Seismic Site Classification from Surface Wave Data to v s, 30 without Inversion

Surface wave methods have been extensively employed to estimate time-averaged shear-wave velocity (Vs) to 30 m (Vs,30) for seismic site classification. However, the inversion of surface wave dispersion data for Vs profiling is inherently computationally intensive. To overcome the challenge involved in surface-wave inversion, a new method is proposed to directly estimate Vs,30 from surface wave fundamental-mode dispersion data without inversion. The new method is based on the concept that the phase velocity of surface waves (VR) at a given frequency (f) is proportional to the average shear-wave velocity of soils within one-half of the wavelength (λ=VR/f). The proposed method starts from calculating the Vs of the top layer as it can be directly estimated from the phase velocity of surface waves whose half wavelengths are smaller than the thickness of the top layer. Likewise, the Vs of the second layer can be deduced from the top layer Vs along with the phase velocity of surface waves whose half wavelengths are within the thickness range of the second layer. Through such a repetition of sequence-deduction, the Vss of the other layers can be calculated. This method is verified with five case studies that cover typical geotechnical sites. Results indicate that the proposed method can obtain a similar Vs,30 with the same site classification for seismic design in comparison with invasive tests and inversion techniques.