Abstract
On the Musashino Upland in Kodaira City, Tokyo, there are depressions approximately 1 to 3 meters deep. In order to clarify the cause of the depression, microtremor measurements were taken inside and outside the depression to understand the underground S-wave velocity structure. As a result of comparison with the geological cross section, the boundary depth between the Tachikawa loam layer and the Musashino loam layer coincides with the depth at which the S wave velocity changes around 200 m/s. The depth at the top of the Musashino Gravel layer coincided with the depth at which the S-wave velocity changes significantly around 300 m/s. The elevation of the upper end of the Musashino gravel layer was not particularly deep, especially in the depression area. Therefore, the possibility that this depression was formed due to structural factors is considered to be small. On the other hand, the thickness of the Musashino loam layer inside the depression was significantly thinner than outside. Furthermore, there was no significant difference in the thickness of the Tachikawa loam layer between inside and outside the depression. Based on these results, we considered the following landform evolution. When the Musashino loam layer was deposited, there was spring water in the depression, which washed away the deposited loam layer, making the Musashino loam layer in the depression thinner than in the surrounding area. Later, when the Tachikawa loam was deposited, there was no spring water, so there was no major change in the thickness of the loam layer inside and outside the depression.
