4:45 PM - 5:00 PM
[MIS11-12] “Maximum possible tsunami” along the eastern Nankai Trough from the perspective of tsunami deposit studies
★Invited Papers
Keywords:Maximum possible tsunami, Nankai Trough, Tsunami deposits, Meio earthquake
To reconstruct the size of past tsunamis from tsunami deposits, it is more important to clarify how far inland the deposits were distributed during deposition rather than the thickness of the deposits. Tsunamis with the longer wavelength run up for a longer time, flooding deeper inland. On the other hand, the thickness of tsunami deposits can vary significantly depending on the unevenness of the ground and the amount of material moved by the tsunami.
To investigate the inundation distance, it is necessary to have a wide lowland extending inland from the coastline. In lowlands in eastern Hokkaido (e.g., the unusually large earthquake of the 17th century; for example, Nanayama et al., 2003) and the Sendai Plain (e.g., the Jōgan Earthquake of 869; for example, Sawai et al., 2012), giant tsunamis with significantly larger run-up distances than those caused by historical earthquakes have been revealed through the tsunami deposit studies. On the other hand, along the Nankai Trough coastline, there are few wide plains, making it difficult to reconstruct the size of past tsunamis from tsunami deposits.
Along the Nankai Trough coast, the western part of Hamamatsu City in Shizuoka Prefecture is almost the only example where it is possible to reconstruct the inland distribution of tsunami deposits and compare the size of past tsunamis. In the western Hamamatsu Plain, multiple washover deposits (deposits left by large waves such as tsunamis that overtop the coastal topographic high) have been found dating back as far as approximately 6,000 years (e.g., Sato et al., 2016; Fujiwara et al., 2020). About 6,000 years ago, the coastline in the Hamamatsu Plain was located about 4 km inland from its current position, and the coastline gradually progressed seaward as sedimentation continued, expanding the plain.
At the wetland site about 3.8 km inland from the present coastline, only washover deposits older than approximately 3200 years are found. At the wetland site about 2.1 km from the present coastline, only washover deposits older than around 2100-1200 years are found. At the wetland site 1.3 km from the present coastline, washover deposits older than the 1498 Meio earthquake are discovered. Additionally, for individual survey sites, the upper (younger) washover deposits tend to be thinner and finer-grained. The distribution of younger washover deposits shifts further seaward, suggesting that as the Hamamatsu Plain expanded seaward, large waves such as tsunamis no longer reached deep inland. This implies that there is no evidence of giant tsunamis with extremely long run-up distances occurring within the past 6,000 years (Fujiwara et al., 2020).
The above-mentioned Meio tsunami deposit was found from a marshland more than 1.2 km away from the coastline and rivers at the time of the earthquake and is the youngest washover deposit in this marshland over the past 2,500 years but are markedly thicker and coarser-grained than the other older washover deposits at this site. Array drilling survey at this site revealed that the Meio tsunami deposit continuously distributed over an area of at least 600 m from east to west and 100 m from north to south. It consists mainly of medium-grained sand, is relatively well sorted, and is 10-15 cm thick. The Meio tsunami may have been exceptionally larger than other tsunamis that have occurred in the past 6000 years. However, no trace of the Meio tsunami has been found inland from this site, and the inundation distance is unknown. Increasing the number of survey sites and reconstructing the run-up area are key to confirm whether the Meio tsunami can be regarded as the maximum possible tsunami in the eastern Nankai Trough region.
References
Fujiwara, O. et al., 2020. Earth-Science Reviews 210; https://doi.org/10.1016/j.earscirev.2020.103333
Fujiwara, O. et al., 2023. Jour. Geography 132, 309-325; doi:10.5026/jgeography.132.309
Nanayama, F. et al., 2003. Nature 424, 660-663; https://doi.org/10.1038/nature01864
Sato, Y. et al., 2016. The Quaternary Research 55, 17-35
Sawai et al., 2012. Geophys. Res. Lett. 39, L21309, doi:10.1029/2012GL053692
To investigate the inundation distance, it is necessary to have a wide lowland extending inland from the coastline. In lowlands in eastern Hokkaido (e.g., the unusually large earthquake of the 17th century; for example, Nanayama et al., 2003) and the Sendai Plain (e.g., the Jōgan Earthquake of 869; for example, Sawai et al., 2012), giant tsunamis with significantly larger run-up distances than those caused by historical earthquakes have been revealed through the tsunami deposit studies. On the other hand, along the Nankai Trough coastline, there are few wide plains, making it difficult to reconstruct the size of past tsunamis from tsunami deposits.
Along the Nankai Trough coast, the western part of Hamamatsu City in Shizuoka Prefecture is almost the only example where it is possible to reconstruct the inland distribution of tsunami deposits and compare the size of past tsunamis. In the western Hamamatsu Plain, multiple washover deposits (deposits left by large waves such as tsunamis that overtop the coastal topographic high) have been found dating back as far as approximately 6,000 years (e.g., Sato et al., 2016; Fujiwara et al., 2020). About 6,000 years ago, the coastline in the Hamamatsu Plain was located about 4 km inland from its current position, and the coastline gradually progressed seaward as sedimentation continued, expanding the plain.
At the wetland site about 3.8 km inland from the present coastline, only washover deposits older than approximately 3200 years are found. At the wetland site about 2.1 km from the present coastline, only washover deposits older than around 2100-1200 years are found. At the wetland site 1.3 km from the present coastline, washover deposits older than the 1498 Meio earthquake are discovered. Additionally, for individual survey sites, the upper (younger) washover deposits tend to be thinner and finer-grained. The distribution of younger washover deposits shifts further seaward, suggesting that as the Hamamatsu Plain expanded seaward, large waves such as tsunamis no longer reached deep inland. This implies that there is no evidence of giant tsunamis with extremely long run-up distances occurring within the past 6,000 years (Fujiwara et al., 2020).
The above-mentioned Meio tsunami deposit was found from a marshland more than 1.2 km away from the coastline and rivers at the time of the earthquake and is the youngest washover deposit in this marshland over the past 2,500 years but are markedly thicker and coarser-grained than the other older washover deposits at this site. Array drilling survey at this site revealed that the Meio tsunami deposit continuously distributed over an area of at least 600 m from east to west and 100 m from north to south. It consists mainly of medium-grained sand, is relatively well sorted, and is 10-15 cm thick. The Meio tsunami may have been exceptionally larger than other tsunamis that have occurred in the past 6000 years. However, no trace of the Meio tsunami has been found inland from this site, and the inundation distance is unknown. Increasing the number of survey sites and reconstructing the run-up area are key to confirm whether the Meio tsunami can be regarded as the maximum possible tsunami in the eastern Nankai Trough region.
References
Fujiwara, O. et al., 2020. Earth-Science Reviews 210; https://doi.org/10.1016/j.earscirev.2020.103333
Fujiwara, O. et al., 2023. Jour. Geography 132, 309-325; doi:10.5026/jgeography.132.309
Nanayama, F. et al., 2003. Nature 424, 660-663; https://doi.org/10.1038/nature01864
Sato, Y. et al., 2016. The Quaternary Research 55, 17-35
Sawai et al., 2012. Geophys. Res. Lett. 39, L21309, doi:10.1029/2012GL053692