Estimating of Subsurface Structures around Liquefied Former River Channels of the Tone/Kinu River with Geophysical Prospecting

Abstract

 During the 2011 off the Pacific coast of Tohoku Earthquake, liquefaction occurred over a wide area of the Kanto plain (Kanto Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism and The Japanese Geotechnical Society, 2011), and many instances were concentrated in the former river channel and filled-up land of the old water area (Koarai et al., 2011; Wakamatsu, 2012 etc.). Observing them on a microscale, regional biases of damage to structures and distribution of sand volcanoes due to liquefaction were observed in the former river channel. In particular, in the former river channel of the Kinu River in Shimotsuma City, Ibaraki Prefecture, damage to structures at the attack slope side of the former river channel was unevenly distributed (Koarai and Nakano, 2013). One of the factors behind the uneven distribution of liquefaction damages is inferred to be the influence of the former riverbed topography, including the thickness of the former riverbed sediment and its layer structure. Therefore, to clarify subsurface structures and hydraulic conditions such as the former riverbed topography and groundwater level distribution in the former river channel, 2-D electrical resistivity imaging and ground-penetrating radar (GPR) survey were conducted across the former river channel in the downstream basin of the Tone River (Kozaki site) and the Kinu River (Kinu site). At the Kozaki site, the paleo-river channel topography of the older period was identified with 2-D electrical resistivity imaging, and the results were correlated with the results of a boring survey. However, no relationship was found between subsurface structures with 2-D electrical resistivity imaging and the distribution of liquefaction (sand volcanoes). Although the groundwater level was detected with the GPR survey, the former riverbed topography was not detected, and no relation between the results of the GPR survey and the sand volcano areas was found. At the Kinu site, the former riverbed was detected with the GPR survey. At a bent in the former river channel, it was confirmed that the former river bed is deeper at the attack slope side than the slip-off slope side. This suggests that there is a correlation between increased liquefaction damage and former riverbed depth.