A post-event survey of the coastal impacts of Typhoon Hagibis was conducted in the Greater Tokyo and Shizuoka areas. The typhoon’s intensive landfall generated the highest-level storm surges recorded in the study area with a maximum sea-level anomaly of nearly 2 m at the head of Tokyo Bay. Highly populated coastal areas in the inner sheltered part of Tokyo Bay were subject to only minor damage partially because the peak storm surge occurred during a low spring tide. High water levels along the coastline facing the Pacific Ocean mostly measured 4–8 m above the mean sea level, the exception being the Seisho coast, where extreme wave runup reached over 10 m above the mean sea level. The high water levels predominantly caused by energetic swells were comparable with the crest heights of defense structures in a broad range of the study area. The intensive typhoon did not trigger catastrophic flood damage, but caused minor flooding due to wave overtopping, and there was substantial damage to coastal infrastructures along the coast. This paper presents both an overview of and detailed information regarding the coastal impacts of the intensive typhoon along the highly altered coastline with complex geometry.

A moment magnitude (M_w) 7.5 earthquake occurred on January 1, 2024, at the northern tip of the Noto Peninsula, Central Japan, triggering a large tsunami. Seismological and geodetic observations revealed the rupture of mapped submarine active faults. While proximal segment ruptures have been well resolved by previous research, far offshore segments have posed challenges for onshore-based inversions. This emphasizes the necessity for a comprehensive study on the fault through tsunami modeling. Here, we aimed to examine tsunami propagation and inundation using four different fault models to identify the general characteristics of the tsunami source and evaluate the complexities of earthquake- and submarine landslide-induced tsunamis. We identified the simultaneous rupture of two active fault systems as the most suitable model for explaining observed tsunami height and inundation; however, some inconsistencies with observations remained. The propagation process did not follow a concentric pattern but aligned with bathymetric heterogeneity. The findings also suggested potential amplification effects responsible for the devastation of the coast of Iida Bay and indicated a possible submarine landslide in southern Toyama Bay. The findings of the present work could benefit the exploration of a more realistic tsunami source model, considering the differences between observations and simulations. Such efforts, in collaboration with paleotsunami research, can contribute to the improved assessment of hazards from submarine active faults.

Typhoon Faxai caused severe damages along the western shore of Tokyo Bay, Japan in September 2019. Retaining a relatively small radius of maximum wind, Faxai passed across the center of Tokyo Bay and caused intensive wind and waves. While the sea level anomaly recorded at several tide gauge stations along Tokyo Bay were at most 1 m, and no significant surge-induced inundation was found, locally concentrated damages and wave-induced hazards were observed around Yokohama, on the middle-western shore of Tokyo Bay. The observed inundation height around Yokohama was TP (Tokyo Peil) 4.2 m on average, and the highest runup, TP 10.8 m, was observed at a small hill directly behind the seawall in Fukuura, Yokohama. The estimated wave overtopping directions at each location varied from the northeast to southeast, and no clear correlation was found between the wave overtopping directions and the extent of observed coastal hazards. Based on these findings and investigations of recorded data such as wind and waves, it was deduced that locally concentrated damages and hazards around the middle western shore of Tokyo Bay may be explained by the unique features of a relatively compact Faxai and the topographical characteristics of Tokyo Bay.

Numerous tsunami numerical models have been proposed, but their prediction accuracies have not been directly compared. For quantifying the modeling uncertainties, the authors statistically analyzed the prediction results submitted by participants in the tsunami blind contest held at the 17th World Conference on Earthquake Engineering. The reproducibility of offshore water level generated due to the tsunami with soliton fission significantly decreased when the nonlinear shallow water equation models (NSWE) was used compared to three-dimensional (3D) models. The inundation depth was reproduced well in 3D models. However, the reproducibility of wave forces acting on the structure and velocities over land was lower in 3D models than that in NSWE models. For cases where the impulsive tsunami wave pressure generated could not be calculated based on the hydrostatic assumption, the prediction accuracy of the NSWE models was higher than that of the 3D models. The prediction accuracies of both models were not improved at small grid-cell sizes. The NSWE model cannot simulate the short-wave component and vertical pressure distribution. Therefore, further developments in 3D models and smoothed particle hydrodynamics methods (SPH) are needed. The presented results contribute to the future development of tsunami numerical simulation tools.

Researchers have developed tsunami inundation models based on nonlinear shallow water equations to estimate tsunami propagation and inundation. However, their empirical results are not in perfect agreement with those of other research institutes, even though the same governing equations are used. Therefore, we quantitatively evaluated the variability of tsunami simulations in this study. Several research institutes have conducted tsunami simulations under the same input conditions using tsunami inundation models adopted for tsunami hazard assessment, resulting in a certain degree of variability among them. By examining the spatial and temporal differences in various physical quantities, we identified the characteristic topography where the variability between tsunami simulations increases. A novel method for calculating statistics from the area integrals of physical quantities was proposed to demonstrate the variability in the overall simulation results. In addition, the effects of different setting parameters and computational environments on the simulation results of a single model were evaluated. The findings of this study are expected to not only serve as a basis to verify the reliability of source codes employed by users of the tsunami inundation model, but also contribute useful technical information to advance probabilistic tsunami hazard assessment in the future.