9:30 AM - 9:45 AM
[HDS09-03] Study of landform evolution of valley plain in East Ibaraki Plateau based on micro tremor observation results for seismic risk assessment
Keywords:valley plain, micro tremor observation, sloop degree, basement depth, East Ibaraki Plateau
Based on landform classification, it is necessary to consider the basal depth and constituents of alluvium in the valley plain. From the viewpoint of landform evolution, it is considered that the steeper slope on the upstream side is thinner and the composition is coarser, and the gentler slope on the downstream side is thicker and the composition is finer. Therefore, it is assumed that the gentler the slope of the valley plain, the higher the risk of ground disaster during an earthquake.
The purpose of this study was to clarify the relationship between the gradient and the thickness of the alluvium in the valley plain that carves the East Ibaraki Plateau in Ibaraki Prefecture. The specific target areas are the area around Mito, Ishioka and Omitama City. First, a cross-section of the valley plain was created using GSI Maps cross section function. Boring data was collected and micro-tremor observation was performed in the valley plain.
As a result of comparing the S wave velocity structure obtained by micro tremor observation with the boring data near the measurement point, it was judged that the depth at which the S wave velocity changes significantly near the S wave velocity of 300 m / s is the base of the offshore stack in the valley plain. Then, the S-wave velocity structure and boring data were arranged in the longitudinal direction to create a vertical cross-sectional view of the valley bottom plain.
The valley plains near Mito have gentle slopes of less than 1/500 in the upstream and downstream, and steep slopes of about 1/100 to 1/300 in the middle stream. In the valley plains of Omitama and Ishioka, the upstream is steep and the downstream is gentle. The result is that the alluvium deposit is thicker in the downstream, but in the valley plains near Mito, the thickness of the alluvium deposits increases sharply at the point where the steep slope of the middle stream changes to the gentle slope of the downstream. This is considered to be the difference in the progress of the erosion stage. In the valley plain near Mito, the steep slopes of the middle stream become an erosion front and are in the middle of retreating to the upstream side, and in the valley plain of Omitama and Ishioka, the retreat of the erosion fronts have advanced to the upstream.
In the wide valley plains in the lower reaches of the Sonobe River and the Koise River, the micro-tremors observations and the collection of boring data were done. As a result, even within the same valley plain, the alluvium deposits are thick in the center of the valley plain, while the alluvium deposits are thin on the side closer to the plateau. It can be seen that the cross-sectional shape of the alluvial deposits is V-shaped rather than U-shaped.
The alluvium deposits of the valley plains in the target area are mainly silt, so it does not correspond to the condition with high liquefaction risk. However, there is a clear tendency that the alluvium deposits are thicker in the lower reaches of valleyed plain or in the center of the valleyed plains. Then the risk of ground disaster during an earthquake should be evaluated according to the positions of the low slopes in the downstream or the middle part of the wide valley plains.
The purpose of this study was to clarify the relationship between the gradient and the thickness of the alluvium in the valley plain that carves the East Ibaraki Plateau in Ibaraki Prefecture. The specific target areas are the area around Mito, Ishioka and Omitama City. First, a cross-section of the valley plain was created using GSI Maps cross section function. Boring data was collected and micro-tremor observation was performed in the valley plain.
As a result of comparing the S wave velocity structure obtained by micro tremor observation with the boring data near the measurement point, it was judged that the depth at which the S wave velocity changes significantly near the S wave velocity of 300 m / s is the base of the offshore stack in the valley plain. Then, the S-wave velocity structure and boring data were arranged in the longitudinal direction to create a vertical cross-sectional view of the valley bottom plain.
The valley plains near Mito have gentle slopes of less than 1/500 in the upstream and downstream, and steep slopes of about 1/100 to 1/300 in the middle stream. In the valley plains of Omitama and Ishioka, the upstream is steep and the downstream is gentle. The result is that the alluvium deposit is thicker in the downstream, but in the valley plains near Mito, the thickness of the alluvium deposits increases sharply at the point where the steep slope of the middle stream changes to the gentle slope of the downstream. This is considered to be the difference in the progress of the erosion stage. In the valley plain near Mito, the steep slopes of the middle stream become an erosion front and are in the middle of retreating to the upstream side, and in the valley plain of Omitama and Ishioka, the retreat of the erosion fronts have advanced to the upstream.
In the wide valley plains in the lower reaches of the Sonobe River and the Koise River, the micro-tremors observations and the collection of boring data were done. As a result, even within the same valley plain, the alluvium deposits are thick in the center of the valley plain, while the alluvium deposits are thin on the side closer to the plateau. It can be seen that the cross-sectional shape of the alluvial deposits is V-shaped rather than U-shaped.
The alluvium deposits of the valley plains in the target area are mainly silt, so it does not correspond to the condition with high liquefaction risk. However, there is a clear tendency that the alluvium deposits are thicker in the lower reaches of valleyed plain or in the center of the valleyed plains. Then the risk of ground disaster during an earthquake should be evaluated according to the positions of the low slopes in the downstream or the middle part of the wide valley plains.