Kazuki ADACHI

Study on the Predictive Performance of Simplified Liquefaction Evaluation Methods for Liquefaction Occurrence in Niigata City Induced by the 2024 Noto Peninsula Earthquake

Takaaki IKEDA, Masataka SHIGA

This study evaluates the simplified liquefaction assessment method (FL method) in Niigata City, Japan based on observations from the 2024 Noto Peninsula Earthquake. Although the earthquake epicenter was more than 150 km away from Niigata city, widespread liquefaction was recorded especially near the toe of the Niigata dune slope and along former channels of the Shinano River. The same area also liquefied severely during the 1964 Niigata Earthquake, suggesting that Niigata City is highly susceptible to liquefaction damage.
The FL method estimates liquefaction potential using the factor of safety. FL is defined as the ratio of the liquefaction resistance (R) to the earthquake-induced cyclic shear stress (L). Owing to its reliance on readily available geotechnical parameters such as SPT N-values, this method is widely used in practice; however, its predictive accuracy for the observed damage pattern in Niigata City has not been verified for 2024 earthquake event.
To identify factors controlling FL performance, we conducted number of analyses including (i) geomorphological comparisons between liquefaction occurrences and terrain conditions, (ii) sensitivity analyses of FL components across design guidelines, and (iii) site-specific FL evaluations using in situ investigation data. Liquefaction occurrence closely follows paleo-topography including dune-margin zones, abandoned channels, and reclaimed lands. According to sensitivity analysis, liquefaction resistance RL for highway bridges (Japanese specifications) increases remarkably in the presence of high fine content (FC).  At four representative sites in the Terao and Tokimeki districts, shallow layers with FL<1 were consistent with surface manifestations of liquefaction, while some sites without surface damage showed false positives. These outliers are highly corresponding to the local stratigraphy, indicating that the depth and thickness of liquefiable layers control the development of surface deformation. Replacing approximate FC with laboratory-measured FC helps to reduce uncertainty of results in classifications of high-FC zone.
Overall, the results support prioritizing mitigation in high-risk zones and cost-effective countermeasure planning within limited budget.