Nonlinear 3D Seismic Site Response Analysis Considering Spatial Variation of Geological Conditions

Site response analysis is important for assessing ground motions and seismic hazards at a specific location by given an earthquake event. 1D site response analyses have been widely used in engineering applications due to its simplicity and computational efficiency by assuming the soil strata is horizontally layered. However, a real site may have diverse and complex geologic conditions, such as irregular topography, highly variable rockhead, varying soil profiles, and formation of sedimentary basin etc., leading to complexed 3D wave prorogation and ground distribution at different locations. Therefore, 3D seismic site response analysis is much needed to incorporate these geological features for better simulation of ground motions. In this study, we develop a large-scale 3D geological model for Ma On Shan area in Hong Kong for seismic site response analysis in order to assess the influence of complex geology on ground motion distribution. The detailed 3D geological model is 1.3 × 1.6 km with a spatial resolution of 10 m. Through the study area, fills are underlain by alluvium, marine deposit, weathered granitic rock etc. A fault zone crosses the study region. Higher weathering in the fault zone results in a deep rockhead up to 120 meters, while marble and granite on both sides of the fault zone have much shallower rockhead. This rockhead feature significantly enhances the ground motion,which cannot be captured by 1D site response analysis.Another feature of this study is to develop a fully nonlinear constitutive model into the large-scale situation to capture the nonlinearity of soils undergoing large strains.The nonlinear 3D site response analysis shows that the spatial variation of geological condition contributes significantly to site amplification of ground motions.

Keywords: Site response, Numerical simulation, 3D site effects, Spatial variation, Nonlinear soil model.