标题：Application of a simplified calculation for full-wave microtremor H / V spectral ratio based on the diffuse field approximation to identify underground velocity structures
摘要：Under the diffuse field approximation, the full-wave (FW) microtremor H / V spectral ratio ( H / V ) is modeled as the square root of the ratio of the sum of imaginary parts of the Green’s function of the horizontal components to that of the vertical one.For a given layered medium, the FW H / V can be well approximated with only surface waves (SW) H / V of the “cap-layered” medium which consists of the given layered medium and a new larger velocity half-space (cap layer) at large depth.Because the contribution of surface waves can be simply obtained by the residue theorem, the computation of SW H / V of cap-layered medium is faster than that of FW H / V evaluated by discrete wavenumber method and contour integration method.The simplified computation of SW H / V was then applied to identify the underground velocity structures at six KiK-net strong-motion stations.The inverted underground velocity structures were used to evaluate FW H / V s which were consistent with the SW H / V s of corresponding cap-layered media.The previous study on surface waves H / V s proposed with the distributed surface sources assumption and a fixed Rayleigh-to-Love waves amplitude ratio for horizontal motions showed a good agreement with the SW H / V s of our study.The consistency between observed and theoretical spectral ratios, such as the earthquake motions of H / V spectral ratio and spectral ratio of horizontal motions between surface and bottom of borehole, indicated that the underground velocity structures identified from SW H / V of cap-layered medium were well resolved by the new method.
