Abstract: The average shear wave velocity to a depth of 30m below the ground surface (V_s30) is an important parameter for evaluating the site seismic condition and the site earthquake response and is widely used worldwide. But there are many bore hole data with depth less than 30m, and these bore hole data cannot be directly used to evaluate the V_s30 so that the correlated research results which include the parameter of V_s30 might not be directly referenced. Based on the data from 590 bore holes with depth greater than 30m in Guanzhong plain of Shaanxi Province, the test data were evaluated by using three models, namely, the bottom layer constant velocity model, the velocity gradient model and the extrapolation model with the travel-time averaged shear-wave velocities at two different depths. Throughout this study the V_s30 estimation model can be therefore localized. The results show that, the constant velocity extrapolation model is more suitable for estimating the V_s30 value when the bore hole depth exceeds 15m; when the depth of the bore hole is less than 15m and the amount of borehole data is large, the linear model in the velocity gradient extrapolation model is more appropriate for estimating the V_s30 value. Considering that the extrapolation model with the travel-time averaged shear-wave velocities at two different depths does not need any regression analysis to derive empirical relations from a large number of data and the total estimation error is small, this model is appropriate for the entire depth range (10-28m) and is more applicable.

Key words: Guanzhong Plain, Shear Wave Velocity, Extrapolation Models of V_s30, Applicability