Spatial variation characteristics of shear wave velocity structure and its application to quaternary deep sediment layers in Suzhou region

The shear wave velocity of soils is one of the most important parameters to evaluate the seismic effects of engineering sites. Based on large numbers of borehole shear wave velocity measurements and the engineering geological characteristics in Suzhou region, the spatial variation characteristics of shear wave velocity structure for the quaternary deep sediment layers are analyzed, and the empirical equations of shear wave velocities with depth in different engineering geological zones are given respectively. Moreover, based on the significant correlation between the average shear wave velocities to different depths, the gradual extrapolation method for estimating the values of deep shear wave velocity from a shallow shear wave velocity profile is established. The validated results show that: (1) The spatial variations of equivalent shear wave velocities for ν_s20 and ν_s30 in Suzhou City exhibit obvious regional dependency. It will be more appropriate to use ν_s30=170 m/s as the threshold value for the site class Ⅲ and Ⅳ with overburden thickness d>80 m. (2) The variations of shear wave velocities with depth in the western and eastern plain zones can be fitted by power-law functions. After subdividing hill zones, the variations of shear wave velocities with depth in the three subdivided hill zones can be fitted by quadratic polynomial functions. (3) The gradual extrapolation method has a good applicability for deep shear wave velocity, but when the shear wave velocities of soils under extrapolation initial depth have dramatic changes, the extrapolation method is inapplicable. (4) The depths (H_rock) of seismic bedrock surfaces with shear velocity not less than 500 m/s or 700 m/s and the corresponding predominant periods (T_g) under microseism can be obtained by the gradual extrapolation method. It is found that the spatial variations for both T_g and H_rock are similar. By choosing the interface of soils with shear velocity not less than 700 m/s as the seismic bedrock surface, the differences of T_g and H_rock among different engineering geological zones significantly increase.