Document Type : Research Article
Authors
1
Department of Geography, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
2
Department of Sedimentary Basins and Petroleum, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
Abstract
Earthquake-induced landslides are among the most significant geological hazards capable of causing substantial loss of life and property damage. The present study was conducted with the aim of evaluating the impact of the Ezgeleh earthquake of November 12, 2017 (Ms = 7.3) on the occurrence of landslides, and quantifying the displacement of landslides within the Tang Hamam and Alvand basins. In this regard, landslides within the study area were first identified and located using satellite imagery and field observations. Subsequently, new landslides were detected by comparing satellite images taken before and after the earthquake. Then, employing multi-temporal radar interferometry, 12 Sentinel-1A images—comprising data from both before and after the earthquake—were processed. The results indicate that, among the identified landslides, the earthquake triggered the activation of four landslides; in addition to surface ruptures, these exhibited significant deformation in the form of displacement along the satellite's line-of-sight (LOS). The largest landslide occurring on the slopes of Mount Shahneshin covering an area of 18,242,677.47 square meters exhibits an average displacement of approximately 42 cm and a maximum displacement of up to about 53 cm along the satellite's line of sight, indicating a high degree of slope instability in this area. Additionally, three other landslides on the slopes of Mount Gamishan despite their smaller extent have exhibited a relatively uniform pattern of displacement along the satellite line-of-sight, averaging approximately 45 centimeters; this indicates a similar response of these slopes to seismic triggering. A simultaneous analysis of radar interferometry results, the distribution of earthquake epicenters, and the locations of active faults in the region indicates that earthquake-induced movements played a significant role in triggering and intensifying landslide displacements.
Extended Abstract
Introduction
Landslides caused by earthquakes are considered one of the most important geological hazards, and due to their rapid occurrence, they can cause extensive loss of life and property. In this regard, the intensity and duration of earthquake motion are considered to be the most important factors stimulating the movement of slopes. In this context, radar interferometry is one of the newest remote sensing methods that, by processing radar images, enables the preparation of maps of the displacement of the earth's crust over a large and continuous surface. One of the main applications of synthetic aperture radar (SAR) remote sensing is the monitoring of areas affected by surface deformation using the differential interferometry of synthetic aperture radar (DInSAR) method. The Ezgeleh earthquake occurred on November 12, 2017, with a magnitude of 7.3, 10 km from Ezgeleh and about 37 km northwest of Sarpole Zahab County in Kermanshah Province. The study of the earthquake's focal mechanism showed that a rupture in a blind fault in the Zagros Fold Thrust Belt (ZFTB) was the main cause of this earthquake, which led to various instabilities, including landslides and rockfalls. Therefore, the present study focused on landslides caused by the aforementioned earthquake, within the Alvand and Tang Hamam watersheds. The primary objective of this study is to evaluate the impact of the Ezgeleh earthquake on landslide activity in the Tang Hamam and Alvand basins. This study aims to determine the earthquake's role in landslide occurrence, assess the displacements caused by the event within the study area, and establish a foundation for future research on geological hazards.
Material and Methods
In this study, landslides were first identified and located using a digital elevation model (DEM) with a spatial resolution of 12.5 m, topographic maps with a scale of 1:50,000, satellite images available in Google Earth, Arc GIS, and Global Mapper software, and field observations. Then, in order to investigate and determine the amount of displacement and movement of landslide masses, radar interferometry and Sentinel-1 radar images were used. A total of 12 Sentinel-1 radar images, including six images before the earthquake and six images after it, were downloaded from https://scihub.copernicus.eu. After importing the images into the ENVI and SARscape software environments, in order to fully cover the study area, the burst corresponding to the Sarpole Zahab region was selected from the Sentinel-1 images to be used in the radar interferometry process. Then, the temporal and spatial relationship between the images was examined using a connection graph. Next, interferometric processing was performed within the framework of the SBAS Interferometric Processing method, which included image co-referencing, view interferometry generation, filtering, and phase unwrapping. The output of this step included filtered view interferometry (FINT), differential view interferometry without topography effect (DINT), and unwrapped phase (UFA). In order to reduce systematic errors and improve data quality, the Refinement and Reflattening step was performed using ground control points (GCP). In the next step, data inversion was performed by executing SBAS Inversion – First Step, and the average land surface displacement rate map was extracted. Then, by executing SBAS Inversion – Second Step, the land surface displacement time series was calculated for each imaging date and atmospheric effects were corrected. Finally, the Geocoding step was performed to transfer the results from the satellite view geometry to the terrestrial coordinate system.
Results and Discussion
An examination of the spatial distribution of old and new landslides alongside active faults and earthquake epicenters shows that the seismic activity of the region has played a decisive role in activating these landslides. Also, an examination of objective evidence of land surface deformation on the slopes of Shahneshin Mountain indicates the occurrence of displacement, local uplift, and rupture of soil and rock masses in the landslide area. The results of the present study also confirm the occurrence of significant deformation in this domain and the important role of seismic stresses in activating and intensifying the domain instability; however, simultaneous examination of the Shahneshin and Gamishan landslides showed that the intensity and pattern of deformation in all landslides caused by this earthquake are not the same. While the Shahneshin landslide shows the largest range of deformation and displacement, the Gamishan landslides, despite being smaller in size, have a relatively more uniform deformation pattern. This uniformity in displacement values could indicate the similar response of the Gamishan slopes to seismic loading and the relatively homogeneous tectonic conditions of this part of the region. In other words, although the intensity of instability in Gamishan is less than in Shahneshin, the mechanical behavior of the slopes against seismic movements follows a more specific and predictable pattern. This difference indicates that the simultaneous assessment of multiple landslides allows for a better understanding of the spatial variability of the response of the slopes to a single seismic event, a topic that is less likely to be investigated in studies focused on a single landslide. The results of radar interferometric analysis based on the SBAS-InSAR method show that the identified landslides, in addition to surface ruptures, were accompanied by significant displacement along the satellite line of sight. The displacement values along the satellite line of sight extracted for the Shahneshin landslide and the Gamishan landslides indicate a high intensity of deformation of the slopes after the earthquake. This displacement rate can indicate the continued deformation of the landslide mass after the seismic event. The difference between the deformation amplitudes in the Shahneshin landslide and the Gamishan landslides can be attributed to the difference in the slip extent, the geometry of the slope, and the concentration of seismic stresses.
Conclusion
The analyses show that four new landslides have occurred in the study area. The largest landslide on the slopes of Mount Shahneshin, with an area of 18242677.47 square meters, had a maximum displacement along the satellite line of sight of about 53 centimeters and an average displacement of 42 centimeters. Three other landslides on the slopes of Mount Gamishan, with areas of 22560.07, 6727.73, and 5528.86 square meters, respectively, show an average displacement of about 45 centimeters, and their deformation pattern has been relatively uniform. The study of the spatial distribution of landslides along active faults and earthquake epicenters indicates the important role of seismic movements as a driving factor in the activation and development of slope instabilities; although the contribution of other effective factors such as geological characteristics, slope morphometry, and environmental conditions requires independent and more detailed studies. The results of InSAR analysis show that the concentration of deformation in the central and lower parts of the slopes shows the highest intensity of displacement along the satellite line of sight, indicating the effective role of seismic stresses on the instability of the slopes.
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