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[MIS11-07] Temporal change of the 2011 off the Pacific coast of Tohoku Earthquake tsunami deposits in the shallow marine area.
Keywords:The 2011 off the Pacific coast of Tohoku Earthquake, tsunami deposit, shallow marine area
The 2011 off the Pacific coast of Tohoku Earthquake (the 2011 Tohoku-oki earthquake) occurred on March 11th, 2011, and the resulting tsunami caused extensive damage in the coastal areas of the Tohoku region. After the 2011 Tohoku-oki Earthquake, tsunami deposits have been researched from marine to land areas, and it has been reported that tsunami deposits are distributed over a wide area. In the shallow marine area of the Sanriku Rias coast, the topographic changes caused by the tsunami, the characteristics and distribution of tsunami deposits, and the depositional process during the tsunami were investigated. In terms of topographic changes, the formation of erosion marks on the seafloor caused by tsunami and changes after 10 years have been confirmed (Yokoyama et al., 2024). For example, the 2011 Tohoku-oki tsunami formed more than 300 current marks on the seafloor at a depth of about 10 to 30 m in Toni Bay. It is also reported that about 10 years after the earthquake, the amount of topographic change is large at depths shallower than about 20 m, and current marks are less visible (Yokoyama et al., 2024). However, it is not clear whether tsunami deposits are preserved or lost in the areas where such changes occur.
In this study, we attempted to understand the change process of tsunami deposits in Toni Bay about 10 years after the 2011 Tohoku-oki earthquake. In Toni Bay, seafloor topography, stratigraphic exploration, and core sampling were conducted from 2012 to 2015, immediately after the 2011 Tohoku-oki earthquake. In addition to these surveys, additional bathymetry (in 2020) and stratigraphy and core sampling (in 2023 and 2024) were conducted to understand the changes in about 10 years after the earthquake.
The results from the post-tsunami period (2012-2015) showed that several reflective surfaces were identified below the seafloor in the seismic data. A characteristic reflective surface (R1) was found just below the seafloor, and its distribution was confirmed at depths from 0 to 40 m throughout the bay. The thickness between R1 and the seafloor (layer A) ranged from about 25 to 110 cm. The core samples consist of the sand layer (Unit 1) and mud layer (Unit 2) from the top, which are separated by a clear boundary. Unit 1 was identified as the 2011 tsunami sediments based on lithology (grading, parallel lamina etc.), and Unit 2 as the normal sediments before the tsunami. These features are consistent with those of the 2011 tsunami deposits in Hirota Bay (Yokoyama et al., 2021). The thickness of the 2011 tsunami deposit was confirmed to be between 19 and 89 cm based on the core samples collected in 2012-2013. Based on the comparison of seismic data and core samples, layer A in the seismic data is considered to represent the 2011 tsunami deposits.
The results of the seismic data from 2023 and immediately after the tsunami showed no significant changes. Comparison of core samples taken in 2012 and 2023 at the almost same site (approximately 17 m depth) showed that the thickness of the tsunami deposit tended to be slightly thicker, 65 cm in 2012 and 76 cm in 2023. However, the overall grain-size pattern was similar in both years. Therefore, the sediments do not show as much change as observed in the bathymetry, suggesting that the shallow marine area well preserved the tsunami deposits. On the other hand, the top layer in 2023 consists of silt to very fine-grained sand, which tends to be slightly finer-grained than that in 2012 based on the grain size and character similar to those in the lower Unit 2. Therefore, it is considered that the composition of the sediments returned to the pre-tsunami in the bay as a result of sedimentation during the 10 years after the tsunami.
[reference] Yokoyama et al., 2021, Journal of the Sedimentological Society of Japan, 79(2)47-69. Yokoyama et al., 2024, the 131st Annual Meeting of the Geological Society of Japan.
In this study, we attempted to understand the change process of tsunami deposits in Toni Bay about 10 years after the 2011 Tohoku-oki earthquake. In Toni Bay, seafloor topography, stratigraphic exploration, and core sampling were conducted from 2012 to 2015, immediately after the 2011 Tohoku-oki earthquake. In addition to these surveys, additional bathymetry (in 2020) and stratigraphy and core sampling (in 2023 and 2024) were conducted to understand the changes in about 10 years after the earthquake.
The results from the post-tsunami period (2012-2015) showed that several reflective surfaces were identified below the seafloor in the seismic data. A characteristic reflective surface (R1) was found just below the seafloor, and its distribution was confirmed at depths from 0 to 40 m throughout the bay. The thickness between R1 and the seafloor (layer A) ranged from about 25 to 110 cm. The core samples consist of the sand layer (Unit 1) and mud layer (Unit 2) from the top, which are separated by a clear boundary. Unit 1 was identified as the 2011 tsunami sediments based on lithology (grading, parallel lamina etc.), and Unit 2 as the normal sediments before the tsunami. These features are consistent with those of the 2011 tsunami deposits in Hirota Bay (Yokoyama et al., 2021). The thickness of the 2011 tsunami deposit was confirmed to be between 19 and 89 cm based on the core samples collected in 2012-2013. Based on the comparison of seismic data and core samples, layer A in the seismic data is considered to represent the 2011 tsunami deposits.
The results of the seismic data from 2023 and immediately after the tsunami showed no significant changes. Comparison of core samples taken in 2012 and 2023 at the almost same site (approximately 17 m depth) showed that the thickness of the tsunami deposit tended to be slightly thicker, 65 cm in 2012 and 76 cm in 2023. However, the overall grain-size pattern was similar in both years. Therefore, the sediments do not show as much change as observed in the bathymetry, suggesting that the shallow marine area well preserved the tsunami deposits. On the other hand, the top layer in 2023 consists of silt to very fine-grained sand, which tends to be slightly finer-grained than that in 2012 based on the grain size and character similar to those in the lower Unit 2. Therefore, it is considered that the composition of the sediments returned to the pre-tsunami in the bay as a result of sedimentation during the 10 years after the tsunami.
[reference] Yokoyama et al., 2021, Journal of the Sedimentological Society of Japan, 79(2)47-69. Yokoyama et al., 2024, the 131st Annual Meeting of the Geological Society of Japan.