Temporal changes in tsunami risk considering future population and sea-level rise under different climate scenarios: a case study in Tokushima, Japan

Recently, concerns have grown regarding increased coastal inundation risk due to climate change. In Japan, tsunamis generated by a Nankai Trough earthquake are expected to cause extensive damage, with urgency increasing as time passes since the previous event. Tokushima is projected to experience severe inundation from the largest-class Nankai Trough tsunamis. However, conventional hazard assessments often neglect projected relative sea-level rise, potentially underestimating tsunami risk in regions where relative sea-level rise is positive. This study clarifies time-dependent tsunami risk in Tokushima by incorporating earthquake probabilities based on the Brownian Passage Time (BPT) distribution, sea-level rise, and population change. Tsunami simulations were conducted for 12 earthquake magnitudes ranging from Mw 8.0 to Mw 9.1 under five sea-level conditions representing present and projected future scenarios. For each case, 100 simulations were performed, yielding 6,000 scenarios in total. Risk curves for inundation area and affected population were then developed. The results reveal contrasting risk curve trends between the neighboring cities of Tokushima and Komatsushima. Sea-level rise has a greater impact on inundation area than changes in earthquake occurrence probability from the BPT distribution. Furthermore, although the population is projected to decline substantially by 2100, intensified hazards could still increase the affected population.