Abstract: To study the engineering characteristics of sub-ground motions,three components of 120 ground motions on surface and in subsoil recorded by six CSMIP geotechnical arrays of the United States from 28 earthquakes were analyzed.The variations of amplitude of earthquake acceleration,normalized response spectrum(NRS) and bi-normalized response spectrum(BNRS) with depth were evaluated.It was concluded that,the amplitude of acceleration of ground motions decreased with depth and the attenuation rate was higher in the shallow strata than that in deeper ones.NRS was strongly related to site conditions and depth,while the BNRS had better consistency among all strong motion records than the conventional NRS under the influence of site condition,depth and motion component.Additionally,features of Fourier amplitude spectrum and variation of duration of ground motions with depth in Treasure Island were discussed.This study not only revealed the engineering characteristics of sub-ground motions,but also provided criterion in determining the seismic design for embedded structures and pipelines.