9:15 AM - 9:30 AM
[MIS13-02] Estimating the sources for 17th century tsunami by using tsunami deposits distributed from the Hidaka coast to Uchiura Bay, Hokkaido, Japan
Keywords:tsunami deposits, 1640 Hokkaido Komagatake eruption, 17th century great earthquake, 1611 Keicho-Oshu earthquake, tsunami simulation, landslide simulation
The 17th century tsunami deposits are widely distributed along the Pacific coast of Hokkaido and Tohoku.Tsunami deposits in the eastern Pacific coast of Hokkaido from the 17th century great earthquake or the 1611 Keicho-Oshu earthquake. In the Tohoku region, tsunami deposits from the 1611 Keicho-Oshu earthquake have been found at several locations. The 17th century tsunami deposits in Uchiura Bay of western Hokkaido from the 1640 Hokkaido Komagatake eruption tsunami. On the other hand, for the Hidaka to Iburi coast, there are possibilities that the tsunami from the 17th century great earthquake, the 1611 Keicho-Oshu earthquake, or other earthquakes.
The western Pacific coast of Hokkaido has been extensively surveyed for tsunami deposits and the distribution of 17th century tsunami deposits has been confirmed in detail (Takashimizu et al.,2007, Takashimizu et al.,2017, Nakanishi and Okamura,2019, etc.).Takashimizu et al.,2013, based on the sedimentological analysis of 17th century tsunami deposits in the eastern Iburi coast, found that at least three tsunamis run-ups in a single event.In this study, (1)the comparison of the distribution of tsunami deposits and the inundation area by tsunami simulation, and (2)the characteristics of tsunami arrivals in the eastern Iburi coast were used as the indicators of reproducibility to estimate the tsunami sources that explains the 17th century tsunami deposits distributed from Hidaka coast to the Uchiura Bay.
The tsunami source and numerical simulation method used in the study. The seismic tsunamis used in this study are the 17th century great earthquake, the 1611 Keicho-Oshu earthquake, and the Northern Sanriku-Oki earthquake. Nonlinear long wave theory was used for the numerical simulations. The 1640 Hokkaido Komagatake eruption tsunami was simulated numerically by combining the tsunami propagation process with the sediment collapse behavior. For landslide simulation, We used code:TITAN2D, which has been validated for its applicability in sediment down-slope experiments. The volume of sand from the collapse of the mountain was changed to 1.5 km3 and 1.7 km3, and the reproducibility of the amount of sediment flowing into the sea and the extent of sediment deposition in the sea by Yoshimoto et al.,2003 was confirmed. For the tsunami simulation, a two-layer model was used with the input condition of the temporal variation of the collapsed sand layer thickness and movement speed obtained from the landslide simulation. Nonlinear dispersive wave theory was used to account for the dispersive nature of the waves.
As a result, we confirmed that the only tsunami source that can explain the 17th century tsunami deposits distributed from Iburi coast to Uchiura Bay is the 1640 Hokkaido Komagatake eruption tsunami. In addition, the tsunami deposits in Artori, Date City (12.1 m above sea level) and Mukawa Town (8 m above sea level), which are located inland and have high distribution elevations, can be explained. We also confirmed that the tsunami could explain the characteristics of tsunami arrivals in the eastern Iburi coast. The reason for the multiple inland run-up of the tsunami is considered to be the edge waves generated by the curved topography of the Iburi coast and the seafloor topography where the continental shelf extends. The 17th century tsunami deposits at Kahari, Hidaka Town, are closer to the coastline (about 150 m inland) and have a lower distribution elevation (3.86 m above sea level) than other sites. Therefore, we confirmed that the tsunami could be explained by the 1640 Hokkaido Komagatake eruption tsunami, the 17th century great earthquake or the Northern Sanriku-Oki earthquake.
Considering the fact that the tsunami deposits distributed from the Hidaka coast to Uchiura Bay are only in the 17th century, and considering the geological evidence that does not show repeated tsunamis in the past, the tsunami that can most reasonably explain these tsunami deposits is the 1640 Hokkaido Komagatake eruption tsunami.
The western Pacific coast of Hokkaido has been extensively surveyed for tsunami deposits and the distribution of 17th century tsunami deposits has been confirmed in detail (Takashimizu et al.,2007, Takashimizu et al.,2017, Nakanishi and Okamura,2019, etc.).Takashimizu et al.,2013, based on the sedimentological analysis of 17th century tsunami deposits in the eastern Iburi coast, found that at least three tsunamis run-ups in a single event.In this study, (1)the comparison of the distribution of tsunami deposits and the inundation area by tsunami simulation, and (2)the characteristics of tsunami arrivals in the eastern Iburi coast were used as the indicators of reproducibility to estimate the tsunami sources that explains the 17th century tsunami deposits distributed from Hidaka coast to the Uchiura Bay.
The tsunami source and numerical simulation method used in the study. The seismic tsunamis used in this study are the 17th century great earthquake, the 1611 Keicho-Oshu earthquake, and the Northern Sanriku-Oki earthquake. Nonlinear long wave theory was used for the numerical simulations. The 1640 Hokkaido Komagatake eruption tsunami was simulated numerically by combining the tsunami propagation process with the sediment collapse behavior. For landslide simulation, We used code:TITAN2D, which has been validated for its applicability in sediment down-slope experiments. The volume of sand from the collapse of the mountain was changed to 1.5 km3 and 1.7 km3, and the reproducibility of the amount of sediment flowing into the sea and the extent of sediment deposition in the sea by Yoshimoto et al.,2003 was confirmed. For the tsunami simulation, a two-layer model was used with the input condition of the temporal variation of the collapsed sand layer thickness and movement speed obtained from the landslide simulation. Nonlinear dispersive wave theory was used to account for the dispersive nature of the waves.
As a result, we confirmed that the only tsunami source that can explain the 17th century tsunami deposits distributed from Iburi coast to Uchiura Bay is the 1640 Hokkaido Komagatake eruption tsunami. In addition, the tsunami deposits in Artori, Date City (12.1 m above sea level) and Mukawa Town (8 m above sea level), which are located inland and have high distribution elevations, can be explained. We also confirmed that the tsunami could explain the characteristics of tsunami arrivals in the eastern Iburi coast. The reason for the multiple inland run-up of the tsunami is considered to be the edge waves generated by the curved topography of the Iburi coast and the seafloor topography where the continental shelf extends. The 17th century tsunami deposits at Kahari, Hidaka Town, are closer to the coastline (about 150 m inland) and have a lower distribution elevation (3.86 m above sea level) than other sites. Therefore, we confirmed that the tsunami could be explained by the 1640 Hokkaido Komagatake eruption tsunami, the 17th century great earthquake or the Northern Sanriku-Oki earthquake.
Considering the fact that the tsunami deposits distributed from the Hidaka coast to Uchiura Bay are only in the 17th century, and considering the geological evidence that does not show repeated tsunamis in the past, the tsunami that can most reasonably explain these tsunami deposits is the 1640 Hokkaido Komagatake eruption tsunami.