The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China A PROCEDURE FOR RISK MITIGATION OF WATER SUPPLY SYSTEM IN LARGE AND POPULATED CITIES Mahmood Hosseini 1 and Hazhir Moshirvaziri 2 1 Associate Professor, Structural Engineering Research Center, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran, Email: hosseini@iiees.ac.ir 2 Graduate Student, Earthquake Engineering Department, Engineering School, Science and Research Branch of the Islamic Azad University (IAU), Tehran, Iran, Email: hazhirm@yahoo.com ABSTRACT : The complexity of lifeline systems and their operation, their high number of components, and the variety of age in an old existing lifeline system, such as Tehran water supply, cause that all parts of the system can not be upgraded to the same level of performance. In this paper a procedure is proposed for mitigating the seismic risk of water supply system in large cities in which all important factors with regard to system components, including seismic resistance to various seismic hazard levels, component’s age, importance of the component role in the function of the whole system, reparability of the component and so on are taken into consideration. Furthermore, as applying the mitigating measures are very costly in case of large cities, and the required technology for applying the measures is not always available in all countries the economical and technological situations of the country at hand are also included in the propose procedure. One of the main features of the proposed procedure is considering the total value of mitigation costs and earthquake induced losses as a target function for minimization, and also considering the variation of costs with passage of time. On this basis the mitigation measures can be divided into three -short-, mid-, and long-term categories, and the optimal upgrading level for every component in every part of the city in each time window can be obtained by a high level of confidence. To show the efficiency of the proposed procedure it has been applied to Tehran metropolis. KEYWORDS: Operation level, Mitigation costs, Technological considerations, Tehran 1. INTRODUCTION Earthquake occurrence in large and populated cities, which are like Tehran highly vulnerable, can be disastrous. Therefore, every mitigative measure which can decrease the seismic vulnerability of a large city is very well acknowledged. Obviously, mitigation is more important with regard to lifeline systems, such as water supply system, as they can have a disaster creative role if they can not withstand the earthquake effects. On the other hand, applying the mitigative measures in an existing lifeline system is much more difficult than a building. In fact, the complexity of lifeline systems and their operation, their very high number of components, and the variety of age in an old existing lifeline system, such as Tehran water supply, cause that all parts of the system can not be upgraded to the same level of performance. In many cases the old components, which do not have acceptable operation situation, are continuously, or on a regular basis, replaced or renewed. Therefore, it is not reasonable and, in many cases, possible to apply the same upgrading measure to even similar component which have different ages. On this basis, it would be reasonable to look for some new upgrading methodology to be employed for such cases. Up to now, several guidelines and commentaries have been presented by researchers with regard to seismic design or upgrade of water supply systems. As one of the earliest works in this regard the monograph which has been published in early 90s, although not dedicated to just water systems, can be mentioned (Central United States Earthquake Consortium, 1993). In that monograph there is one chapter discussing on water and sewage systems, however, it gives just some general guidelines which is not enough for upgrading purposes, particularly for cases mentioned above. It is also worth mentioning that there are two relatively recent