During the Iwate-Miyagi Nairiku Earthquake (M7.2) of June 14, 2008, seismic motions with a maximum acceleration of 1,024 cm/s2 in the stream direction were recorded at the foundation bedrock of Aratozawa Dam, a rockfill dam located approximately 16 km from the epicenter. However, the maximum response acceleration in the same direction near the center of the dam crest was 525 cm/s2, and the acceleration amplification ratio of the dam body was far lower than that normally considered for a rockfill dam. Furthermore, it was measured that the crest settled down 19.8 cm after the earthquake. In this study, the dynamic properties of the embankment materials were identified using reproduction analysis of past earthquakes, and the recorded behaviors of the dam body during the mentioned strong earthquake were simulated. The generating mechanism of the peculiar earthquake behavior was investigated based on the results of the earthquake response analysis. Furthermore, in order to understand the deformation mechanism, sliding stability analysis and cumulative damage analysis were performed. According to the results, the residual deformation of the dam body after the strong earthquake is inferred to be caused by the shaking settlement of the embankment materials.

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The earthquake records of a concrete gravity dam with eight seismographs were spectrally analyzed to clarify the characteristics of the seismic motions at each monitoring station. Based on the analysis results, whether it is appropriate to use such earthquake records to generate the input ground motion in assessments of the seismic performance of dams was discussed. It was found that the earthquake records at the dam base show reduced amplitudes at frequencies corresponding to the natural vibration modes of the dam. When these earthquake records are used to generate the input ground motion in an assessment of the seismic performance of a dam, sufficient consideration should be given to this feature. Suggestions on where to install seismographs at dam sites are presented. Based on the numerical simulation of the seismic response of the dam, it is suggested that the engineering bedrock should be considered to be located at a depth of at least 1.5 times the dam height, and the shear wave velocity of the rock at that depth should preferably be 2000 m/s or more. Since this study uses seismic motions recorded across the entire dam site, including the deep rock, which is globally rare, it is considered that the basic conclusions obtained in the article can provide a reference for other concrete dams.

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