An irregularly layered subsurface structure (hereafter irregular site) amplifies earthquake motions sometimes more than a stratified media (hereafter flat site) due to, for example, a focusing effect of seismic waves. An exploration of a depth distribution of a structural boundary is too costly at each site in practice. A preliminary examination is desirable to discriminate sites where an amplification factor can be approximately estimated with stratified media from irregular sites. Focusing on spatial variation of horizontal to vertical spectral ratios (hereafter HVSRs) of microtremors, we performed 3 investigations with respect to a relationship between coefficients of variation of HVSRs' peak periods (hereafter CVs) and effects of irregular interfaces of a subsurface structure.
First, we conducted densely mobile microtremor measurements at 4 sites, which consist of 2 irregular sites and 2 flat sites. The values of CVs on the irregular sites are significantly larger than those on the flat sites, and they can be obviously separated.
Second, in order to reveal the characteristics of shapes of sediment interfaces affected the values of CVs, we analyzed sensitivity to CVs by numerical simulations for wave propagation with complex media. The basic subsurface structure model was constructed based on results of the drilling method at Nabari site where mobile microtremor measurements were also conducted. CVs of simulated motions with the basic model are almost consistent with the CVs by the observation. We found that slope angles and wave number of irregular boundaries of layers respectively affected amplitudes of CVs and inflection distances making a smaller inclination of CVs.
Third, we compared CVs with irregularity of subsurface structure, using results of simulated microtremors and subsurface structure models. For a comparison, we converted CVs to a power spectral density (hereafter PSD) via a semivariogram. The PSD estimated from CVs showed a good agreement with the PSD calculated from the subsurface structure model in the wave number range corresponding to interstation distances of the CVs.
Through the above investigations regarding the difference of CVs of irregular and flat sites, we concluded that CVs can be a proxy to represent an irregularity effect of sediment interfaces.
