10:45 AM - 11:00 AM
[HDS09-06] Geotechnical hazard risk of the Sakuragawa Lowland estimated by micro tremor observation
Keywords:Geotechnical hazard risk, microtremor obsavation, Sakuragawa Lowland
In the Sakuragawa Lowland located in the Kasumigaura basin of Ibaraki Prefecture, the underground structure estimated by microtremor observation, in order to examine the risk of geotechnical disasters and attempt to reconstruct the landform evolution. This presentation will focus on the risk of geotechnical disasters.
The microtremor observations were carried out at 128 sites in the Sakuragawa Lowland using the method of Cho and Senna (2006). The S-wave velocity structure was determined at each measurement point, and the depth at which the S-wave velocity exceeded 300 m/s was defined as the engineering basement rock depth, and the average S-wave velocity up to a depth of 30 m was defined as AVS30.
The low terraces have developed in the middle reaches of the Sakuragawa Lowland, but their relative height from the flood plain remains the same compared to natural levees, making it difficult to distinguish between the two based solely on topographic form.
As a result of comparing the boring data in the Sakuragawa Lowland with the S-wave velocity structure determined by microtremor observations, it was found that there is a thick gravel layers in the Sakuragawa Lowland at a depth of around 10 m and around 20 m, and that the upper gravel layer has S wave speed exceeded 200 m/s, and exceeded 300 m/s in the lower gravel layer. Using these results as a reference, the underground structure was estimated in the Sakuragawa Lowland, including no boring data area.
Low terraces develop in the middle reaches of the Sakuragawa Lowland, and the basement depth is shallow, about several meters, and the AVS30 is about 250 to 350 m/s both on the low terraces and on the surrounding flood plains. It is estimated that buried terraces are spreading around the low terraces. On the right side and the left side with the buried terrace, there are measurement points with a basement depth of around 20 m and AVS30 of around 150 to 250 m/s. It is estimated that there are two buried valleys that are thought to have been carved the basement rock. The Kimishima area on the upstream side and the Mushigake area on the downstream side are areas where low terraces are not seen, and natural levees are developed. The measurement results on natural levees show that AVS30 is small, and the basement depth is deep in an extension of the estimated buried valley, and AVS30 is large and the basement depth is deep in an extension of the buried terrace.
Even in the Tsuchiura urban area of the lower Sakuragawa River basin, the AVS30 is large and the basement depth is shallow in the olden central part, so buried terraces are thought to exist. It is judged that a buried valleys exists in the area along the Sakuragawa River and the area along the Shinkawa River,
because the AVS30 is small and basement depth is deep.
The above results were comprehensively summarized, and the locations of buried terraces and buried valleys were organized in a plan view. In terms of landform classification, natural levees and flood plains have different geotechnical disaster risks due to differences in underground structure, so by incorporating the results of microtremor observation, it is possible to evaluate disaster risks that do not rely solely on terrain type.
The microtremor observations were carried out at 128 sites in the Sakuragawa Lowland using the method of Cho and Senna (2006). The S-wave velocity structure was determined at each measurement point, and the depth at which the S-wave velocity exceeded 300 m/s was defined as the engineering basement rock depth, and the average S-wave velocity up to a depth of 30 m was defined as AVS30.
The low terraces have developed in the middle reaches of the Sakuragawa Lowland, but their relative height from the flood plain remains the same compared to natural levees, making it difficult to distinguish between the two based solely on topographic form.
As a result of comparing the boring data in the Sakuragawa Lowland with the S-wave velocity structure determined by microtremor observations, it was found that there is a thick gravel layers in the Sakuragawa Lowland at a depth of around 10 m and around 20 m, and that the upper gravel layer has S wave speed exceeded 200 m/s, and exceeded 300 m/s in the lower gravel layer. Using these results as a reference, the underground structure was estimated in the Sakuragawa Lowland, including no boring data area.
Low terraces develop in the middle reaches of the Sakuragawa Lowland, and the basement depth is shallow, about several meters, and the AVS30 is about 250 to 350 m/s both on the low terraces and on the surrounding flood plains. It is estimated that buried terraces are spreading around the low terraces. On the right side and the left side with the buried terrace, there are measurement points with a basement depth of around 20 m and AVS30 of around 150 to 250 m/s. It is estimated that there are two buried valleys that are thought to have been carved the basement rock. The Kimishima area on the upstream side and the Mushigake area on the downstream side are areas where low terraces are not seen, and natural levees are developed. The measurement results on natural levees show that AVS30 is small, and the basement depth is deep in an extension of the estimated buried valley, and AVS30 is large and the basement depth is deep in an extension of the buried terrace.
Even in the Tsuchiura urban area of the lower Sakuragawa River basin, the AVS30 is large and the basement depth is shallow in the olden central part, so buried terraces are thought to exist. It is judged that a buried valleys exists in the area along the Sakuragawa River and the area along the Shinkawa River,
because the AVS30 is small and basement depth is deep.
The above results were comprehensively summarized, and the locations of buried terraces and buried valleys were organized in a plan view. In terms of landform classification, natural levees and flood plains have different geotechnical disaster risks due to differences in underground structure, so by incorporating the results of microtremor observation, it is possible to evaluate disaster risks that do not rely solely on terrain type.