Paleotsunami research along the Nankai Trough and Ryukyu Trench subduction zones – Current achievements and future challenges

Located near Japan's most densely populated and industrially active region, the Nankai Trough subduction zone has long been highlighted as a high-risk area for damaging earthquakes and tsunamis. In contrast, less attention has been paid the adjacent Ryukyu Trench because historical and geological records are scarce. In order to develop better quantitative estimates of the timing and size of the earthquakes and tsunamis generated along these subduction zones, comprehensive studies using geological, seismological and historical methods have been conducted. Since the 1990s, studies of tsunami deposits in this region have contributed to our current understanding of the history of tsunamis over the last 6000 years. Following the 2011 Tōhoku earthquake and tsunami, paleotsunami research has especially focused on guiding and enhancing tsunami disaster management and mitigation measures. The last nine years have seen a rapid increase in paleotsunami data from the Nankai Trough and Ryukyu Trench coasts. These recent studies reveal that there are significant differences in the size and recurrence pattern of earthquakes and tsunamis along these subduction zones. For instance, large earthquakes have repeatedly occurred along the Nankai Trough every 100–200 years. On the other hand, large earthquakes and tsunamis may not have occurred over the last few thousand years at the junction between these subduction zones. There is also no evidence for the occurrence of large earthquakes along the northern and central Ryukyu Trench over this time period. Large earthquakes and tsunamis may have occurred every few hundred years at the southern end of the Ryukyu Trench. Nevertheless, data on the occurrence of these earthquakes are still insufficient, both in quantity and quality, to estimate the maximum size or recurrence pattern. •Comparative review of tsunami deposits in the Nankai and Ryukyu subduction zones.•Recurrence mode of great earthquakes and tsunamis based on geological evidence.•Updating the model describing the recurrence mode of great earthquakes.•Discussion about the largest tsunami ever in the region from geological evidence.