Swash Zone Response to the Tsunami Triggered by the 2024 Noto Peninsula Earthquake

Abstract On January 1, 2024, a destructive earthquake struck the Noto Peninsula in Japan, triggering a tsunami in the Sea of Japan. A 3D lidar, which was installed on a gravel beach 150 km from the epicenter before the earthquake, successfully measured nearshore waves and topographic changes during the tsunami event. This study is focused on the analysis of a novel data set to elucidate the swash zone processes caused by the tsunami, with a particular focus on the combined effects of the tsunami and wind waves on high wave runup and morphological changes. The observation data show that the mean sea level was elevated by 1 m when the largest tsunami arrived at the beach. In addition to the tsunami, wind waves reaching a height of 2 m induced wave setup of approximately 1 m and swash exceeding a height of 2 m. The empirical formulas considering the observed change in foreshore slope reasonably reproduced the observed runup heights, suggesting that an increase in the tsunami water level indirectly amplified wind wave runup by increasing the foreshore slope on the concave profile beach. High wind wave runup largely eroded the gravel beach, with the elevation of the upper beach face decreasing by 0.5 m during the tsunami event. We found that the area on the beach face affected by wind waves expanded in response to changes in the tsunami water level.