11:00 AM - 1:00 PM
[HTT20-P01] Comparison between the underground structure in Fukuyama plain estimated by the microtremor measurements and the boring survey results
Keywords:microtremor measurement, Fukuyama plain
The Fukuyama plain in eastern Hiroshima prefecture is the reclamation land that has been artificially developed since the Edo era. Before the reclamation, the shallow sea with the gentle slope was formed by the sediments from the Ashida river, and some small islands and reefs were scattered in the area. I have performed the microtremor measurements since 2015, and estimated the dense spatial distribution of the dominant frequencies and the underground structures. In this study, I compared the estimated underground structures with the borehole data obtained by the MLIT (Ministry of Land, Infrastructure, Transport and Tourism), Water and Sewerage Bureau in Fukuyama city and so on. I verified the adequacy of the estimated underground structures through the comparison.
I have intermittently performed the microtremor measurements in the Fukuyama plain and the surrounding areas since 2015. At present, I got the measurement data at total 351 sites. Mukai (2021) estimated the S-wave velocity and thickness of the surface layer by using the H/V spectral ratio. The estimated S-wave velocities were around 350 m/s at every sites, which agree with the JIVSM (Japan Integrated Velocity Structure Model) in the Fukuyama plain published by the Headquarters for Earthquake Research Promotion. On the other hand, the thicknesses of the surface layer were estimated to be thinner in the mountainous area in the northern part of the plain and larger in the southeastern part near the coastline. Major part of the reclaimed plain has the surface layer thicker than 40 m. In particular, the thickness of the surface layer is more than 80 m in the southeast area. It was also revealed that the surface layer was estimated to be extremely thin in the area where is thought to be the reefs before reclamation. This variation of the surface layer thickness is considered to show the seafloor geography before reclamation.
Kamata (1985) estimated the ground properties in the Fukuyama plain by using the standard penetration tests from the borehole investigations in Fukuyama city. In his paper, the geological columnar sections at 11 sites were published. I took the depth at which the N-value was greater than 30 at each site. The depths show positive correlation with the surface layer thickness in Mukai (2021). Kamada (1985) made the east-west geological cross section and showed the thick surface layer in the east of the small mountain in the northern part of the plain. This characteristics agree with the spatial distribution of the surface layer thickness in Mukai (2021).
We can get the borehole data along the Ashida river in Fukuyama plain from the MLIT. The depth at which the N-value was greater than 30 has some positive correlation with the layer thickness in Mukai (2021). In both cases, the southeast area near the coastline has larger values, which is considered to show the seafloor slope before reclamation.
As described above, the surface structure estimated by the dense microtremor measurements in Mukai (2021) is considered to be consistent with the borehole surveys. In this study, I tried to verify the adequacy of the estimated surface structure by comparing with the borehole data of Water and Sewerage Bureau in Fukuyama city as well.
I have intermittently performed the microtremor measurements in the Fukuyama plain and the surrounding areas since 2015. At present, I got the measurement data at total 351 sites. Mukai (2021) estimated the S-wave velocity and thickness of the surface layer by using the H/V spectral ratio. The estimated S-wave velocities were around 350 m/s at every sites, which agree with the JIVSM (Japan Integrated Velocity Structure Model) in the Fukuyama plain published by the Headquarters for Earthquake Research Promotion. On the other hand, the thicknesses of the surface layer were estimated to be thinner in the mountainous area in the northern part of the plain and larger in the southeastern part near the coastline. Major part of the reclaimed plain has the surface layer thicker than 40 m. In particular, the thickness of the surface layer is more than 80 m in the southeast area. It was also revealed that the surface layer was estimated to be extremely thin in the area where is thought to be the reefs before reclamation. This variation of the surface layer thickness is considered to show the seafloor geography before reclamation.
Kamata (1985) estimated the ground properties in the Fukuyama plain by using the standard penetration tests from the borehole investigations in Fukuyama city. In his paper, the geological columnar sections at 11 sites were published. I took the depth at which the N-value was greater than 30 at each site. The depths show positive correlation with the surface layer thickness in Mukai (2021). Kamada (1985) made the east-west geological cross section and showed the thick surface layer in the east of the small mountain in the northern part of the plain. This characteristics agree with the spatial distribution of the surface layer thickness in Mukai (2021).
We can get the borehole data along the Ashida river in Fukuyama plain from the MLIT. The depth at which the N-value was greater than 30 has some positive correlation with the layer thickness in Mukai (2021). In both cases, the southeast area near the coastline has larger values, which is considered to show the seafloor slope before reclamation.
As described above, the surface structure estimated by the dense microtremor measurements in Mukai (2021) is considered to be consistent with the borehole surveys. In this study, I tried to verify the adequacy of the estimated surface structure by comparing with the borehole data of Water and Sewerage Bureau in Fukuyama city as well.