250TH ANNIVERSARY OF THE 1755 LISBON EARTHQUAKE Seismic risk evaluation for an urban centre

Risk has been defined, for management purposes, as the potential economic, social and environmental consequences of hazardous events that may occur in a specified period of time. However, in the past, the concept of risk has been defined in a fragmentary way in many cases, according to each scientific discipline involved in its appraisal. From the perspective of this article, risk requires a multidisciplinary evaluation that takes into account not only the expected physical damage, the number and type of casualties or economic losses (direct impact), but also the conditions related to social fragility and lack of resilience conditions, which favour the second order effects (indirect impact) when a hazard event strike a urban centre. The urban seismic risk evaluation is proposed from a holistic point of view; that is, an integrated and comprehensive approach to guide decision-making. Evaluation of the potential physical damage (hard approach) as the result of the convolution of hazard and physical vulnerability of buildings and infrastructure is the first step of this method. Subsequently, a set of social context conditions that aggravate the physical effects are also considered (soft approach). In the method here proposed, the holistic risk evaluation is based on urban risk indicators. According to this procedure, a physical risk index is obtained, for each unit of analysis, from existing loss scenarios, whereas the total risk index is obtained by factoring the former index by an impact factor, based on variables associated with the socio-economic conditions of each unit of analysis. Finally, examples of the model application are given for two urban centres: Bogotá and Barcelona. METHODOLOGY OF EVALUATION The report Natural Disasters and Vulnerability Analysis (UNDRO, 1980) proposed the unification of disaster related definitions as hazard (H), vulnerability (V), exposed elements (E) and risk (R) and suggested one expression to associating them, that is considered a standard at present, V H E R ⋅ ⋅ = (1) Based on this formulation several methodologies for risk assessment have been developed from different perspectives in the last decades, and recently a holistic approach for the case of urban centres (Cardona and Hurtado 2000; Masure, 2003). Cardona (2001) developed a conceptual framework and a model for seismic risk analysis of a city from a holistic perspective. It considers both “hard” and “soft” risk variables of the urban centre, taking into account exposure, socioeconomic characteristics of the different districts (units) of the city and their disaster coping capacity or degree of resilience. The model was made to guide the decision-making in risk management, helping to identify the critical zones of the city and their vulnerability from different professional disciplines. This article presents an alternative method for urban risk evaluation based on Cardona’s model (Cardona, 2001; Barbat and Cardona, 2003), using a holistic approach and describing seismic risk by means of indices. Expected building damage and losses in the infrastructure, obtained from future loss scenarios are basic information for the evaluation of physical risk in each unit of analysis. Starting from these data, a physical damage index is obtained. The holistic evaluation of risk by means of indices is achieved affecting the physical risk with an impact factor or aggravating coefficient, obtained from contextual conditions, such as the socioeconomic fragility and the lack of resilience, that aggravate initial physical loss scenario. Available data about these conditions at urban level are necessary to apply the method. An explanation of the model is made further and also some examples of application for the cities of Bogotá, Colombia, and Barcelona, Spain, are described to illustrate the benefits of this approach that contributes to the effectiveness of risk management, inviting to the action identifying the hard and soft weaknesses of the urban centre. Figure 1 shows the theoretical framework for the alternative model. Fig. 1. Theoretical framework and model for holistic approach of disaster risk (adapted from Cardona and Barbat, 2000) From a holistic perspective risk, R, is a function of the potential physical damage, Dj, and an impact factor, If. The former is obtained from the susceptibility, γDi, of the exposed elements to hazards, Hi, regarding their potential intensities, I, of events in a period of time t, and the latter depends on the social fragilities, γFi, and the issues related to lack of resilience, γRi, of the disaster prone socio-technical system or context. Using the meta-concepts of the theory of control and complex system dynamics, to reduce risk it is necessary to intervene in corrective and prospective way the vulnerability factors and, when it is possible, the hazards directly. Then, risk management requires a system of control (institutional structure) and an actuation system (public policies and actions) to implement the changes needed on the exposed elements or complex system where risk is a social process. In this paper the proposed holistic evaluation of risk is performed using a set of input variables, herein denominated descriptors. They reflect the physical risk and the aggravating conditions that contribute to the potential impact. Those descriptors, listed forward, are obtained from the loss scenarios effects and from socioeconomic and coping capacity information HAZARDS