5:15 PM - 6:45 PM
[MIS20-P05] Numerical Validation of the Tsunami Deposit using the 1498 Meio-Tokai Tsunami Source Model Estimated from Tsunami Trace Heights
Keywords:Historical earthquake, Tsunami deposit, The 1498 Meio-Tokai earthquake, Sediment transport numerical simulation
The tsunami associated with the 1498 Meio-Tokai earthquake, which occurred in the Nankai Trough subduction zone, caused the catastrophic damages along the Pacific coast from Wakayama Prefecture to Chiba Prefecture. The tsunami source models were estimated by Annaka et al. (2003) and Abe (2017), and Kitamura et al. (2020) suggested that submarine landslides may have caused the tsunami. The Meio Tokai tsunami traces are few compared to the tsunamis associated with the 1854 Ansei Tokai and the 1854 Ansei Nankai, and the 1707 Hoei earthquakes. Therefore, in this study, we examine sediment transport numerical simulation (TUNAMI-STM; Takahashi et al., 1999; Sugawara et al., 2019) using a tsunami source model (Kusumoto et al., 2023) estimated by the trace height distribution (Tsunami trace database; Tohoku University, Nuclear Regulation Authority) for the Kogareike, Minamiise Town, Mie Prefecture (Shimada et al. 2023), the Shirasuka, Kosai City, Shizuoka Prefecture (Komatsubara et al. 2008), and Hamatome, Yaizu City, Shizuoka Prefecture (Kitamura et al. 2020). Moreover, we compare with observation facts and simulated results.
Kogare lake is a coastal lake with an approximately 5 m high of embankment between the lake and the open sea. Shirasuka and Hamatome are coastal lowlands less than 5 m above sea level, and main roads and coastal structures have been developed along the coastline. In both cases, major highways and coastal structures were removed to bring the coastal topography closer to the one at the time of the earthquake from the digital elevation model published by the Geographical Survey Institute of Japan, and the digital Ino map (supervised by Murayama, 2015). As the tsunami wave source model, we adopted the five-subfaults source model of Kusumoto et al. (2023). The calculation time was set to 3 hours after the earthquake occurred. In addition, the grain size of the tsunami deposits corresponding to the Meio-Tokai Earthquake consists of fine-grained sand to coarse-grained sand at all three locations, thus numerical simulations were performed with the average grain size used as 0.267 mm.
Tsunamis with a maximum height of 5.8 m attack in Hamatome and overcame the beach embankment at least eight times to enter coastal areas. Sediment was repeatedly deposited and eroded each time a tsunami inundation, and approximately 28 cm of sediment had been deposited three hours after the earthquake. The thickness of the tsunami deposits corresponding to the Meio-Tokai earthquake in the sediment core was reported to be a maximum of 35 cm, which was generally consistent with the calculated results.
Tsunami waves of up to 6.8 m attack in Shirasuka and overcame the beach embankment at least seven times to enter coastal lowland. The amount of sediment deposited was largely contributed by the fifth and seventh waves, which arrived approximately 110 and 130 minutes after the earthquake, and approximately 49 cm of sediment was deposited 3 hours after the earthquake. The thickness of the tsunami deposits corresponding to the Meio-Tokai Earthquake in the sediment core was reported to be a maximum of 50 cm, which was generally consistent with the calculated results.
Tsunamis of up to 4.6 m high attack in Kogare lake and overcame the beach embankment at least three times to enter Kogare lake. The largest contribution to the amount of sediment deposited is the first wave, which arrives approximately 22 minutes after the earthquake. Approximately 40 cm of sand had deposited in the pond three hours after the earthquake occurred. Conversely, the thickness of the tsunami deposits corresponding to the Meio-Tokai earthquake in the sediment core was reported to be less than 1 cm, which was a large difference from the calculated results. This is because the elevation of the beach ridge at the time of the earthquake is greatly uncertain, suggesting that a tsunami larger than the actual one invaded and caused excessive sedimentation.
From the above verification, the simulated results and observed facts are consistent, indicating the possibility of using tsunami deposits for the tsunami source model. Conversely, the results for Kogare lake revealed the importance of topographical restoration (e.g., elevation of beach embankments, position of shoreline) in the event of a tsunami. Utilizing tsunami deposits for tsunami source models, such as adjusting the beach ridge elevation of Kogare lake and conducting tsunami sediment transport simulation in the future, and quantitatively restoring of past beach ridge elevations based on ground-penetrating radar observations.
Kogare lake is a coastal lake with an approximately 5 m high of embankment between the lake and the open sea. Shirasuka and Hamatome are coastal lowlands less than 5 m above sea level, and main roads and coastal structures have been developed along the coastline. In both cases, major highways and coastal structures were removed to bring the coastal topography closer to the one at the time of the earthquake from the digital elevation model published by the Geographical Survey Institute of Japan, and the digital Ino map (supervised by Murayama, 2015). As the tsunami wave source model, we adopted the five-subfaults source model of Kusumoto et al. (2023). The calculation time was set to 3 hours after the earthquake occurred. In addition, the grain size of the tsunami deposits corresponding to the Meio-Tokai Earthquake consists of fine-grained sand to coarse-grained sand at all three locations, thus numerical simulations were performed with the average grain size used as 0.267 mm.
Tsunamis with a maximum height of 5.8 m attack in Hamatome and overcame the beach embankment at least eight times to enter coastal areas. Sediment was repeatedly deposited and eroded each time a tsunami inundation, and approximately 28 cm of sediment had been deposited three hours after the earthquake. The thickness of the tsunami deposits corresponding to the Meio-Tokai earthquake in the sediment core was reported to be a maximum of 35 cm, which was generally consistent with the calculated results.
Tsunami waves of up to 6.8 m attack in Shirasuka and overcame the beach embankment at least seven times to enter coastal lowland. The amount of sediment deposited was largely contributed by the fifth and seventh waves, which arrived approximately 110 and 130 minutes after the earthquake, and approximately 49 cm of sediment was deposited 3 hours after the earthquake. The thickness of the tsunami deposits corresponding to the Meio-Tokai Earthquake in the sediment core was reported to be a maximum of 50 cm, which was generally consistent with the calculated results.
Tsunamis of up to 4.6 m high attack in Kogare lake and overcame the beach embankment at least three times to enter Kogare lake. The largest contribution to the amount of sediment deposited is the first wave, which arrives approximately 22 minutes after the earthquake. Approximately 40 cm of sand had deposited in the pond three hours after the earthquake occurred. Conversely, the thickness of the tsunami deposits corresponding to the Meio-Tokai earthquake in the sediment core was reported to be less than 1 cm, which was a large difference from the calculated results. This is because the elevation of the beach ridge at the time of the earthquake is greatly uncertain, suggesting that a tsunami larger than the actual one invaded and caused excessive sedimentation.
From the above verification, the simulated results and observed facts are consistent, indicating the possibility of using tsunami deposits for the tsunami source model. Conversely, the results for Kogare lake revealed the importance of topographical restoration (e.g., elevation of beach embankments, position of shoreline) in the event of a tsunami. Utilizing tsunami deposits for tsunami source models, such as adjusting the beach ridge elevation of Kogare lake and conducting tsunami sediment transport simulation in the future, and quantitatively restoring of past beach ridge elevations based on ground-penetrating radar observations.