[S08-2] Paleoseismology and paleotsunami studies: Their potential and limitation II

[S08-2-04] Has the unusual “mega-tsunami" ever occurred along the Nankai Trough?

^○Osamu Fujiwara (Geological Survey of Japan, AIST, Tsukuba, Japan)

invited

Following the 2011 Tohoku-oki earthquake and tsunami, Japanese government reviewed their policies for earthquake and tsunami disaster management and mitigation measures. This review includes the idea of defining maximum magnitudes for earthquake and tsunami in the Nankai Trough subduction zone; M9 earthquake is supposed as the maximum possible class of earthquake in this area. However, at least in the eastern Nankai Trough region, this supposition does not concern the paleotsunami data such as tsunami inundation area reconstructed from the tsunami deposits. As a case study, we have researched the tsunami deposits by the excavation survey and reconstructed the Holocene tsunami inundation history in the Hamamatsu plain, facing the eastern Nankai Trough. Because of the progradation of the beach system during the last 7000 years, about 4 km seaward migration of the coastline has occurred in Hamamatsu plain. We reconstructed the location of the former coastlines based on the beach ridges, and estimated their ages by using the radiocarbon dating of the beach deposits. We found some tsunami sand layers intercalated in the inter beach ridge marsh deposits and determined their ages by radiocarbon dating method. Taking the location of contemporary coastline into account, we have not recognized the tsunami deposit suggesting an exceptionally large inundation area in the last 4000 years. It was found from the result that “mega-tsunamis" with much larger size than historical tsunamis have not occurred in this region at least in the last 4000 years.