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Individual reaction to extreme events and the emergence of collective effects in urban systems: mitigation, response and recovery

Goals of the workshop

The focus of the workshop is the individual reaction to extreme events in relation to urban systems. Through the network of social relations, individual reactions emerge in the collective response to the extreme event. Thereby, we consider complex system theory as the key for the management of extreme events. This workshop is also suppose to be the first step for the organization of an European network for the study, prevention and management of urban disasters, which should link centers devoted to the study of complex systems such as the CSDC, IZKS in Bonn, ETSLAB in Tel Aviv and the Max Planck Institute for Complex Systems in Dresden.

Through the workshop, we plan to put together experts of networks, of collective and nonlinear systems with people involved in control and organization of public resources, in order to build up a synergistic work-group dedicated to the study of these phenomena. One main result of this collaboration could be the construction of a simulation environment in which strategies could be tested and observation tool constructed. A "realistic" environment should take into consideration the many aspects of a "real" environment: land usage, networks, psychological and social effects, economics and logistic aspects, just to cite a few.

The methodology should be that of studying disasters in the past, trying to extract a model simple enough to be simulated on a computer.


Disasters are events whose effects goes beyond "normality". They can be caused by exceptional circumstances (earthquakes, tsunami, war operations, terrorist attacks) by simultaneous occurrence of "normal" emergencies, or by "resonance" of a normal emergence with some critical state. The main characteristics are that the bigger the event the less likely it is, but the greater the social and economic costs it causes.

A disaster is a large perturbation of a system, or of a major component of the system, that disrupts, temporarily or permanently, established relationships and patterned behavior. Namely, urban disasters occur when an extreme event causes extensive damage to some or all of the built environment, the people and institutions, and the relationships among those people and institutions and the outside world, thus destroying the network of relations on which urban life is based.

The two aspects of the problem we want to focus on are the network and the individual response. The network represents the structure of human and technological relation, while the individual response is located at the nodes of this network. Modern society is composed by many network structures, built, reassembled and exploited by humans. For instance, a city is a network of streets, of transportation systems, of communication devices, of energy, food, waste, transportation, and over all of social contacts among people. A disaster may affect one or more of these networks. Let us for instance think of the diffusion of an epidemics. It spreads on the network of social contacts, but the consequences may affect all other networks when people start to fell ill. The network may be destroyed by the extreme events, such as the social network or the electric power network in case of an earthquake: so that a network is also dynamically affected by the extreme event.

Individual actions and reactions, are placed on the nodes of the network. Their heterogeneity, and the links of the network determine the response to the extreme event. Individual reactions try to create a new network of relations, and the human activities act in a kind of feedback loop with the occurrence of extreme events. It is for this reason that, in disaster management, planning should take into account how people and organizations are likely to act, rather than expecting them to change their behavior to conform to the plan.

Reactions to the extreme events pass through three phases:

  • prevention, mitigation, which is related to the way people evaluate the risk;
  • response: immediately after the event;
  • recovery and reconstruction of the social structure in a medium period.

    Prevention, mitigation

    The assessment, perception management and prevention of the risk are linked with each other. Risk assessment evaluates the likelihood and consequences of prospective risks. Risk perception is concerned with the psychological and emotional aspects of risks. Risk management involves developing strategies for reducing the likelihood and/or consequences of extreme events.

    The discussion of risk management strategies focuses on insurance and mitigation as two complementary strategies for reducing future losses and providing funds for recovery, and addresses the role of public-private partnerships in this regard.

    Mitigation in a network may be achieved by decentralization. A black-out in communications is often the result and the trigger of a disaster: while social networks are in general robust in a blind removal of nodes, there are hubs that are vital for the working of the networks itself. These hubs may fail as a result of a targeted attack, or overloading due to panic, which may be the result of peer-to-peer diffusion of uncontrolled information. A robustness could be achieved by using local resources: an energy black-out is less probable if the energy production is diffused.

    Concerning insurance, the underestimation of rare natural disasters (low-probability, high-risk events) is well documented in the literature on risk psychology. Surveys of people living in areas at risk of earthquakes and floods have repeatedly shown that individuals systematically underestimate the likelihood of such events. Events thought to lie in the distant future are strongly disregarded and no technical or financial provisions are made. The lack of demand leads to problems on the supply side. Both these factors together, and each one independently, drive up the costs and the price of insurance. But the rising price depresses demand. So any reduction in demand (whatever its cause) has a multiplier effect, depressing further demand. Education of individuals, monitoring of the right indicators and safety protocols may help in reducing or preventing the occurrence of a disaster. However, major disasters can be extremely costly to individual insurance companies. In fact they differ from common insurance in which the yearly premium covers the yearly costs: the cost of an extreme events may be unaffordable for a newly established insurance company. For this reason, reinsurance programs, which allow even wider spreading of risk, have recently prove essential.


    The first phase of the event is crucial for the mitigation of the effects. The effects of a disaster can be amplified or reduced by the collective behavior of people involved (escape/isolation/panic vs. cooperation/altruism).

    Extreme events disintegrate the social structure, because they change the usual environment. This is a process of self-organized disintegration. While many (or most) processes of self-organization indeed evolve toward higher levels of integration, others proceed in the reverse direction, namely, from high level of integration toward disintegration.

    Humans and animals exhibit three typical forms of behavior when facing an emergency situation: fighting back, freezing and become passive, and fleeing from the dangerous place, event or situation. Empirical studies further show that the 3F forms of behavior are present also in cases of extreme events that took place in cities:

  • flee: some immediately evacuate the city;
  • fight: some stay and continue their normal space-time routine;
  • freeze: some become "paralyzed" and helpless in their homes and shelters.

    These three forms of behavior give rise to emergent phenomena when their diffusion is amplified by social networks.

    The re-organization of the network destroyed by the event, or the establishment of a new special network for the management of the rescue are some of the main tasks of this phase. Decentralized, peer-to-peer communications could weaken the effects of a disaster, for instance by allowing communications in the presence of a failure of centralized structures.


    In the recovery phase the complex character of society plays a preeminent role.

    Often recovery and re-construction are seen as a planning activities whose goal is the rebuilding of a new urban fabric. Moreover, public expenses made in communities that experience extreme events are often based on oversimplified notions of what is needed for recovery (usually rebuilding or restoration). When a community is severely damaged, however, there is no return to what was before. Instead, members of the community struggle to achieve viability in a new context and a new environment. The community system morphs to another system state, which may or may not be preferable to the pre-event state.

    This happens because the extreme event does more than damage physical artifacts in the system. It also damages or destroys relationships among the individuals and institutions that include the life of the system. Recovery would be simple if only restoring the built environment would be needed. Recovery is complex and difficult because it requires establishing or re-establishing important relationships within the system and between the system and its environment.

    For this reasons, local government can influence, but not determine, whether the community recovers. Whether a community system survives and becomes viable in the post-event setting depends in part on the individual choices of a critical mass of people and institutions in the community. These persons will respond in various way. Each will define rules for making choices about whether to stay in or leave the system and, if they choose to stay, for making choices about what to do and how and where to do it. It is from the interaction of these individuals that the recovery will happen as an emergent phenomenon. So that achieving post-event viability is a function of the individual choices made by the constituent elements of the community concerning their roles, actions, and relationships.