A simple but high-resolution ground exploration method for building design

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The objective of this research is to establish a simple yet highly precise exploration method for shallow S-wave velocity structures of ground by using microtremors. For this purpose, the observation method and observation equipment were first improved, which would enable us simple and mobile observation. Then using that equipment we develop a system that can predict quite simply site amplification characteristics. In the first year by combining a notebook PC and PCMCIA card type AD converter, we develop a data acquisition system that makes it possible to see data online. In the second and third year microtremor measurements at observation points of K-NET and KiK-net were carried out using this system, and we analyzed these data together with the observed data obtained in the past. Comparisons with the S-wave site amplification characteristics obtained by the spectral inversion analysis and theoretical H/V characteristics by the S wave velocity structures, which were inverted from the … More site amplification characteristics, were carried out. As a result, it turned out that about 1:1 relation between the peak frequency of microtremor H/V ratios and the peak frequency of the site characteristics of the earthquake motions acquired by the spectral inversion method exists on the average. However, the variation was large and logarithmic standard deviation was found to be 2.12 times or 1/2.12 times. When we plot relationship between the earthquake motions' peak frequencies versus those of microtremor H/V ratios, it is found that we can well predict peak frequency within ±30% of error if the H/V peak level of microtremors exceeds 4 times. However, when the peak levels of microtremor H/V ratios and those of earthquake motions were compared, the levels of microtremor peaks were remarkably smaller than those of earthquake motions, and there is no good correlation between them. Thus we found that the H/V ratios of microtremors can be reliably used for detecting peak frequency if the impedance contrast of soil ground is large, but their peak levels could not be used as indicator of the earthquake site amplification. Less