Explaining the concept of risk and assess the seismic risk of urban areas(case study: Najafabad City)

 Iran is one of the most vulnerable countries in the world against the earthquake at the present time (Zabardast, 2013: 1331). According to the seismic hazard map of Iran, almost the whole territory of the country is in relatively medium to high hazard area(Ziari, 2006: 283). Najaf Abad city is located near the Isfahan with the population more than 221000 individual (in the year 2011) and the most important service center after Isfahan within the urban set of Isfahan. In terms of geology, Najaf Abad is located within Sanandaj-Zirjan zone regarded as one of the most active zones (Darvishzadeh, 2013: 118). Based on the conducted studies, the occurrence of earthquake over this zone is constantly possible (Shahpasandzadeh & Heidari, 2007: 64). 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 and 20th century over Isfahan extension as well as the basic trend of faults and dynamic structures in this zone indicate the relatively high rate of seismicity of this area (Ambraseys & Melville, 1991: 133), (Safaei, 2003: 15). 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 to the possible earthquake. Basically, dedicating meaningful weights to physical , social,...attributes for evaluating the seismic vulnerability and providing a local model make the practical application of seismic risky analysis possible.  The data collection as library including using the maps, the information of 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 were used and the maps and informational layers provided in ArcGIS environment. Hence, with reference to the previous conducted studies and making benefit from the views of some experts in this regard, 5 criteria and 18 sub-criteria were considered to prepare the vulnerability map of the town to theseismic risk. In addition, having the maps and information related to the relatively earthquake risk over the zone and regarding the mentioned descriptions given in theoretical concepts section, the earthquake risk within the viewed area will be studied and analyzed based on the proposed model. 3-1- Studying of earthquake risk over the zone - Studying tectonic and faults conditions of the zone The contact of this town with the surrounding mountains is usually from the type of the fault that threats the town less and more due to its being active. The super-faults of Qom-Zafreh, Zagros and Rokh are located surrounding this district. Evidence shows that whenever they start to vibrate, they will certainly affect this area (Nabavi, 1976: 70). Moreover, according to the studies recently done on the seismic condition of Isfahan zone, a lot of important active faults, some with a higher than 100 km length have been detected within an area with a 100 km radius of Isfahan city (Safaei, 2005: 109). - Studying seismic condition of zone With respect to two references; Iranian historical earthquakes ((Ambraseys & Melville, 1991: 133) and Iranian earthquakes catalogues (Berberian, 1994), several historical earthquakes (higher than 5 Richter) and 95 earthquakes higher than 2.5 Richter at time interval 1962-2014 3-2- Analysis of the town vulnerability using ANP model This research regards the vulnerability as an integration of the risky factors (environmental, anatomic, social etc.) and the coping capacity of community (aid and management indices) and studies the vulnerability of Najaf Abad town through considering 5 basic criteria and 18 sub-criteria. Therefore, firstly the maps related to sub-criteria were prepared, converted to Raster and reclassified. Then, to weight the above indices, AHP model was used into Super Decision software environment. Finally, through introducing the obtained weights to the relevant layers, the maps were overlapped and the general vulnerability map of the town provided.3-3- Evaluation of the Urban risk to the earthquakeThe proposed model was used in this survey emphasizing that a risk is the result of integrating two factors; hazard and vulnerability. In this model, a risk based on the linear relationship between these factors as Risk = Hazard × Vulnerability is determined. Studying the tectonic conditions of the zone showed that there is the continuous possibly of earthquake hazard in this zone. On the other hand, due to the small area of under study zone, if one of the faults of this zone is activated promptly and/or the occurrence of an earthquake is possible, the whole county may be affected and the rate of hazard for the whole points is almost the same. Hence, with respect to the risk map of the town, around 33% of undeveloped area of the town is with a high risk involving the central core of the town and burnt brick buildings and some of the cement block buildings with more than 30 years old. In addition, 27% of this area has a medium risk including the buildings with masonry materials (brick and iron), without proper foundations and often with more than 30 years old.  Studying different models showed that risk is the combination of hazard and vulnerability. In this survey, initially the study of the zone tectonic condition showed that the threat of earthquake hazard is inevitable for this town. The results show that about 60% of developed areas of the town were found to be with a medium to high risk. These areas were located in the central district of the town that constructed by masonry and reinforced masonry buildings. Finally, the synthesis of the population distribution map and the risk map indicate that more than 51000 individuals are in the high to very high risk zone.