Spatial variability of earthquake ground motions at the dam–foundation rock interface of Saint Guérin: experimental and numerical investigations

The goal of the present study is to quantify the spatial variability of seismic ground motions (SVGM) at the arch dam–foundation rock interface, using data coming from a case study, and the effect of the presence of the structure and the local topography on it. For this purpose, a dense seismological network, consisting of 19 velocimeters, was deployed during 1 year in the Saint Guérin arch dam in France. Fifty-five recorded events are used for the analysis. Both amplitude (in terms of differences of Fourier amplitudes) and phase variability (in terms of lagged coherency) are evaluated at the dam–foundation rock interface and in the free-field. Two lagged coherency models, Menke et al. (Bull Seismol Soc Am 80:430–449, 1990 ) and Ancheta et al. (Proceedings of 4th IASPEI/IAEE international symposium on effects of surface geology on seismic motion, Aug 23–26, 2011 ) amplitude variability model are compared with the observations in Saint Guérin and the overall fit is satisfactory. The comparison with the free field motions and the existing models imply that local topography at the interface does not contribute to a significant increase of SVGM. However, higher phase and amplitude variability is observed around the frequencies of vibration of the structure. The numerical model of the dam and a simple representation of the surrounding site confirm the in situ observations.