Journal of Spatial Analysis

Urban planning has to perform seismic pathology of urban streets in seismic cities. Streets and roads are the most important spaces and urban elements in the cities which should be considered not only in space occupation and connecting spaces and urban activities but also in seismic vulnerability and on this basis it is planned to reduce environmental hazards and on top of earthquake-related. Many physical and functional characteristics of urban spaces and the distribution and concentration of the urban population take shape to comply with the location, capacity and function of the city streets network. Therefore, one of the most essential and the most important topics in the study of seismic cities is understanding of the relation between seismicity and urban streets through seismic vulnerability studies. This paper aims to assess factors and patterns of seismic vulnerability of urban networks with a prevention planning view in the 3rd district of Tabriz City.

    This research has descriptive-analytic method and the statistical population is street network of 3rd district of Tabriz city. Data and layers of information have been prepared by documentary method and have been processed using the Delphi method and the method of ranking and rating IHWP in GIS. The main factors and indicators influencing streets vulnerability have been selected based on the eight indicators. These indicators include distance and proximity to faults, quality of buildings, the degree of closeness (width of the wall), building density, population density, the traffic service or traffic volume toward roads capacity, access to health centers and services and the land use system. The final map of seismic vulnerability has been produced by combining eight layers of information related to above mentioned indicatorsand based on it the seismic vulnerability levels and factors of the street network has been analyzed.

    The final results of the seismic vulnerability of streets have been categorized in the 5 classes of vulnerability including very low, low, medium, high and very high. From total area 18.4% is estimated very low, 29.37% low, 31.77% medium, 14.21% high and 6.22% very high. Thus, taking into account the streets with medium, high and very high degree as vulnerable axes, it is concluded that 52.2% or more than half of the streets are seismic vulnerable and other half are relatively stable.

    Within the vulnerable and unstable network, more than 20% of the streets are in high and very high vulnerable classes. Street network with high and very high vulnerability are mainly arterial streets with commercial and service land uses in the scale of trans-regional or secondary roads leading to artery of trans-regional which have high population density. These streets compose a high degree of closeness, increase in traffic service level, population density and land use system with the concentration of commercial, recreational and trans-regional land uses are the main causes of vulnerability. But, in the narrow streets (8 to 10 meters), the degree of closeness of arterial streets, traffic parameters and user system have increased the seismic vulnerability index. Spatial pattern of streets vulnerability has an increasing trend from East to West and from North to south. The results show Spatial intensity of vulnerable streets is located at the center of the district and on Vali Asr, Shariati, Aref  and Razi Streets. Thus, the efficient and sustainable streets are located in the East of the under studied district.

    The results also show that high vulnerable streets has less distance to fault and more distance from medical centers. In addition, they have high traffic and lower quality buildings and high risk land uses (electric and gas infrastructure) are located there. Since the wide streets are more often subject to less obstruction, this characteristic in seismic time cause to transfer the traffic of narrow passage to the main streets. Grid pattern of streets and frequency of intersections by slowing down the speed of the vehicle increase the volume of traffic and lead to an increase in seismic vulnerability.

As an important factor to be considered, rapid population growth, lack of resources and appropriate management has led the natural hazards threatening human societies increasingly. Although it is impossible to eliminate the effects of natural hazards, however, risk reduction and risk cities against natural phenomena has become the main topics of urban planning and design in recent years. Iran is one of the countries that are faced with numerous natural hazards. With Location and geographical characteristics, Iran is a main country located in earthquake belt; therefore earthquake is one of the main natural hazards in human settlements. Now, more than 70 percentage of Iran are at risk of earthquake. This study investigated spatial effects of urban roads and network on vulnerability in Amirieh neighborhoods. The aim of this study, identification and isolation of factors affecting the vulnerability of urban streets and quantify the effect of each factor is the vulnerability. Amirieh neighborhood with 10 (he) area, located in center of Sabzevar city. Amirieh is part of the detorated urban fabric in Sabzevar, therefore, earthquakes it is one of the main threats of this urban historic neighborhood. As a holistic approach, safety and immunization of the city is in regard with the recognition of constituent elements of urban structure completely. Comprehensive identification is aimed at reducing the vulnerability of urban and urban elements. In order to, one of the most important elements is the road network and impacts on the vulnerability of urban neighborhoods. Neighborhood is smallest unit of urban spatial planning that has the most important role in the planning and reduction at the risks of natural hazards. The spatial relationships between the components of an urban system that can fit through association with the whole city would be reduced environmental hazards, particularly earthquakes.-From planning perspective, any activity be organized in small-scale and size, will increase the possibility of its constituent elements in crisis management. The vulnerability of urban networks in related to spatial structure and impact on other infrastructure directly. The nature of the vulnerability of urban streets can be based on three factors: the structure, origin and traffic. As a structure, form and pattern of urban access associated with the vulnerability that this pattern is in related to urban network movement geometry and topological properties. Road network and access can be analyzed spatially by both composition and configuration. Composition of road network affected by the physical geometry and presented in different scales and defined by location, form, length, angle and direction. While the configuration is sets of the points witch defined by the related lines. Roads determine accessibility to critical points, and are including topological features, displacement, time travel or transport costs.

In analysis process of data and maps, scientific methods and models were used such as geographic information systems (GIS), the Analytic Hierarchy Process and method (AHP) and weighted overlaying map. Research method involves the following steps:

1. Introduction of indicators: In order to determine the vulnerability of the network in the various aspects needed to be based on the criteria established to determine the vulnerability and damaging. In this study, selected Indicators include: Type of road, the width of road, construction quality, density, population density and age of the buildings.
2. To determines the importance and ranking criteria: Each of the above criteria has the sub-criteria which based on expert opinions, and comparing them with field studies. The (AHP) was used to weight sub-criteria for the experts and paired comparison.
3. To weight the criteria: At this stage, the selection criteria are weighted by research. To determine weights, the criteria and sub-criteria, were quantified by which is determined measure the intensity excellence criterion of i to j. At this step, the above criteria and sub-criteria in the form of a questionnaire was given weight by the Group of Experts. Then, weights of each criterion was determined the final weight by Expert Choice software.
4. Layers integration and production of Vulnerability final map: in order to produce the final map of vulnerability, the command Raster Calculator and weighted overlap method was used in the GIS environment. Density calculation is one of the suitable methods of spatial analysis. we calculated the density to represent the value of points or lines in the form of levels. In this study was used Kernel density equation for converting line to surface value, due to represent of spatial value. Map applying numerical value to each pixel density is formed in the periphery.

In Amirieh neighborhood, Results show that width of streets, land use, population density, quality of construction and age of building will be in the range of medium to high vulnerability. In this study, 50% of the length of passages, more than 73 percent of the quality of the existing structure, 69% of land uses, and more than 40 percent of population density were classified in the range of high to very high vulnerability. The final vulnerability map shows that more than 58% of the total area is in the range of high and very high vulnerability. The areas with moderate vulnerability involve 19 percent of the entire neighborhood approximately. The final map shows that areas with low vulnerability appropriate width placed adjacent to the passages open while the passages the end and low width are critical zone of significant congestion. Too, the results showed that the topological characteristics of the network involved in the formation of critical points. So that in the event of a crisis and then could impair relief and evacuation of the neighborhood. From spatial perspective, vulnerability is influenced by two urban network properties:

1. Urban network structure: The street network is determined based on geometric features. This communication and spatial distribution of the points and roads in the neighborhood.
2. Spatial hierarchy: Spatial hierarchy access to the neighborhood of the important points is that the crisis could guarantee public services.