Effect of the 2D Spatial Variability of Linear Soil Properties on the Variability of Surface Ground Motion Coherency

Spatial incoherence of earthquake ground motion (SIEGM), modelled by a "coherency function", can have significant effect on the dynamic response of engineering structures such as bridges, dams, nuclear power plants, and lifeline facilities. This paper deals with the study of near-surface heterogeneities effect on the coherency of surface ground motion. A set of numerical experiments are designed based on spatial variability of shear-wave velocities. The 2D spatially variable shear-wave velocities are modeled as homogeneous stationary random field and discretized by the EOLE method (expansion optimal linear estimation). Seismic ground motions were simulated within 1 Hz and 25 Hz using FLAC2D for a plane wave excitat ion with SV polarization. The horizontal and vertical autocorrelation distances that represents the size of the ground heterogeneity, range between 5 and 20 m and between 1 and 2 m, respectively. The coefficient of variation of the shear-wave velocity is varying between 5% and 40%. Computed time series clearly outline locally diffracted surface waves at the ground heterogeneity, which lead to large spatial variation of coherence that are mainly controlled by the coefficient of variation of the shear-wave velocity; the horizontal autocorrelation distance has an impact on the coherency calculation at short distances. The reliability of existing coherency models in the literature is briefly analyzed.