SEGJ Technical Conference

Estimation of damping factors of deep sedimentary layers from spectral ratio of S-wave observed in vertical arrays in western part of Kanto basin

Abstract Spectral ratios of earthquake records in vertical borehole arrays at two sites of the KiK-net in Saitama prefecture in the western part of the Kanto basin were inverted to 1D profiles of S-wave velocity and Q-value of sedimentary layers for a reliable estimation of amplification factors at high frequency. We used the Markov Chain Monte Carlo method by Yamanaka et al. (2013) in the inversion to determine final model parameters with their resolutions. At first, earthquake data at the two KiK-net stations were analyzed for the spectral ratios between the surface and the bottom at a depth of -100 meters approximately of the boreholes. The spectral ratios were inverted to 1D-profiles. S-wave velocities and the parameters of Q-values for sampled models in the MCMC method show narrow frequency distributions indicating high resolutions. In particular the S-wave velocities for the shallow part have narrow distributions. We estimated S-wave amplification factors using the sampled models. Since the uncertainties for the S-wave velocities and the parameters for the Q-values are very small, the amplification factors of 1D S-waves are also stable with small fluctuations.