Effective preparedness and countermeasures against great earthquakes require considering not only single-hazard scenarios, such as strong motion, but also multi-hazard situations and their cascading effects. This study focuses on coseismic subsidence, especially the resulting expansion of land below sea level. Land below sea level cannot naturally drain water, so it is vulnerable to the long-term flooding. There are concerns that long-term damage may occur not only from tsunami inundation caused by great earthquakes but also from other inundation caused by disasters other than earthquakes, such as storm surges and river flooding. To understand the characteristics of the expansion of land below sea level, we analyzed the likelihood and extent of such expansion due to coseismic subsidence for great earthquakes along the Nankai Trough. Additionally, prior research has examined a limited number of earthquake scenarios and does not adequately consider the variation across great earthquakes along the Nankai Trough in terms of magnitude, source area extension, and the number of occurrences that compose a seismic cycle. Considering this variety, we analyzed coseismic subsidence for more than 900,000 occurrence patterns. We then discussed the likelihood of land falling below sea level and their total areas for four regions using two different approaches. From the viewpoint of multi-hazards and their cascading effects, by comprehensively addressing our results and hazard information on possible inundation areas attributable to other hazards, we expect that the results of this study will contribute to effective preparedness and countermeasures against long-term flooding, such as determining which areas require countermeasures and prioritizing investments.

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