Tsunami inundation hazard across Japan

Japan faces the world's highest tsunami hazard and risk due to its tectonic environment, high population density and exposure concentration along its coastlines. It is therefore of paramount interest to quantify and differentiate absolute and relative tsunami hazard and risk on a countrywide scale. We quantify tsunami hazard in terms of inundation depth for the entire Japanese coastline with a probabilistic tsunami hazard assessment utilizing the earthquake source information of the 2017 Japanese seismic hazard model. We simulate a stochastic event set for offshore earthquake sources and generate, for each single source with M-W >= 7.5, non-uniform finite-fault slip models. We calculate elastic seafloor and landmass deformations that serve as initial conditions to high resolution numerical modelling of tsunami wave propagation and coastal inundations by solving the nonlinear shallow water equation. Variable land surface roughness based on land cover data is used to simulate accurate hydraulics of coastal inundation. We differentiate tsunami hazard by inundation depth hazard curves and inundation depth return period maps aggregated to city ward polygons as meaningful administrative boundaries. We find mega-thrust events on the subduction interfaces constituting the largest hazard. However, events down to M-W >= 7.5 can contribute to substantial hazard in several regions along the coast. In particular for city wards within the Tokyo bay area, we find that earthquakes occurring on the Sagami trough and not the largest mega-thrust events on the Nankai trough, contribute to the highest inundation hazard. Our results also illustrate that tsunami hazard on the western Japanese coast is not negligible.