ESTIMATION OF S-WAVE VELOCITY STRUCTURES BY USING MICROTREMOR H/V SPECTRA TECHNIQUE BASED ON BOTH RAYLEIGH- AND LOVE- WAVES

摘要

It is important to clarify basement structures of urban areas from a viewpoint of earthquake engineering. Microtremor H/V spectra techniques had been conventionally used to estimate the basement depth based on medium responses of horizontal to vertical amplitude ratio of Rayleigh wave. It is insufficient to consider only Rayleigh wave because microtremor consists of not only Rayleigh wave but also Love wave. This study tried to estimate the velocity structures from microtremor H/V spectra based on the theory of surface wave H/V spectra [Arai and Tokimatsu [1]]. This theory needs both Rayleigh and Love waves to explain microtremor H/V spectra. But, the power ratio of Rayleigh and Love waves in microtremors had not been estimated. We apply three-component spatial auto-correlation analysis, which we have developed, to obtain the power ratio of Rayleigh to Love waves in microtremors. By using this analysis we can obtain the power ratio based on tri-axial microtremor array records empirically. We carried out microtremor observations at three sites in Morioka City, the northern Honshu, Japan. We employ a circular seismic array with seven tri-axial sensors. The array observation aims to obtain the dispersion curve of phase velocity of Rayleigh wave and the power ratio of Rayleigh to Love wave, respectively. Estimated power ratios were around 1.0 at all sites. Therefore, we assumed that the ratios were 1.0 in order to calculate surface wave H/V spectra. The microtremor H/V spectra were estimated from the average of spectra at all sensors. We used genetic algorithm inversion technique to determine velocity structures from microtremor H/V spectra because the inversion of structures from microtremor H/V spectra is a non-linear problem. They also agree with the structures estimated from the dispersion curves of Rayleigh phase velocity by using a classical array technique. And synthetic surface wave H/V spectra calculated from the structure are concordant with observed microtremor H/V spectra. The results suggest that the use of both Rayleigh and Love wave is necessary to estimate velocity structures using microtremor H/V spectra.