11:15 AM - 11:30 AM
[HQR04-03] S-wave velocity structures and ground motion characteristics along valley bottom lowlands within an upland: The case of the Musashino Upland, central Tokyo
Keywords:valley bottom lowland, microtremor observation, central Tokyo
Soft muddy and humic deposits beneath valley bottom lowlands often cause the amplification of earthquake ground motion and uneven subsidence (Yasuda, 2017). However, the thickness of these deposits decreases toward the upstream along the valleys, and in that case, even the lowlands are expected to show hard ground, reflecting the physical property of the basement. In this study, for better understanding how the ground characteristics of the valley bottom lowlands change from downstream to upstream, along the Kanda River (Zenpukuji River) and the Furukawa River (Shibuya River) that dissect the Musashino Upland in central Tokyo, microtremor observations were carried out along several survey lines across the valley bottom lowlands.
The average S-wave velocities in the valley bottom lowlands in the lower reaches of both rivers were quite low. This is because the soft valley bottom deposits with an S-wave velocity of about 100 m/s is distributed relatively thickly in the downstream part. Especially in the areas where the thickness reaches 10 to 15 m, a peak was observed at a slightly low frequency (around 1.5 Hz) of the H/V spectrum showing the ground motion characteristics. These areas almost correspond to those severely damaged by the 1923 Kanto Earthquake (Takemura, 2003).
On the other hand, in the valley bottom lowland in the upper reach of the Kanda River (Zenpukuji River), the average S-wave velocity to the depth of 30 m (AVS30) was as high as 300–400 m/s, and the H/V spectrum showed peaks at high frequencies or flat shapes in the high-frequency range. In the upper reach of this river, aeolian sediments as base material of the Kanto Loam is constantly removed by the running water of the river (Kubo, 1988), and the Musashino Gravel Bed is distributed in the very shallow part beneath the valley bottom lowland. In this case, AVS30 was higher in the valley bottom lowland than in the surrounding upland because the soft Kanto Loam with the same thickness as the height of the upland was distributed in the upland.
Similarly, in the middle reach of the Furukawa River (Shibuya River), AVS30 was as high as 350 m/s in the valley bottom lowland, and the H/V spectrum showed peaks at high frequencies or flat shapes in the high-frequency range. In the middle reach of this river, the valley bottom deposits were hardly accumulated, and the Tokyo Gravel Bed was distributed in the very shallow part beneath the valley bottom lowland. Since the surrounding upland is composed of soft mud of the Tokyo Formation and the Kanto Loam, the upland exhibited lower values of AVS30 rather than those of the valley bottom lowland.
Thus, along small rivers such as the Kanda and Furukawa rivers that dissect the Musashino Upland, soft valley bottom deposits are distributed relatively thickly in the lower reach, but the deposits are thin or hardly distributed in the middle/upper reach. In addition, as the river erosion often stop at the upper surface of Pleistocene gravel beds, the valley bottom lowland in the middle and upper reaches consists of relatively hard ground, rather than the uplands consisting of the soft Kanto Loam and Tokyo Formation. Consequently, along the small rivers in the upland, the relative fragility of grounds between uplands and lowlands is entirely reversed in the lower and middle/upper reaches. It is important for strong ground motion prediction and earthquake disaster prevention to accurately grasp the change in the thickness of the valley bottom deposits from the downstream to the upstream and the positional relationship between the gravel beds and the erosion surface of the valley bottom.
Kubo, S., 1988, Geogr. Rev. Japan, Ser. A, 61, 25–48.
Takemura, M., 2003, J. Japan Assoc. Earthquake Eng., 3 (1), 1–36.
Yasuda, S., 2017, Quat. Res. (Daiyonki-Kenkyu), 56, 217–225.
The average S-wave velocities in the valley bottom lowlands in the lower reaches of both rivers were quite low. This is because the soft valley bottom deposits with an S-wave velocity of about 100 m/s is distributed relatively thickly in the downstream part. Especially in the areas where the thickness reaches 10 to 15 m, a peak was observed at a slightly low frequency (around 1.5 Hz) of the H/V spectrum showing the ground motion characteristics. These areas almost correspond to those severely damaged by the 1923 Kanto Earthquake (Takemura, 2003).
On the other hand, in the valley bottom lowland in the upper reach of the Kanda River (Zenpukuji River), the average S-wave velocity to the depth of 30 m (AVS30) was as high as 300–400 m/s, and the H/V spectrum showed peaks at high frequencies or flat shapes in the high-frequency range. In the upper reach of this river, aeolian sediments as base material of the Kanto Loam is constantly removed by the running water of the river (Kubo, 1988), and the Musashino Gravel Bed is distributed in the very shallow part beneath the valley bottom lowland. In this case, AVS30 was higher in the valley bottom lowland than in the surrounding upland because the soft Kanto Loam with the same thickness as the height of the upland was distributed in the upland.
Similarly, in the middle reach of the Furukawa River (Shibuya River), AVS30 was as high as 350 m/s in the valley bottom lowland, and the H/V spectrum showed peaks at high frequencies or flat shapes in the high-frequency range. In the middle reach of this river, the valley bottom deposits were hardly accumulated, and the Tokyo Gravel Bed was distributed in the very shallow part beneath the valley bottom lowland. Since the surrounding upland is composed of soft mud of the Tokyo Formation and the Kanto Loam, the upland exhibited lower values of AVS30 rather than those of the valley bottom lowland.
Thus, along small rivers such as the Kanda and Furukawa rivers that dissect the Musashino Upland, soft valley bottom deposits are distributed relatively thickly in the lower reach, but the deposits are thin or hardly distributed in the middle/upper reach. In addition, as the river erosion often stop at the upper surface of Pleistocene gravel beds, the valley bottom lowland in the middle and upper reaches consists of relatively hard ground, rather than the uplands consisting of the soft Kanto Loam and Tokyo Formation. Consequently, along the small rivers in the upland, the relative fragility of grounds between uplands and lowlands is entirely reversed in the lower and middle/upper reaches. It is important for strong ground motion prediction and earthquake disaster prevention to accurately grasp the change in the thickness of the valley bottom deposits from the downstream to the upstream and the positional relationship between the gravel beds and the erosion surface of the valley bottom.
Kubo, S., 1988, Geogr. Rev. Japan, Ser. A, 61, 25–48.
Takemura, M., 2003, J. Japan Assoc. Earthquake Eng., 3 (1), 1–36.
Yasuda, S., 2017, Quat. Res. (Daiyonki-Kenkyu), 56, 217–225.