Alternative Resilience Measures for Urban Systems Subjected To Seismic and Landslide Hazards

Abstract eng:
Seismic events represent nowadays a major challenge for the worldwide scientific community. Besides, novel methodologies for a prompt and efficient response are needed to ensure adequate levels of resilience to global communities, allowing they to be capable of recovering from a disaster to a target performance level which is at least equivalent to the pre-event condition. Resilience can also be evaluated as a proxy of the recovered urban damage. Despite the traditional approach to the singlestructure damage assessment, the overall urban environment and mutual connectivity among its components may be considered. As a point in matter, the human part of a city is the most important one, as citizens rule urban dynamics being the principal end-users of local services. Hence, when studying urban environments, a multi-faceted and human-centric approach is required to account for both physical and human components. This means that infrastructure damage can be assessed through a systemic methodology where the loss of functionality of single buildings and services as well as the entire city are considered. To this aim, an integrated framework for quantitative resilience assessment is used, where any urban system can be modelled through the graph theory. The social and infrastructure sub-systems of the urban centre are separately modelled as complex networks and overlaid to account for their interrelations as a hybrid social-physical network (HSPN). In this study, first of all efficiency measures are evaluated on the HSPN and systemic damage is assessed as a proxy of the city efficiency, soon after the earthquake occurrence. Resilience is evaluated by means of two alternative approaches proposed in the literature. Further resilience measures are proposed to evaluate urban resilience accounting for the initial state of the city damage and the ratio between the numbers of displaced citizens and inhabitants. Given that many urban systems are subjected to multiple hazards, the proposed resilience metrics are evaluated in the case of a urban centre threatened by both seismic and landslide hazards. The municipality of Sarno, Italy, which lies in an earthquake-prone region and was affected by a severe hydrogeological event in 1998, is assumed to be a case-study. Resilience is computed in relation to a hazard-compatible seismic scenario and a landslide scenario to assess the robustness and efficiency of the proposed resilience metrics under different scenarios. As a matter of fact, earthquakes are lowprobability/high-consequence events whereas landslides have got a high occurrence rate in Italy, particularly in the form of flow-type events. Furthermore earthquake damage typically involves the whole urban system whereas flow-type landslides strike some sectors only. Fragility curves are used to model the vulnerability of reinforced concrete and masonry buildings located in the case-study urban centre, both in the case of seismic and landslide risks. Also, the probability of street links to get inaccessible is considered as a function of the buildings’ height. The results show that the response and resilience of the urban system strongly changes with the type of disaster, emphasizing the role of systemic damage and population involved.

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