2025 Volume 25 Issue 12 Pages 12_121-12_140
We have identified microtremor Horizontal-to-Vertical (H/V) spectral ratios at numerous grid observation points in southeastern Saitama Prefecture in an area within a rectangle with a side length of approximately 35 km; average interval of about 1 km; 853 grids. Peak frequencies were read from the obtained spectral ratios 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. By 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) that was constructed based on numerous borehole data (∼7338 boreholes). They are in good agreement in topographical and geological distributions. For example, in areas with thick alluvial deposits ( > 20 m) in the Arakawa and Nakagawa lowlands, the peak frequency averaged a low value of 1.3 Hz, whereas in areas without alluvial deposits, the average peak frequency was much higher at 4.7 Hz. The peak frequency of the H/V spectral ratios can be considered to be an indicator for quantifying qualitative assessments of ground conditions assumed based on topography and 3D geological structure, as well as for forecasting earthquake hazards. The resulting peak frequency maps will be released on the website of the urban geological map to enable detailed comparisons with geological distributions. We have a plan to construct and release similar peak frequency maps for the other regions involved in the urban geological map in the future.
