5:15 PM - 6:30 PM
[HTT17-P02] Estimation of underground structure beneath the debris control dams constructed during the Edo era based on the microtremor measurements
Keywords:microtremor measurement, debris control dam
In Fukuyama city, Hiroshima, many debris control dams "Sunadome" had been constructed during the Edo era and have been left. The largest one in such dams is “Dodo-gawa Rokuban Sunadome”. I performed the microtremor measurements at Dodo park constructed on the sedimentation of Sunadome, and estimated the underground structure beneath the park by using the H/V spectral ratio. The soft surface layer was estimated to be thicker around the Dodo river flowing in the middle of the park and be thinner nearby the mountain slope. In this study, I estimated the underground structures in other debris control dams along the river by using the microtremor measurements in order to reproduce the topography before the constructions of Sunadome.
Major geology in Hiroshima prefecture is granite, which can be easily turned to the decomposed granite soil due to weathering. Therefore, the surface layer in this area tends to collapse into landslide disaster. Fukuyama city has suffered the serious landslide disasters repeatedly. In May, 1673, the large-scale debris flow occurred along the Dodo river due to the local heavy rainfall and swept away the Kokubunji, houses and farms. That was the reason that the Fukuyama domain started the construction of Sunadome and had installed many debris control dams in this city. “Dodo-gawa Rokuban Sunadome” with dam height of 8.8m started to be constructed in 18th century.
I have performed the microtremor measurements at Dodo park since 2017. This park is about 70m x 250m in size along the Dodo river. I recorded 5 minute microtremors measured by the three-component seismometer at 70 sites which were set up at intervals of about 15m. I applied FFT to the measurements to obtain the H/V spectral ratios, and estimated the dominant frequency at each site.
Two dominant frequencies could be found at every sites in this park. One is a low frequency 0.1-1 Hz, and the other is a high frequency of 1-10 Hz. The H/V spectral ratio is larger at the latter one and can be definitely extracted from the noise. I assumed two lateral layer as the underground structure and estimated P wave velocity and thickness of the surface layer by using the higher dominant frequency and its H/V spectral ratio. The other parameters such as density and S-wave velocity were assumed to be constant.
As a result of the estimation of underground structure, the surface layer was found to be thicker around the Dodo River flowing through the middle of the park, and be thinner near the mountain slopes on both sides of the river. In addition, I could see the surface layer tended to be thinner from the downstream to the upstream of the river. It might be indicated that the subsurface structure estimated in this study shows the topography of the valley before the construction of Sunadome. These results suggest that we can estimate a complicated underground structure in the lateral scale of 10m, even if a simple horizontally stratified structure is assumed for estimation of the underground structure. In this study, we have performed the microtremor measurements at other Sunadome along the river and tried to reproduce the past topography of the valley before the construction of Sunadome.
Major geology in Hiroshima prefecture is granite, which can be easily turned to the decomposed granite soil due to weathering. Therefore, the surface layer in this area tends to collapse into landslide disaster. Fukuyama city has suffered the serious landslide disasters repeatedly. In May, 1673, the large-scale debris flow occurred along the Dodo river due to the local heavy rainfall and swept away the Kokubunji, houses and farms. That was the reason that the Fukuyama domain started the construction of Sunadome and had installed many debris control dams in this city. “Dodo-gawa Rokuban Sunadome” with dam height of 8.8m started to be constructed in 18th century.
I have performed the microtremor measurements at Dodo park since 2017. This park is about 70m x 250m in size along the Dodo river. I recorded 5 minute microtremors measured by the three-component seismometer at 70 sites which were set up at intervals of about 15m. I applied FFT to the measurements to obtain the H/V spectral ratios, and estimated the dominant frequency at each site.
Two dominant frequencies could be found at every sites in this park. One is a low frequency 0.1-1 Hz, and the other is a high frequency of 1-10 Hz. The H/V spectral ratio is larger at the latter one and can be definitely extracted from the noise. I assumed two lateral layer as the underground structure and estimated P wave velocity and thickness of the surface layer by using the higher dominant frequency and its H/V spectral ratio. The other parameters such as density and S-wave velocity were assumed to be constant.
As a result of the estimation of underground structure, the surface layer was found to be thicker around the Dodo River flowing through the middle of the park, and be thinner near the mountain slopes on both sides of the river. In addition, I could see the surface layer tended to be thinner from the downstream to the upstream of the river. It might be indicated that the subsurface structure estimated in this study shows the topography of the valley before the construction of Sunadome. These results suggest that we can estimate a complicated underground structure in the lateral scale of 10m, even if a simple horizontally stratified structure is assumed for estimation of the underground structure. In this study, we have performed the microtremor measurements at other Sunadome along the river and tried to reproduce the past topography of the valley before the construction of Sunadome.