Comparative Study of Vulnerability of Grid and Semi-Grid Urban Form against Earthquake from the Perspective of Passive Defense in the City of Salmas

The present age is referred to as the age of urban vulnerability, because cities face natural hazards and technological crises from various aspects on the one hand and socio-security crises on the other as urban life becomes more complex. The issue of passive defense is no longer defined as a mere research topic but as a vital requirement for governing any country, both upon crisis and at other times, for maintenance of its material and spiritual values. Therefore, the theoretical and practical position of defense and defense against the crisis is very important in this area. On that basis, the application of passive defense and consideration of its principles in urban planning can greatly reduce the destructive effects of such crises. The checkered city of Salmas, Iran, a mid-sized city, according to Iranian Space Agency, with a population of 91239 people, is located in a region with high relative risk based on the relative earthquake risk zoning of Iran, from the study of the National Physical Plan of Iran. Due to its location on a fault, evidenced by the devastating 7.2-magnitude earthquake in 1930, it is very important in this city to observe the principles of passive defense in urban planning. Accordingly, this study was conducted with the aim of measuring and modeling the vulnerability of grid and semi-grid urban forms in Salmas against earthquakes from the perspective of passive defense.

 Theoretical Framework

With respect to the type of fabric, there is less vulnerability and greater relief in case of earthquake in regular continuous fabrics over flat lands featuring roads with low or medium confinement and blocks with one or two regular rows of construction. The grid urban form of the roads is also effective in the provision of relief due to easy access. Regular and stepped discontinuous fabrics over foothill lands are moderate in terms of efficiency and vulnerability, and irregular continuous fabrics over flat areas are less effective against earthquakes. The indicators of urban fabric in the assessment of vulnerability against earthquake include the method of attachment of adjacent segments to the passage, adjacency of the open and constructed spaces of each section to the passage, extent of confinement of the fabric, pattern and size of the urban blocks, and pattern of combination of the roads and urban blocks. 

In this applied analytical research, data collection was carried out through library studies, existing articles, field studies, and 2016 census information from the Statistical Center of Iran. Thus, the effective indicators of vulnerability were extracted from the perspective of passive defense after the relevant documents and resources were studied, and fourteen indicators were then selected from among various influential factors according to their availability for specification of vulnerability in Salmas to obtain the research output. Given that each of the indicators effective in the specification of the vulnerability of the coefficient has a different importance, the opinions of the elite were used in this research to determine the weight (coefficient of importance) of each indicator. To weigh the indicators according to the BWM method, ten questionnaires with contents based on a pairwise comparison of the indicators were formulated given the preference of the best indicator over the others and the preference of the other indicators over the worst. In the next step, the data from the questionnaires were entered into the GAMS software and calculated and analyzed. The weight calculated with the value of λε obtained for the ten questionnaires was 0.097, which indicates the stability and consistency of the calculated weights due to its proximity to zero. For spatial analysis, the information layers of the indicators were first digitized and edited in the GIS software, and each of the indicators was multiplied by the significance coefficient calculated by the BWM method through conversion of the information layers into rasters and their standardization with large and small fuzzy functions and Boolean logic. Using the weighted sum of the indicators to measure vulnerability, the grid and semi-grid urban forms in Salmas were addressed separately. 

Among the calculated weights of the indicators in GAMS, the highest concerned urban from pattern, with 0.164, and the lowest pertained to building façade, with a significance coefficient of 0.030. The average value of λε obtained for the ten questionnaires was 0.097, which indicates the stability and consistency of the calculated weights due to its proximity to zero. The results of combining the indicators in the grid urban form in Salmas demonstrated that there were 4866 parcels in the very low vulnerability zone in the grid urban form, 2719 parcels in low vulnerability, 2862 in medium vulnerability, 3435 in high vulnerability, and 430 parcels in the very high vulnerability zone. In the semi-grid urban form, there were 611 parcels in the very low vulnerability zone, 2598 in low vulnerability, 3669 in medium vulnerability, 5350 in high vulnerability, and 3057 parcels in the very high vulnerability zone.
 

In general, it can be stated that the level of vulnerability in the grid urban form is less than that in the semi-grid urban form. The semi-grid urban form was found to be more vulnerable than the grid urban form based on the indicators of area, parts pattern, building density, building age, building quality, distance from rescue centers, distance from hazardous centers, building facade, population density, and building structure. Moreover, the grid urban form was found more vulnerable than the semi-grid urban form based on the indicators of distance from open spaces, range of worn fabric, distance from faults, and distance from the main thoroughfares of the city.