Evaluation of seismic wave coherency considering spatial variability of model ground

This study experimentally evaluates seismic wave coherency with respect to the spatial variability of model ground using a 1g shaking table and a Laminar Shear Box (LSB). Two types of model grounds, silica sand and weathered soil, were constructed to analyze the effect of spatial variability on seismic wave coherency. The spatial variability was assessed by measuring the shear wave velocity using a Miniature Cone Penetration Test (Mini-CPT). The silica sand model ground exhibited a coefficient of variation (CV) of 18.73 %, indicating relatively homogeneous characteristics, whereas the weathered soil model ground showed a CV of 45.71 %, reflecting high heterogeneity. Unlagged and lagged coherency calculations were performed using 30 scaled seismic records with a peak acceleration of 0.03 g. The results showed that coherency decreased with increasing frequency and separation distance, with a particularly pronounced reduction in the high-frequency range. The weathered soil model ground exhibited a steep decline in coherency above 20 Hz due to its heterogeneous composition, while the silica sand model ground maintained relatively stable coherency across all frequency ranges. The coherency regression analysis was performed in the atanh domain and back-transformed to the original scale for comparison with the measured data. This study highlights the importance of considering spatial variability in seismic design, particularly in high-frequency regions where coherency reduction becomes significant. These findings contribute to developing more reliable site-specific seismic design methodologies.