عنوان مقاله [English]
Iran is one of the most vulnerable countries of the world to the earthquake. According to the seismic hazard map of Iran, almost the whole territory of the country is in relatively medium to high hazard area. Najaf Abad town is located in 30 km of Isfahan with the population more than 221000 people (in the year 2011) and is a middle town. It has the most important service center following Isfahan within the urban set of Isfahan. In terms of geology, Najaf Abad is located within Sanandaj-Sirjan zone regarded as one of the most active zones. Based on the conducted studies, the occurrence of earthquake over this zone is constantly possible. With respect to the seismo-tectonics map of Iran, this county is located within a zone with a relatively medium hazard and the event of historical earthquakes in 20th century. The basic trend of faults and dynamic structures in this zone indicate the relatively high rate of seismicity of this area. Regarding the importance of the issues related to the earthquake, the main objective of this research is to study and analyze the risk of Najaf Abad city exposed to the possible earthquake. Basically, meaningful weights have been dedicated to physical and social attributes for evaluating the seismic vulnerability to provide a local model for practical application of seismic risky analysis.
The data gathered in library surveys are including the maps, development plan and urban planning, aerial images, and statistical data related to population census in 2011. The procedure is based on a multiple decision-making approach. In order to analyze the data, Analytical Network Process (ANP) and Super Decision software have been used as maps and informational layers in ArcGIS environment. Hence, we have selected 5 criteria and 18 sub-criteria were considered to prepare the vulnerability map of the town to the seismic risk. The criteria selection is based on previous studies and the views of some experts in this regard. The earthquake risk of the study area have been analyzed using theoretical concepts and proposed model.
Results and discussion
Studying tectonic and faults conditions of the zone
Contact of the mountains surrounding the city with an active fault can threaten the city. The super-faults of Qom-Zafreh, Zagros and Rokh are located surrounding this district. The activity of the earthquake will certainly affect this area (Nabavi, 1976). Moreover, according to the studies recently done about the seismic condition of Isfahan, a lot of important active faults, some with a higher than 100 km long, have been detected within a radius of 100 Km of the Isfahan city (Safaei, 2005).
Studying seismic condition of Najafabad area
According to Iranian historical and Iranian earthquakes catalogues, there were several historical earthquakes (with magnitude higher than 5) and 95 earthquakes higher than 2.5 in magnitude in 1962-2014.
Urban vulnerability analysis by ANP model
The vulnerability of the city against the earthquake have been assessed as an integration of the effective factors including environmental, anatomic, social etc. and the coping capacity of community (emergency and management indices) in Najaf Abad city through 5 basic criteria and 18 sub-criteria. Therefore, the maps related to sub-criteria were initially prepared and converted into raster. Then, to weight the above indices, ANP model was used into Super Decision software environment. Finally, obtained weights have been applied to the relevant layers and the maps were overlapped as general vulnerability map of the city.
Evaluation of the earthquake risk to urban areas
The proposed model used in this survey emphasizes that the risk is resulted from two factors of hazard and vulnerability. In this model, the risk is based on the linear relationship between these factors as Risk = Hazard × Vulnerability. Study of the tectonic conditions of the zone showed that there is always the possibly of earthquake event in this area. Entire the area is affected by a possible earthquake. Hence, with respect to the risk map of the town, nearly 33% of undeveloped area of the city is under the high risk. In addition, about 27% of this area has medium risk in the buildings with brick and iron materials and those without proper foundations, often more than 30 years old.
The results of different models have indicated that the risk is the combination of hazard and vulnerability. In this survey, analysis of the zone tectonic conditions showed that the threat of earthquake hazard is inevitable for this city. The results have revealed that about 60% of developed areas of the town were in a medium to high risk level. These areas were located in the central district of the town mainly constructed by masonry buildings. Finally, the combination of the population distribution map and the risk map has also indicated that more than 51000 people are in the high to very high risk zone.