Investigation on the geomorphological effects of active tectonics and zoning Earthquake risk with emphasis on seismicity of faults (Case study: Namin, Astara, Talesh)

Introduction According to the results of this study, Tabriz metropolis does not have a favorable situation in terms of earthquake risk, and most densely populated parts of the city, especially the northern and central parts, are in very high and high vulnerability zones. Milani and Nemati (2015), studied the geological indices, tectonics and seismicity of faults in Lut and Jazmourian basins. Their work’s results showed that both basins have active tectonics, but the western margin of Shahdad basin has high to medium activity and Jazmourian basin has little activity. Shayan and Zare (2013) investigated and determined the seismic hazard ranges in active alluvial fans by barbarian spatial analysis method, and the results of their research showed that Garmsar and Sorkh Sokoot faults have the highest seismic potential and Lalehzar and lower Qaleh mountain faults have the lowest. Rajabi and Aghajani (2010) investigated faults and seismicity and seismic hazard in the northeast of Lake Urmia. Their studies in the region led to three important classifications. Rabati et al. (2015) studied active tectonics using geomorphological indices in the Sefidrud basin of western Alborz and their results indicated that sub-basins corresponding to dense fault zones show high measurement indices. Data and Method In order to achieve the objectives of this study, the data of topographic map 1: 25000 of the surveying organization, region’s geological map of 1: 100000, region’s aerial photographs of 1: 50000, 1: 20000 and 1: 40000, Landsat 8 satellite images related to the year 2020, as well as digital elevation model map were used. Using the existing relationships, geomorphological indices were calculated and necessary analyses were performed. The seismic potential of active faults in the region was calculated using the formulas of Zare (1995), Ashjaei and Vorozi (1978), and Wells and Coopersmith (1994). In addition, the spatial analysis method was used to analyse and determine the seismic risk. In this study, the method proposed by Barbarian et al. in spatial analysis was used. The steps of this method are as follows: first, faults with a length of more than 10 km are identified. Then, faults with a length of 5 to 10 km are identified and added to the first category of faults, and configuration operations are applied based on the distances to the source of the earthquake and the faults. Then, the extracted map is overlapped with the land use map to determine the location of human settlements in the danger zones of fault lines, and the map resulting from the sensitivity of the region's formations from the point of view of resistance, and finally the final map of earthquake danger configuration and the position of highly populated centres is obtained based on the level of risk-taking. Results and Discussion The most important factor in the structural development of Alborz is the existence of driven and inverted faults that are located along this mountain range. Astara fault is one of Alborz’s most important faults. This fault is one of the most important geological structures in the northwest of the country, and the occurrence of destructive historical earthquakes has increased its importance. In this regard, the activities of major and minor faults of this region and the resulting seismic potential were investigated and analysed in this study. In the present work, based on the analyses performed by remote sensing of satellite and GIS and the implementation of processes such as principal component analysis, band composition, Hillshid and filtering of satellite images indicating structures, there are definite line structures in the study area. In this regard, 25 fault lines were identified in the study area. The presence of fault lines in the study area indicates tectonic activities there. In fact, the fault lines extracted from the satellite image show different faults, especially in the north-eastern, north-western, western and south-eastern parts of the study area. Yamani and Alizadeh (2016) also achieved similar results in the study of tectonic activities of the Karaj watershed using satellite imagery and geomorphological indicators. Also, based on the results of the study, among the methods used to identify faults in the study area, the methods of applying directional filters, principal component analysis and Hilshid have had a great impact on the proper identification of faults. This can be due to the feature of edge highlighting in directional filters, and also the presence of more than 80% of the information in the first band of the principal component analysis. In addition, fault lines that were not identified by the above two methods were extracted using the Hillshid method. Based on the results of the used indicators, most of the basins have high tectonic activities in the study area, and the morphological characteristics of the studied catchments are under the influence of fault lines in the area. ConclusionThe results of the relatively active tectonic index showed that basins 1 and 2 in the eastern parts of the study area had the highest tectonic activities. In the present study, the calculations performed in GIS and Excel software were calculated using the equations provided by Norouzi and Ashjaei, Zare, and Wells and Coopersmith, and the average seismic power of Astara fault was equal to 6.9 Richter, Neur fault equal to 7 Richter, Hir fault equal to 6.2 Richter, and Sangour fault equal to 4.8 Richter. In general, the average seismic power for the main and secondary faults in the study area based on Nowruz and Ashjaei, Nowruzi, and Wells and Coopersmith equations was estimated to be 5.844, 5.710 and 6.517, respectively. Also, in addition to calculating the seismicity, spatial analysis method was used to determine the risk of faults. The results showed that most of the settlements in the study area are located at a distance of 3000 to 10,000 meters from the fault lines. The results indicate three hazardous zones in the study area. Shayan et al. (2013), in a similar study, determined the earthquake risk zones in the Garmsar alluvial fan using the GIS and concluded that most of the residential centres in their investigated study area are located at a distance of 3000 to 7000 meters from the fault lines.