The role of earthquakes in the occurrence of landslides on the Tefin-Dagaga axis in Kurdistan province using radar interferometry techniques

Extended Abstract



Introduction

In general, the movement of the earth's constituent materials, including soil and rock, that occurs in sloping areas under the influence of gravity downwards is called a landslide (this definition includes types of collapse, slip, flow, etc.). In general, there are different classifications for naming and terminology of landslides, but the first classification is related to Warrens (quoted by Roustai, Shahram, 2004). Classifications after Warrens made very few changes in the terminology of landslides. In Warrens' classification, the type of material displaced and the type of sliding are referred to. Therefore, each landslide can be named and categorized into two types: first, a name that expresses the type of material displaced and second, a name that describes the mechanism and type of movement. There are different methods for measuring the movements of the earth's crust caused by the occurrence of a landslide. These include geodetic methods using precise leveling and GPS observations. The modern use of several remote sensing technologies, including synthetic aperture radar (SAR), optical measurements, and light detection and ranging (LiDAR), represents a valuable complementary data source to conventional mapping methods.

Methodology

In this study, various types of data were used, including Sentinel-1 satellite radar images with InSAR (Table 1) and data collected from the Global Positioning System (GPS) during field operations. Also, 1/50,000 topographic maps from the Iranian Surveying Organization and 1/100,000 geological maps from the Iranian Geological and Mineral Exploration Organization were used to study the geology and morphology of the region. The software used in this study is: scape SAR version 5.2, ArcGIS version 10.6, ENVI version 5.3, which will be used to prepare and process radar images and prepare a displacement map related to landslides. In this study, the temporal and spatial baseline for SAR images, 8 pairs of images from 2017 to 2022, have been identified for subsequent processing after controlling the parameters obtained from the baseline and coherence, and will be the basis for preparing a map of land surface displacement in unstable slopes in the study area, and finally the displacement rate for the mentioned years was calculated. Figure (2) shows the location map of the radar image of the study area.



Results and Discussion

Earthquakes are usually caused by the propagation of seismic waves that lead to physical changes in the Earth's surface. One of the results of these changes can be subsidence or elevation in different areas. Radar interferometry, which is used to calculate ground displacements, is one of the most powerful tools for studying ground changes due to earthquakes. Reviewing and comparing ground displacement data over several time and space periods (from 2017 to 2019), including 8 pairs of Sentinel-1 InSAR radar images, shows significant changes in the behavior of the ground along the Tefin-Dagaga road. This comparative analysis can clearly identify the process of subsidence and other geographical changes over these years. Tectonic structures such as faults, folds, and shear zones significantly contribute to landslides. Faults increase susceptibility to landslides due to severe shear stress and weakening of nearby lithology. Both minor and major faults play a significant role in slope instability. The Main Recent Fault and the Main Zagros Reverse Fault are two important faults that affect landslides in the Degaga Basin and the Zagros Range in general. The Degaga Basin landslide complex is located in the Sanandaj-Sirjan structural zone, where the Serpentinites of Sarvabad (Solava) ophiolites (the so-called Kurdistan ophiolite) along the main recent Zagros fault play an important role in their occurrence. These serpentinites are exposed along the Sanandaj-Marivan road and are thrust beneath the Cretaceous slabs and phyllites of Sanandaj-Sirjan. The branches of the recent main fault and its branches with the mechanism of strike-slip and strike-slip are observed along the Azad and Sirvan rivers. These faults have caused the fracture and crushing of serpentinites and other rocks. In some landslides on the Marivan-Sarvabad route, the main control in their occurrence seems to be the activities of the main reverse fault of the Zagros cut by the main strike-slip fault of the Younger and its branches, which intensify the absorption of water by the crushed serpentinites. Due to the continuous movement of these landslides, the Marivan road is constantly being destroyed.



Conclusion

In this study, the interferometric radar (InSAR) technique was used with Sentinel-1 satellite data to measure ground displacement during 2017 to 2019. This method detects changes in the ground surface with high accuracy and calculates the amount of displacement by comparing multi-temporal radar images. Radar images were collected and analyzed at different time intervals to accurately assess changes in ground elevation, subsidence, and landslides. The results of this method showed that ground displacement data during 2018 and 2019 indicate significant changes in the ground condition along the Tefin-Dagaga road. In 2018, ground displacements were mostly reported as minor subsidences, with an average displacement of -0.0235 m in August, indicating a relative improvement in ground condition compared to 2017. Also, limited subsidence with a minimum displacement of -0.0501 m was observed, indicating instability in some specific locations. In contrast, the displacement situation became more severe in 2019. The average displacement reached -0.0376 m, indicating an increase in ground subsidence compared to 2018. The minimum displacement in this year reached -0.1094 m, which raised further concerns about ground instability in some locations. The larger standard deviation in 2019 (with a value of 0.0189 m) also indicated that the displacement changes in this year were more diverse and the ground instability was more widespread.

Consequently, the comparison of displacement data and regression analyses shows that the study area, especially along the Tefin-Dagaga road, is subject to continuous geological changes. While 2018 showed a relative improvement in ground conditions, 2019 data indicate a return of more severe subsidence and widespread instability. This suggests that continuous and careful monitoring, along with urgent protective measures, is essential to prevent further risks in this area.