Redundancy index of lifelines for mitigation measures against seismic risk

Basic consideration is made on how comprehensively we can evaluate potential seismic risk of lifeline systems and gain an insight into the safety by way of information entropy, which transmits information of various kinds under uncertainties. It is clarified that a redundancy index RE defined via Shannonʹs information entropy can be an index to represent redundancy of a system and whose value plays a role to choose the best alternative for designing a system or for finding the best damage mitigation measure against earthquake hazard. The redundancy index RE takes a value between zero and unity with zero for no redundancy, and unity for maximum redundancy. After the validity of RE is justified to represent a redundancy measure, a case study is conducted for a Kobe water supply system to evaluate a mitigation measure against earthquake risk where the 1995 Hyogoken-Nanbu earthquake is used as the input hazard.