ABSTRACT
The term “spatial variability of seismic ground motions” denotes the differences in amplitude and phase content of seismic motions recorded over extended areas or within the dimensions of a structure. The goal of the present study is to quantify, using in-situ measurements, the spatial variability of the ground motions at the interface of an arch dam and its foundation rock and do the comparison with the free field variability. 58 events are used herein, coming from a very dense seismological network installed during the period of one year in the 69 m-high double curvature Saint Guérin arch dam (Beaufort, Savoie, France). Both amplitude (in terms of differences of Fourier amplitudes) and phase (in terms of lagged coherency) variability of the ground motions are evaluated at the dam-foundation rock interface and compared with the free-field motions and with existing variability models. The comparison with the free field reveals that local topography at the interface contributes to a small increase of spatial variability of the ground motions at the interface. Additionally, higher phase and amplitude variability is observed around the frequencies of vibration of the structure. The good fit of the observations with models calibrated on flat sites without the presence of an engineered structure implies that the contribution of local topography and the presence of the arch dam, although they are additional causes of spatial variability of the ground motions, they remain secondary.
