2025 Volume 25 Issue 4 Pages 4_241-4_253
Microtremor measurements were conducted at a reclaimed site on the Tama Hills to investigate the distribution of ground-motion characteristics in this area. The area is divided into three levels, with elevations ranging from 110 to 150 meters. The upper level consists of naturally deposited loam layers, while the middle and lower levels are composed of subsurface soils and fills. It was found that the peak period of the H/V spectral ratio of microtremors (Tp) increases with the thickness of the subsurface soils at the borehole points. The result enabled us to conduct high-density microtremor measurements on a cliff edge and in a plaza of the middle level, then depict the spatial distribution of Tp. Measurements at the cliff edge indicated that ground motion was likely amplified in the direction perpendicular to the edge. The results showed that spatial changes in Tp correspond well with the old topography. Additionally, it was found that the H/V spectral ratio of fill areas where landslides occurred in the past exhibits marked peaks, and Tp evaluated at the station where seismic amplification was observed corresponds to the peak period of strong motion spectra.
