During the last decade, critical infrastructure systems such as electric power, water distribution, transportation and telecommunications have been stressed by significant natural hazards and human errors leading to malfunction. Earthquakes, blackouts and satellite failures have evidenced the vulnerability of these growing networked systems threatening the continuity of the United States' economy. The potential for malicious perturbations to these systems is also an important current concern. However, the underlying susceptibility of these networks to disruptive events is in large part due to the increasingly complex pattern of interdependencies that tie these civil infrastructures together. This paper mainly investigates the effect of seismic disruptions on the performance of real interdependent networks. Maintenance-induced malfunctions and coordinated attacks are also studied for completeness. Several degrees of interconnectedness are explored. Interdependent network fragility curves are introduced to display the effect of these different strengths of coupling. Characterization of the performance displayed by the networks is utilized to propose mitigation actions and study propagation of their effects. It is shown that minimal strategic interventions propagated through the network can generate a more resilient interdependent entity. reliable and constant supply of power. However, the event of 2003, besides increasing the awareness to possible deliberate attacks, pointed out another serious problem: incomplete understanding of the interdependencies among different infrastructures.Recent advances have been made to characterize the topological properties (i.e. those that solely depend upon their geometric layout) of complex independent networks. Some examples include the western US power grid [3], general communication systems [4], railroad networks [5], Internet routes [6], and the World Wide Web [7,8]. Other studies focus on finding optimal flow patterns within the networks when exposed to external disruptions and constrained by situational factors, for instance in highway systems [9,10]. In the context of perturbed systems, characterization of the seismic response of independent civil infrastructures such as power and water distribution systems, has followed two major approaches. The first approach relies upon numerical methods such as Monte Carlo simulation [11][12][13] while the second approach relies on analytical seismic reliability frameworks [14]. Although these studies represent important advances in the networks approach to lifeline analysis, modelling interdependent networks is still an open research field. Awareness of interdependencies has increased within several academic disciplines and institutions, including emergency management and governmental organizations [15].Current modelling attempts to capture network interdependencies include the use of economic input-output frameworks [16], or complex-adaptive simulations (CAS) [17,18]. Approaches based on CAS treat network nodes as agents with locatio...

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