of tectonic forces. One of the important natural hazards in mountainous areas is landslides, which has serious consequences for human life. Studies conducted around the world also indicate that landslides in tectonically active areas have much greater cuts and in mountainous areas where there is greater density, education can be witnessed.

Methodology

In this research, the main goal is to identify landslides and investigate the effect of active tectonic parameters on domain instability in the sub-basins of the southern part of Zagros (Ilam-Mehran range). Therefore, initially, 257 small and large landslides, including definite and suspected cases, were identified and characterized using a digital elevation model with a resolution of 12.5 m, topographic maps of 1:50,000, satellite images in Google Earth, Arc GIS, and Global Mapper software. Then, through more detailed studies in images with higher spatial resolution and field visits, the study area was studied on a case-by-case basis and the suspected cases were verified (Figure 3). Next, to evaluate the spatial relationship of landslide occurrence, the tectonic activity of the sub-basins was extracted using Arc Hydro software. Finally, to estimate the relative tectonic activity index Iat, morphometric indices such as hypsometric integral (Hi), river longitudinal gradient (SL), basin asymmetry (AF), basin shape ratio (Bs), valley floor width (Vfw) and mountain front sinuosity (Smf); and to investigate the uneven conditions of sub-basins, slope indices (S), slip area ratio to basin area (LA), slip density (LD) and Melton index (Me) were calculated and the statistical relationship between the indices was estimated using the Pearson correlation test. Also, data related to the epicenter of earthquakes were received from the database of the National Seismological Center.

Results and Discussion

A study of the spatial distribution of 257 identified landslides shows that the largest number of them was observed in the Ilam, Mehran 1, Mehran 2, and Salehabad basins, respectively. In terms of the ratio of landslide area to the total basin area, Mehran 2 basin has the largest share with 4.66 percent, which indicates the high sensitivity of this basin to the occurrence of landslides. In contrast, Salehabad basin has the lowest landslide area with 2.35 percent. Analysis of the relationship between landslides and faults shows that more than 57 percent of landslides occurred within a distance of less than 2 km from faults. This finding indicates that fault activity plays a fundamental role in creating unstable conditions on slopes. Also, the investigation of the distribution of landslides in relation to earthquake centers shows a direct relationship between landslides and earthquake occurrence. About 68.87 percent of the landslides occurred less than 4 kilometers from the earthquake focus with a magnitude of more than 2.5 on the Richter scale. The findings show that with increasing distance from the earthquake focus, the percentage of landslides decreases significantly, which indicates a decrease in the impact of seismic waves on the occurrence of these phenomena. In fact, the vibrations caused by earthquakes play a major role in the activation of landslides, and in areas where the faults have less activity or there are no earthquake centers, the frequency of landslides is reduced and their dispersion is increased. The results of morphometric indices show that the Ilam, Mehran 1 and Salehabad basins are in the medium tectonic activity class, and the Mehran 2 basin, being in the high tectonic activity class, has a higher level of tectonic activity. On the other hand, the distribution of landslides in the basins, considering their size and area, confirms the existence of active tectonics in the region. Especially in the Mehran 2 basin, despite its smaller size, it has a high level of tectonic activity and has the highest frequency of landslides in relation to its area. Also, the results of the analysis of the ratio of the area of landslides to the area of sub-basins indicate that this ratio increases significantly with an increase in the tectonic activity class. This indicates that in sub-basins with higher tectonic activity, the extent of landslides is relatively greater and their probability of occurrence increases.

Conclusion

The findings show that landslides are mostly concentrated in the vicinity of active faults and close to earthquake epicenters, and proximity to structural fractures and seismic activity has played an important role in triggering landslides. By moving away from these tectonic elements, the frequency and intensity of landslides decrease. Therefore, it can be said that faults and earthquake foci, as the main drivers, have a direct impact on the occurrence and spread of landslides in the region. Also, the study of geomorphological indices and morphotectonic index (Iat) shows moderate activity of Ilam, Mehran 1 and Salehabad basins and high activity of Mehran 2 basin. The distribution of landslides in the studied basins shows that the phenomenon of slope instability has occurred significantly in all basins, but its rate is higher in some basins than in other areas. The significant positive correlation between landslide density indices and the ratio of landslide area to the basin hypsometric integral and Melton index indicates the effective role of tectonic conditions and the level of geomorphological dynamics of the basin in increasing the number and extent of landslides. In fact, in sub-basins with high tectonic activity, the area of landslides increases relative to the area of the sub-basin. Therefore, some basins, such as Mehran 2, despite their smaller basin size, witness a greater number and extent of landslides, which indicates their high sensitivity to factors affecting instability, especially tectonic factors. In general, the integration of tectonic, seismic, and morphometric data of the basins shows that the studied area is geodynamically active and landslides, as a clear consequence of these activities, have been formed under the direct influence of seismic activity, faults, and tectonic conditions of the region.