Natural hazard risk of complex systems – the whole is more than the sum of its parts: II. A pilot study in Mexico City

Abstract. Assessing the risk of complex systems to natural hazards is an important and challenging problem. In today's intricate socio-technological world, characterized by strong urbanization and technological trends, the connections, interdependencies and interactions between exposed elements are crucial. These complex relations call for a paradigm shift in collective risk assessments, from a reductionist approach to a holistic one. Most commonly, the risk of a system is estimated through a reductionist approach, based on the sum of the risk of its elements individually. In contrast, a holistic approach considers the whole system as a unique entity of interconnected elements, where those connections are taken into account in order to more thoroughly assess risk. To support this paradigm shift, this paper proposes a new holistic approach to assess the risk in complex systems based on Graph Theory. The paper is organized in two parts: part I describes the proposed approach, and part II presents an application to a pilot study in Mexico City. The choice of Mexico City as the study case allows demonstrating the importance of modelling connections and interdependencies to assess risk in a complex urban environment which, in this case, is characterized by increasing risk of urban flooding due to soil subsidence and the presumable failure of the drainage system. In the application, the complexity of Mexico City is depicted by modelling certain selected typologies of elements (i.e. population, fire stations, hospitals, fuel stations, schools and crossroads) and assuming certain rules of connection between them. The relevant graph properties are computed and interpreted from a natural hazard risk perspective, and a simple hazard scenario is then integrated in the analysis. This study highlights both the potential and the relevance of the graph-based approach for natural hazard risk assessment.