2026 年 26 巻 2 号 p. 2_150-2_172
We have identified microtremor horizontal-to-vertical spectral ratios (HVSRs) at numerous grid observation points in southeastern area of Saitama Prefecture within a rectangle with side lengths of approximately 35 km, at an average interval of about 1 km for 853 grids. Peak frequencies were read from the obtained HVSRs within a frequency range from 0.5 to 20 Hz, from which their spatial distribution was deduced (i.e., a peak frequency map). When multiple local peaks were observed within the analysis frequency range for reading, the amplitude difference between the peak and the corresponding trough was evaluated. Taking into account this information, we set a peak selection threshold and a weighting criterion and then selected a single peak to read the frequency. The resulting peak frequency map was compared with a pre-existing 3D geological structure model (i.e., the urban geological map, the UGmap) constructed based on numerous borehole data (< 7338 boreholes). They are in good agreement in their topographical and geological distributions. For example, in areas with thick alluvium (> 20 m) in the Arakawa and Nakagawa Lowlands, the peak frequency averaged a low value of 1.3 Hz, whereas in areas without alluvium, the average peak frequency was much higher at 4.7 Hz. The peak frequency of the HVSRs can be considered to be an indicator for quantifying qualitative assessments of ground conditions. The resulting peak frequency maps will be released on the website of the UGmap to enable detailed comparisons with geological distributions. We plan to construct and release similar peak frequency maps for the other regions involved in the UGmap in the future.
