Introduction
Isfahan, as one of the old cities of Iran, has 2200 hectares of worn out and deficient fabric and old neighborhoods and has located in the vicinity of a nuclear site and there are 94 major and minor earthquake faults with a 200 km radius. In this regard, this city is one of regions at risk. For this purpose, it is necessary to make necessary preparations to deal with possible hazards. The present study aims to find optimal locations for multipurpose shelters in this city based on the principles of passive defense and to determine the effective indicators and criteria and their role in locating shelters. In this regard, we try to answer the following questions: What are the most important indicators for determining optimal locations for multipurpose urban shelters in Isfahan? What are the best places in Isfahan to build multipurpose urban shelters?

Methods
This is descriptive study. The information were collected by the library method and field surveys. The study indicators for locating best areas for urban shelters were categorized as demographic factors (distance from the crowded places and the city centers), physical factors (distance from main roads, industrial centers, worn-out fabric, and historical buildings), natural/environmental factors (distance from the faults, channels, rivers, land slopes, etc.) and functional factors (distance from the centers prone to enemy attack and the crisis support centers). In the next step, a database related to these criteria was prepared in ArcGIS software, version 10.5 After determining the vulnerability distances for each criterion, a spatial distance map was drawn using the euclidean distance tool.
Considering the different coefficients of these indicators in locating areas for urban shelters, the multi-criteria decision-making method was for weighting the indicators. According to Jodaki and Hassanpor (2018), 5-15 experts are enough for a comparative study. In this study, 20 experts and university professors were selected and commented on the superiority of one indicator over another using a questionnaire. Then, a matrix of pairwise comparisons was created.
After determining the weights of indicators, the output entered into ArcGIS software. After using the Raster Calculator command, the layers were combined and turned into a single layer by the Fuzzy Overly command and the final map of the optimal locations was prepared. Data analysis was done in a descriptive-analytical way using the analytic network process (ANP) in Super Decisions software, version 2.0.8 and ArcGIS software. 
To determine the conditions of the existing shelters, the shelters were classified into five groups: Completely unsuitable, unsuitable, moderate, suitable and completely suitable. The shelters whose condition was below the average were considered as unsuitable shelters and those with a condition higher than the average were considered as suitable places.

Results
Regarding the most important indicators affecting the process of locating optimal places for urban shelters in Isfahan, the results of the ANP method showed that demographic and physical criteria with weights of 0.429 and 0.056 respectively were the most and the least important effective criteria. Also, two indicators of distance from the crowded places and distance from the centers prone to the enemy attack were the most important indicators with the weights of 0.303 and 0.126, respectively, while the two indicators of distance from land slope and the roads were the least importance indicators with weights of 0.023 and 0.024, respectively.
The results showed that the best places to build urban shelters based on the principles of passive defense in Isfahan were located in the northeast (regions 10 and 14 ) and southwest (zone 5) of the city due to the existence of educational centers, the old fabric of the city, and the non-adjacency to incompatible land uses. The southeast area (region 4) was also an optimal region for urban shelters due to the concentration of office buildings on 22 Bahman Street, the density of urban and residential tissue, green space, the presence of educational and medical centers, and barren lands.

Conclusion
The current research was conducted with the aim of locating best places for multipurpose urban shelters in Isfahan based on the principles of passive defense and with the ANP method. The results showed that the northeastern and southeastern areas were the best places for building multipurpose urban shelters.

Ethical Considerations
Compliance with ethical guidelines
In this study, no experiments on animal or human samples were conducted. All publication ethics were observed.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgments
The authors would like to thank the University of Isfahan for their support.

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