Damage to geotechnical structures induced by combined earthquake and tsunami loading during the 2024 Noto Peninsula earthquake

The 2024 Noto Peninsula Earthquake (Mw 7.6) and the subsequent tsunami caused extensive damage to coastal and riverine geostructures in the eastern Noto Peninsula, Japan. This paper presents field investigation results focusing on tsunami–ground interaction from a geotechnical engineering perspective. Severe damage was observed at Ukai Fishing Port, the Ukai River levee, Iida Port, and the Awazu coastal area. Field observations, borehole data, groundwater conditions, and portable dynamic cone penetration test results indicate that liquefaction of the foundation ground, and in some cases the embankment body itself, occurred at several sites. Seismic-induced settlement reduced crest elevations of levees and breakwaters, thereby facilitating tsunami overtopping and significantly amplifying inundation damage. At Iida Port, the eastern breakwater exhibited an atypical displacement mode that differed from previously reported tsunami-induced failure patterns. In the Awazu area, sinkholes near a coastal parapet were interpreted as resulting from piping triggered by transient hydraulic gradients during rapid drawdown following tsunami overtopping. Such combined external loading is not explicitly considered in the current design standards, and detailed design that simultaneously considers both a large earthquake (Level 2 earthquake) and a large-scale tsunami is technically challenging at present. The observed damage patterns suggest that when seismic loads exceed the levels assumed in conventional seismic design, it is important that the structure does not collapse instantaneously but retains ductility and toughness, so as to mitigate damage caused by subsequent external forces.