Abstract: Reduced order bounding surface model was employed to study 3-D seismic ground response of site considering the nonlinearity of soft soils in Shanghai. A nonlinear procedure for response analysis of horizontal layered sites subjected to multidirectional earthquake loading was used to study vertical and horizontal ground responses, respectively, under unidirectional and multidirectional base excitations (Taft). Also, the development of ratio of vertical and horizontal peak acceleration and the effects of the variation of groundwater table on the characteristics of seismic responses were studied. The study results show that the characteristics of vertical seismic ground response are much different from those of horizontal seismic ground response; the amplification effect is greatly increased under multidirectional base excitations compared with that under unidirectional base excitations; greatly difference is induced by the rise of groundwater table between vertical and horizontal seismic response, as result of the rise of groundwater table, seismic amplification effect at ground surface increases in horizontal direction, but decreases in vertical direction，which are important for geotechnical engineering aseismic design in Shanghai region.

Key words: 3-D nonlinearity, seismic ground response analysis, reduced order bounding surface model, multidirectional earthquake loading