Estimation of Vs soil structures from microtremor measurements and its verification by synthetic microtremor array

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This research attempts to verify the possibilities and the limitations of estimating layered soil structure (Vs soil profile). by using Rayleigh-wave dispersion characteristics obtained from observed and synthetic microtremors.Microtremor measurements using 7 stations in triangular array were performed at 19 KiK-net seismological observation sites in Akita Prefecture. The Rayleigh-wave dispersion characteristics at each site were obtained by applying the f-k spectral method. By using the Monte Carlo method for inverse analysis, the possible Vs soil profiles were estimated and compared with the available PS well log records. The estimated Vs soil profiles when comparing with PS well log record showed low scattering for S-wave velocities slower than the fastest observed Rayleigh-wave phase velocity, and showed large scattering for S-wave velocities faster than the fastest observed Rayleigh-wave phase velocity. Most of values for phase velocities obtained by the f-k spectral method were l … More ower than 1000 m/s, meaning that the estimation of Vs soil. profile of unconsolidated soil layers is possible.To verify the above mentioned tendencies from observed microtremors, synthetic microtremors at 7 fictitious stations in triangular array were computed for 6 ground systems, which consisted of increasing number of layers from the surface ground. The 1st ground system consisted of the first upper layer overlying the subsequent second layer, considered in this ground system as half-space ; and so on, up to the 6th ground system, consisting of 6 layers overlying a half-space whose S-wave velocity is 2500 m/s.The synthetic Rayleigh-wave dispersion curve at each of the 6 ground systems were obtained after applying the f-k spectral method in their corresponding 7 fictitious vertical components of simulated microtremors. Little differences between Rayleigh-wave dispersion curves from a specific ground system were obtained. Also, large scattering for estimated Vs-soil structures were obtained for S-wave velocities faster than the fastest Rayleigh-wave phase velocity used in inverse analysis. In conclusion, the propagation of Rayleigh-waves could be restrained from a certain deeper layer. Less