During the 2024 Noto Peninsula Earthquake, severe liquefaction damage occurred along the Shinano River in Niigata City, approximately 160 km from the epicenter. The area affected by liquefaction corresponded to the former riverbed of the Shinano River, reaffirming that former riverbeds are more prone to liquefaction than floodplains. Furthermore, during the 1964 Niigata Earthquake, liquefaction was observed in the same former riverbed area that liquefied this time. This highlights that the susceptibility to liquefaction is strongly influenced by microtopography.In this study, we conducted a simple liquefaction assessment using a voxel model based on a 100 m × 100 m grid, created using high-density boring logs from the Tokyo lowland. The study area includes sites where liquefaction was confirmed during the 1923 Great Kanto Earthquake and features characteristic coastal plain microtopography, such as former riverbeds, natural levees, and deltas. The liquefaction potential index (PL) exhibited similar trends for deltas and natural levees. However, the values for the former riverbeds tended to be lower than those for the deltas and natural levees. Further analysis of the soil properties and N-values in the surface layer revealed that the former riverbed area contained a reclaimed soil layer several meters thick with a thin layer of low N-value, weak soil, particularly near the groundwater level. PL is a weighted integral of the liquefaction safety factors distributed with depth and is calculated up to a depth of 20 m. Consequently, its sensitivity to liquefaction susceptibility in thin layers near the surface was found to be reduced. Therefore, when evaluating the depth of liquefied layers and their impact on the liquefaction risk assessment for small-scale buildings and lifeline facilities, reconsidering the applicability of the PL is necessary.
Ishikawa et al. (Thu,) studied this question.