2025 Volume 25 Issue 1 Pages 1_175-1_185
Subsurface structure model is indispensable in ground motion estimation. There are various methods to estimate the subsurface structures above the seismic bedrock. One of them, velocity model identification based on horizontal-to-vertical spectral ratios of microtremors (MHVRs), is often utilized due to its convenience. However, one-dimensional subsurface structure is often assumed when analyzing MHVRs although directional dependence can be seen in some of the observed MHVRs. The directional dependence is assumed to be related to the lateral heterogeneity of the subsurface structure but investigation of the effect of heterogeneity of subsurface structure on MHVRs is still insufficient. In this study, we perform microtremor measurements around Gokasho, Uji, Kyoto, and examine the effect of lateral heterogeneity on the directional dependence of MHVRs and phase velocities obtained from microtremors. As a result, the spatial variation of the peak frequency and the directional dependence of the amplitude of MHVRs correspond to the inclination of the Tamba group, which is the bedrock layer in the study area. In addition, it was found that the phase velocity of the Rayleigh wave is dependent of the direction of the back azimuth and the phase velocity propagating from the south is slower compared to those from the west for the frequency of 1.2 Hz and it is assumed that the velocity is slower in this frequency range in the study area.
