Methods for constructing VS30 maps from geology, borehole and topography data
Keywords:VS30, Ho Chi Minh city, geology, borehole, topography slope, soil classification
The average shear wave velocity in the upper 30 meters of a soil profile (VS30) is used in building codes to separate sites into classes for the purpose of earthquake engineering design. In this paper, a method of Thompson E.N. and Wald D.J. (2012) was applied to build a VS30 map of Ho Chi Minh city using, many different data such as geologic, topographic and shear wave velocities derived from the Standard Penetration Test (SPT) data of 104 boreholes. The VS30 values are mainly within two ranges of 0 -180 m/s and 180 - 240 m/s corresponding to two site classes of type D and C, respectively, according to the soil classification in Vietnam Construction Code TCVN 9386:2012. The site class of type C is mainly distributed in the districts located in the northern area of Ho Chi Minh city, while the soft soil with VS30 < 180 m/s is concentrated in the south of the city. This result shows that this method can reduce the uncertainty of VS30 map by up to 11.4% relative to the topography slope-only approach. Moreover, the average topographic slope of Ho Chi Minh city area is about 0.0024. It implies that Ho Chi Minh city area is in a stable continental region, which is suitable to apply ground motion prediction equations developed for seismic stable regions for the purpose of seismic hazard assessment. Our study is a prerequisite for the wide application of this method to build VS30 maps for other areas in Vietnam.
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