The “Seismic Behaviour” of Urban Complex Systems

The analysis of losses suffered by city from seismic events is very often carried out focusing on human losses and damages to buildings and to network infrastructures. Up to the present, studies aimed at understanding the “systemic” dimension of the damage of the city as a whole are still few. However, such studies emphasize that the damage of a city due to an earthquake cannot entirely be expressed through the sum of physical damages of urban system elements or through the consequent human sufferings. There is, in fact, a “functional” dimension of the urban damage which is not entirely referable to the physical damage. It depends on spatial and functional organization of the city, or in other words on the complexity of the hit system. With reference to these considerations, this paper delineates the bases of the “seismic behaviour” of urban systems hit by a seismic event. The first part of the paper underlines, through international case-studies about impacts of earthquakes on different cities, the different aspects of the damage suffered by urban systems from seismic event, in order to underline the “functional” dimension of the damage. Basing on analysis of the phenomenology of the earthquake, the theory of the “seismic behaviour” of the city has been worked out. The latter aims at understanding how the cities, considered as complex systems, react to an earthquake. The theory bases on the analogy among evolution of the city hit by an earthquake and the evolution of complex systems hit by an external perturbation. Such evolution is determined by some factors which affect the possible answer of urban system to the external stress represented by the earthquake: restoration of the equilibrium, death of the system, evolution toward a new organization. In the last part of the paper, a performance model, aimed at knowing the efficiency deficit of the system hit by an earthquake has been outlined. It is based on the comparison between the “demand” of the urban population hit by the earthquake and the “supply” of the urban system, measured in terms of specific urban performances. Basing on this knowledge, strategies and actions aimed at facing the seismic problem in its “urban” dimension can be worked out. The theory of the seismic behaviour of the cities, in fact, points out that the mitigation of seismic risk requires both structural interventions on the building stock and actions facing the seismic problem in its “urban” dimension, highlighting the role of Urban Planning in the seismic risk mitigation.