Topographic amplification of ground motions incorporating uncertainty in subsurface soils with extensive geological borehole data

Most three-dimensional simulations of seismic topographic amplification use generic topographic features and soil profiles, because geological data are often very limited. The resolution of topography and subsurface data inevitably influences the accuracy of numerical simulation. Due to its complexity, uncertainties and spatial variability in subsurface soils have not yet been considered in most seismic topographic amplification analyses. In this study, efforts have been devoted to reconstruct a high-resolution 3D subsurface soil model for the Hong Kong Island, by leveraging the most extensive geological borehole data and a digital elevation model. A group of computational cases are designed to characterize different levels of uncertainties associated with subsurface soils, with particular emphasize on key influential factors such as variation in soil strata depth, irregular mckhead and spatially varied shear-wave velocity. Results show that topographic amplification at high frequencies is significantly influenced by the detailed subsurface model. The effect becomes more pronounced when an irregular geometry of rockhead and soil strata distribution are considered. The study demonstrates incorporating the uncertainties can increase averagely by 25% of topographic amplification at high frequencies, and highlights the importance of an accurate description of subsurface lithology through complete geological investigation to make detailed analyses.