Investigation of influence of dipping structures on microtremor exploration from case study research

Shallow S-wave velocity structures are estimated from dispersion curves of phase velocity, H/V spectral ratios, etc., using microtremor exploration technique. However the estimation is originally based on the assumption that layers of media are horizontally stratified. So, if layers of media incline or are discontinued, the estimated structures have errors to some extent. In this study, we tested a procedure of microtremor exploration for a dipping structure and examined the influence of the horizontal stratification assumption on the estimated structure. The followings are brief description of our procedure. At first, we determined S-wave velocity structures apart from the dipping area as references using the SPatial Auto Correlation (SPAC) method. The corresponding fundamental peak frequency of Rayleigh wave ellipticity was also obtained. Using the information from the reference structures, the dipping structure was determined by the H/V spectral ratios which were obtained along the dipping structure under the assumption of the horizontal stratification. We found that the fundamental peak frequency shifts toward lower frequency smoothly as the structure becomes deeper. Using the determined structure, we numerically calculated influence range of the dip on the wave filed. As a result, it was revealed that the structure within one wavelength likely gives effect on the wave field and disturbs the H/V spectral ratios.