Mapping seismic vulnerability at urban scale: Discussion on relevant cartography representations and smoothing for urban planning purposes on the Oran case study

Abstract

Earthquake damage and loss scenarios are considered as a powerful tool to help in the design of efficient seismic mitigation policies. Moreover, at a city scale, adequate urban planning may significantly contribute to improve resilience through requalification of inherited urban fabrics and/or appropriate land use. In this perspective, the present paper investigates what could be the optimal graphic representation of probable losses at urban scale, which be most relevant for seismic risk management. The goal is to provide urban planners with guidelines on how to combine the use of vulnerability index-based damage estimates and GIS tools, to identify urban fabrics that need requalification. The proposed methodology consists in starting from damage estimates at the individual building scale and using GIS (Geographic Information Systems) capacities to aggregate the expected damage on neighborhoods of increasing size, from blocks to large urban districts, in order to delimitate homogeneous urban zones. The criteria thus for choosing the optimal aggregation level are based on the need to obtain legible maps combining robust damage estimations – which implies statistics over a large enough number of buildings –, and a clear identification of the most vulnerable urban areas on which dedicated actions should be focused in priority.These methodologies are presented and tested on the example of the city of Oran (Algeria), and on the basis of a prior vulnerability study based on GNDT and RISK-UE vulnerability index approaches. The main outcome is the identification of different urban zones, which exhibit some homogeneity regarding both their average seismic response and their urban function. The most vulnerable urban fabrics require not only individual retrofitting measures at building scale; they also need urban requalification because it is more convenient to deal simultaneously with urban and safety considerations.