Seismic Hazard Analysis Based on Peak Ground Acceleration Using the Deterministic Approach (Case Study: Semnan, Iran)

Iran due to geological and geographic conditions, has always been exposed to various natural hazards. One of these disasters is the earthquake, which has caused significant damage to the country's economic and social complex. This issue is a serious threat, especially in areas with active faults and Every year, it brings about life and financial losses. Therefore, attention to the importance of locating and zoning of high risk areas in terms of having a hazardous hazard potential is essential and significant. Based on this, the present study uses a deterministic approach to determine the high risk areas of seismicity.
Study Area: The present study was conducted in Semnan city with an area of 11017.8 km2 From that is limited west to Garmsar city and Firooz Kooh district, east to Damghan city, south to central Iran desert, Nain city and Isfahan province, and north to Sari city in Mazandaran province, there have been. The maximum altitude of the city is 2065 meters and it was located at a minimum easterly distance of 52 degrees and 46 minutes and a minimum latitude of 35 degrees and 15 minutes. On the other hand, the population concentration of and unsuitable constructions in this province, indicates that the occurrence of such a major earthquake could be very dangerous. In this paper, seismic hazard analysis using definite method is performed for this area. For this reason, single faults and important fault systems in the region at first step, were identified using Landsat 8 satellite imagery using remote sensing techniques. In the next step, due to the geological and tectonic characteristics of the area, geological fragmentation and seismicity of the study area were investigated and were identified fault mechanisms and fault map was created in the Arc GIS software environment. Then, was calculated and averaged by using valid empirical equations, the maximum expected magnitude of the major faults in the region. Finally, by selecting several valid decreasing relationships, maximum severity and horizontal earthquake acceleration for the study area, calculated using a deterministic approach and the average of calculated values as the maximum intensity and ground acceleration associated with each fault was used to analyze the hazard of the study area. Result and Discussion: The results of study indicated that the maximum intensity of the earthquake was about 8 and the lowest of it was 5. Also, the maximum horizontal motion of the earthquake in the study area was 0.37 and the lowest value of it was 0.048. According to the results, the areas with low risk, medium risk, high risk and very high risk based on maximum values of earthquake severity form 17.45, 24.81, 33.62 and 24.10%, respectively. On the other hand, the areas with low risk, medium risk, high risk and very high risk based on the maximum values of the earthquake motion form 36.34, 29.77, 17.77 and 16.16 percent of surveying area of the region, respectively. Finally, in order to investigate the accuracy of the research, the results were matched with the instances of ground reality, and the results showed that there is a conformity between the results and the ground reality. This study concludes that most PGA occurs in the area with NW to SE trend, which fits very well with the area fault strike. The results of this study showed that the area of with high risk and very high risk with regard to population density was significant. It can be stated that the reason for the high risk of seismicity in the northern part of the city is the existence of active faults related to the quaternary period. There are several faults in the northern part of the city which among the biggest faults in this area is the Attari fault with a length of 85 km that which dates back to the Quaternary period and runs across the west to the east from the north of the city. This issue should be carefully considered by officials, planners, and engineers.