[H-DS06] Tsunami and tsunami forecast

[HDS06-P14] Causes of large tsunamis inferred from tsunami hazard curves in southern Shikoku

*Yusuke Sumida¹, Naoki Tanaka¹, Toshitaka Baba¹ (1.Department of Civil and Environmental Engineering, Division of Science and Technology, Graduate School of Sciences and Technology for Innovation, Tokushima University)

Keywords:tsunami hazard curves, The annual occurrence frequency of the earthquake, tsunami simulation

The Cabinet Office of Japan has conducted tsunami predictions using the earthquake fault models with M9.1 at the Nankai Trough. The maximum tsunami height is 34.4 m. Although the predictions are for the worst-case scenarios with M9.1, tsunamis caused by M8-class earthquakes are more frequent. Therefore, we investigated tsunami hazard curves in the Nankai Trough, which include a wide range of tsunamis from M7.6 to M9.1. For the tsunami calculations required for constructing the tsunami hazard curves, we used the 3480 earthquake scenarios in the Nankai Trough proposed by Fujiwara et al. (2020). We solved the nonlinear long wave equations using the finite difference method for all tsunamis from the 3480 earthquake scenarios. Three-layer nesting of computational grids of the minimum grid interval of 2 arc seconds reduced the computational load. The annual occurrence frequency of the earthquake came from the G-R law. Based on the information, we constructed tsunami hazard curves at seven sites in southern Shikoku.

The annual exceedance frequency of tsunamis in the K site is the maximum among the other sites investigated in this study. To clarify the trend of tsunami heights, we made the frequency distribution of tsunami heights at each site. We found that the tsunami heights in the K site are larger than those in the other locations, regardless of the earthquake magnitude. We further performed a detailed tsunami simulation using a fault model with a super-large slip zone off the coast of Shikoku. The tsunami simulation showed that the tsunami energy is concentrated around the offshore area of the K site, where a V-shaped valley exists at the seafloor. The V-shaped valley and the position where the tsunami focused are almost coincidental. Accordingly, the shape of the seafloor topography is one of the causes of the larger tsunami at the K site.