Normalized Input-Output Minimization (NIOM) analysis was conducted for the long-term earthquake observation records at Tokuyama Dam in Gifu prefecture, Japan operated by the Japan Water Agency, and the propagation times of the seismic waves and the dynamic soil properties in the dam body were investigated. The results revealed that (1) the propagation time in the upper part of the dam body increased with an increase in the reservoir water level during the test impoundment, and decreased with elapsed time after the test impoundment, whereas (2) the propagation time in the lower part of the dam body decreased monotonically and independently of the reservoir water level, and (3) almost no degradation of the dam body was found during the analyzed period in this study.

To establish a method for structural soundness diagnosis of concrete dams hit by earthquakes by focusing on changes in vibration characteristics of a dam body, time series of transfer functions calculated from seismic motion records obtained at the foundation and the crest of dams were analyzed. The results revealed that the natural frequency of several gravity dams hit by strong motions but suffered no obvious damages temporally decreased perhaps due to a behavior of transverse joints during earthquakes, while the analysis about an arch dam structurally damaged by a strong earthquake revealed that the natural frequency of the dam decreased discontinuously during the earthquake and did not recover even in later years.

Structure design using partial safety factors is being introduced in some civil structures. Safety of concrete gravity dams designed by existing design criteria during big earthquakes is discussed theoretically. Introduction of partial safety factors in structure design of concrete dams is proposed.