Influence of Site Parameters on Spatial Coherency of Seismic Ground Motion: Evidence from Chlef City, Algeria

The dynamic response of extended structures, particularly on heterogeneous soils, is strongly influenced by the spatial variability of seismic ground motion (SVGM), yet most existing coherency models either neglect site effects or rely on a single parameter. This study integrates two key site proxies—the fundamental soil frequency (f0) and the average shear wave velocity in the top 30 m (Vs30)—to evaluate their combined influence on lagged coherency using seismic data from 15 earthquake events recorded by a nine-station array in Chlef City, Algeria. The effect of f0 is first assessed observationally for three components (EW, NS, and vertical), followed by nonlinear regression to develop a Vs30-based coherency model. Results indicate that lagged coherency increases with higher f0 and Vs30 (stiffer soils) and decreases with increasing frequency and separation distance, with both parameters providing complementary insight into site effects. The vertical component generally exhibits higher coherency than horizontal components, emphasizing the importance of three-dimensional ground motion. Overall, the proposed framework improves the representation of the site effects in coherency modeling and offers a more realistic basis for the seismic analysis and design of extended structures subjected to spatially variable ground motions.