Special issue “structures, earthquakes and tsunami hazards in the Sea of Japan”

The Sea of Japan covers the back-arc basins of the Japanese island arc and shows intense tectonic activities.The formation of the back-arc basins in the Sea of Japan is believed to be related to the subduction of the Pacific and Philippine Sea plates.Coastal areas of the Japanese islands facing the Sea of Japan are positioned at the edge of the back-arc basins (Sato 1994).In the past, large earthquakes with magnitudes larger than 7 occurred in the coastal areas of the Sea of Japan and caused damages to societies repeatedly by strong ground motions and large tsunamis (e. g.Abe 1975;Satake 1985;Tanioka et al. 1995).On March 11, 2011, large tsunamis and strong ground motions from the 2011 Tohoku-oki earthquake caused devastating damage on the Pacific coasts of the Japanese islands, and it motivated research on seismic and tsunami hazards of the entire Japanese coasts.MLIT (2014) constructed the largest tsunami source fault models in the coastal areas of the Sea of Japan, for calculating tsunami inundation prediction.However, the knowledge and information for distribution and geometries of offshore active faults, crust-mantle structures, and the history of past tsunamis in the coastal regions of the Sea of Japan were limited and not sufficient for more accurate estimation of future earthquake and tsunami hazards.The research project named "Integrated Research Project on Seismic and Tsunami Hazards Around the Sea of Japan" (the Japan Sea project) had been carried out by the Ministry of Education, Culture, Sports, Science and Technology, Japan, for eight years since 2013 (MEXT 2021).As a result of the Japan Sea project, the seismic source fault models along the coast of the Japan Sea were constructed (Sato et al. 2021).Under these backgrounds, research on the structures of the crust and upper mantle, seismic activities, tsunamis, and active tectonics in the Sea of Japan and its coastal regions has made significant progress, and seismic and tsunami hazards can be constrained more quantitatively.In addition, research on databases for scientific results and social scientific studies on the hazards view had been performed.In this special issue, 13 papers discuss state-of-the-art methods and results of geophysical and geological observations and modeling, and outcomes of earthquake and tsunami hazards from a background in both earth sciences and social sciences.Seismic velocity structures provide fundamental information for other geophysical and geological studies.Matsubara et al. ( 2022) conducted a seismic tomography study for the entire Japanese islands including the Sea of Japan and the Pacific Ocean, using arrival times from reflection surveys and the permanent seismic networks.The shallow structures below the Sea of Japan from the off-Yamagata region to the Noto Peninsula were obtained by using controlled seismic source data.They found that the past large earthquakes in the coastal area of the Sea of Japan occurred at the boundary between high-Vp and low-Vp zones.A deep structure beneath the Sea of