5:15 PM - 7:15 PM
[HDS11-P04] Estimation of ground structure around the former river channels in the Kinu district of Shimotsuma City and the Yoshino district of Joso City, using microtremor observation
Keywords:microtremor observation, liquefaction, the 2011 off the Pacific Coast of Tohoku Earthquake, former river channel
The 2011 off the Pacific Coast of Tohoku Earthquake caused liquefaction in a wide area of the Kanto Plain, and liquefaction damage occurred in the Kinu district of Shimotsuma City and in the Yoshino district of Joso City. In the Kinu district, liquefaction damage occurred mainly on the former river, and subsidence, house leaning damage and jetted sands tended to be more pronounced on the undercut slope side of the former river channel. Nakano et al. (2017) conducted a ground penetrating radar survey on the former river channel and confirmed that the depth of the former riverbed was deeper on the undercut slope side of the former river channel, suggesting a relationship between the depth of the former riverbed and the increase in liquefaction damage on the undercut slope side. In the Yoshino area, liquefaction damage occurred on the road at the boundary between the natural levee and the back slough, around the former river channel, and in the area located on the pointbar of the former river channel, which remains as the crescent lake. In this study, the ground structures around the former river channel in the Kinu area and the Yoshino area were estimated using microtremor observation, for discuss the relationship between the ground structures and liquefaction damage.
In this study, the structure of S-wave velocity was investigated by microtremor observation(microtremor array). Since S-wave velocity propagates slower in soft ground, soft ground that causes ground disasters can be detected. Using this property, the areas where soft ground is concentrated around the former river channel and the thickness of the soft layer were estimated. A total of 50 observations in the Kinu area and 17 observations in the Yoshino area were conducted. In addition, the characteristics of S-wave velocity structure of liquefied ground are also discussed using boring data from an area on the former river channel in the Kinu district, which was damaged by liquefaction.
Cross sections of the S-wave velocity structure were prepared based on the S-wave velocity structure analyzed from the microtremor observations in the Kinu area. As a result, soft layers are thickly deposited on the undercut slope side of the former river channel, and the risk of liquefaction may be particularly high in the former river channel. In addition, the meandering cross sections of the former river channel tend to be locally cut on the undercut slope. This tendency is a result of the river cutting the undercut slope at the bend during the period when the former river was carrying water. According to the boring data of the Kinu district, a sand layer, which seems to have liquefied, exists at depths shallower than 8m, and a silt layer is found beneath the sand layer. Comparing this boring data with the S-wave velocity structure, inversion layers with smaller S-wave velocity are distributed at the depth where the stratigraphic facies change from upper sand layer to lower silt layer. This indicates that the shaking is amplified in the soft silt layer and liquefaction is caused in the overlying sand layer at the time of the earthquake. Based on the S-wave velocity correspondence obtained from this comparison, there is a possibility that the tilt of the lower edge of the sand layer coincides with the direction of house tilt, and the liquefied sand layer flowed in the direction of the tilt of the sand layer at the time of the earthquake was considered, and the houses tipped in the direction facing each other. In the Yoshino area, the S-wave velocity structure near the liquefaction point shows a clear inversion layer.
References
・NAKANO, T., KOARAI, M., SUGAI, T. and YOSHIDA, T. (2017): Estimating of Subsurface Structures around Liquefied Former River Channels of the Tone/Kinu River with Geophysical Prospecting. Journal of Geography, 126(6), 749-765.
In this study, the structure of S-wave velocity was investigated by microtremor observation(microtremor array). Since S-wave velocity propagates slower in soft ground, soft ground that causes ground disasters can be detected. Using this property, the areas where soft ground is concentrated around the former river channel and the thickness of the soft layer were estimated. A total of 50 observations in the Kinu area and 17 observations in the Yoshino area were conducted. In addition, the characteristics of S-wave velocity structure of liquefied ground are also discussed using boring data from an area on the former river channel in the Kinu district, which was damaged by liquefaction.
Cross sections of the S-wave velocity structure were prepared based on the S-wave velocity structure analyzed from the microtremor observations in the Kinu area. As a result, soft layers are thickly deposited on the undercut slope side of the former river channel, and the risk of liquefaction may be particularly high in the former river channel. In addition, the meandering cross sections of the former river channel tend to be locally cut on the undercut slope. This tendency is a result of the river cutting the undercut slope at the bend during the period when the former river was carrying water. According to the boring data of the Kinu district, a sand layer, which seems to have liquefied, exists at depths shallower than 8m, and a silt layer is found beneath the sand layer. Comparing this boring data with the S-wave velocity structure, inversion layers with smaller S-wave velocity are distributed at the depth where the stratigraphic facies change from upper sand layer to lower silt layer. This indicates that the shaking is amplified in the soft silt layer and liquefaction is caused in the overlying sand layer at the time of the earthquake. Based on the S-wave velocity correspondence obtained from this comparison, there is a possibility that the tilt of the lower edge of the sand layer coincides with the direction of house tilt, and the liquefied sand layer flowed in the direction of the tilt of the sand layer at the time of the earthquake was considered, and the houses tipped in the direction facing each other. In the Yoshino area, the S-wave velocity structure near the liquefaction point shows a clear inversion layer.
References
・NAKANO, T., KOARAI, M., SUGAI, T. and YOSHIDA, T. (2017): Estimating of Subsurface Structures around Liquefied Former River Channels of the Tone/Kinu River with Geophysical Prospecting. Journal of Geography, 126(6), 749-765.