2:30 PM - 2:45 PM
[MIS11-04] Characters of the ostracod shells in the Tsunami deposits and provenance of tsunami deposits in the 2024 Noto Peninsula earthquake.
Keywords:2024 Noto Peninsula Earthquake, Tsunami deposits, ostracods
Ostracod, a kind of Crustacean, are found in sublittoral bottom sediments frequently. Particularly, high abundance and diversity are recognized from the coastal area in the recent seas around Japan. Ostracod shells are useful tools to estimate the area that tsunami deposits supplied, because they are identified from tsunami deposits during the Holocene. However, a few samples were used and no records collected from sea bottom sediments along the line that tsunami came. Thus, origin of the tsunami deposits supply was vague. Whereas faunal changes in the tsunami deposits of the core (Mischke et al., 2012) and in the sea bottom (Irizuki et al., 2019) were investigated, there are few the research for the characters of ostracod shells in the terrestrial tsunami deposits. Our purposes of this study are 1) to estimate a specific area for the origin of the tsunami deposit supply, 2) to identify ostracod characters in the terrestrial tsunami deposits.
Samples we used were terrestrial tsunami deposits around Kurikawashiri River, Noto Town, and seven sea bottom sediments cores taken by gravity corer. Terrestrial tsunami deposits were taken from three sites in the right bank of the Kurikawashiri River, and from one site in the left bank of the river, respectively, on April 2024. Tsunami deposits of width of 7.5 cm and thickness of 1 cm were cut in 1 cm thick vertically. The sea bottom cores were taken at July 2024 in the areas of water depths of 5.1, 10, 14.2, 18, 22, 23.5, and 32.4 m, along the line of tsunami invasion from the river mouth. The lowest 1 cm thick sediments in each core were used.
Sand bottom dweller and phytal ostracod species were identified from all samples. Total 36 terrestrial samples and four sea bottom samples containing more than 50 ostracod specimens were used for Q-mode cluster analysis. As a results, five biofacies were recognized. Thirty-six terrestrial samples were found in biofacies 2 and 3. Genus Aurila, Cythere, Loxoconcha and Neonesidea show relative high abundance in both biofacies. Four sea bottom samples were found in biofacies 1, 3, 4, and 5, respectively. Similarities in ostracod species compositions between terrestrial sediments and sea bottom sediments suggest that terrestrial tsunami sediments were supplied from sea bottom in the water depth of 5–15 m located 0.7–2 km far from river mouth. This water depth is consistent with estimated water depth from tsunami deposits of 2011 off Tohoku earthquake tsunami.
Tsunami deposits in the terrestrial sites are composed of laminated sand with 8–16 cm thickness. The 1 to 4 cm thick layers consist by conglomerate and plant materials were found in the lower part of the tsunami deposits. Further 2 cm thick mud layer is recognized in the upper most part from the site at the left bank of the river. The number of ostracod individuals per 1g sediments increased upward from the bottom, then decreased once and increased again in all terrestrial sites. The decreasing horizons correspond to slight decrease in mud content, and lower boundary of the plant fragment layer. Few ostracods occurred from mud layer in the top of the tsunami deposits at the left bank. For the terrestrial tsunami deposits, Biofacies 2 was recognized above the biofacies 3 at the sites excepting for one site that no biofacies 2 was found. This observation suggests that tsunami transported sea bottom sediments from shallower area first, then from deeper area. Biofacies 3 was found in all sites and in most horizons of the tsunami deposits in 3 sites at the right bank. However, biofacies 2 was identified dominantly in the site at the left bank. We may discuss the behavior of the tsunami flood and depositional process by tsunami in this site, comparing our ostracod records to sedimentary knowledge and estimates from model simulations.
Samples we used were terrestrial tsunami deposits around Kurikawashiri River, Noto Town, and seven sea bottom sediments cores taken by gravity corer. Terrestrial tsunami deposits were taken from three sites in the right bank of the Kurikawashiri River, and from one site in the left bank of the river, respectively, on April 2024. Tsunami deposits of width of 7.5 cm and thickness of 1 cm were cut in 1 cm thick vertically. The sea bottom cores were taken at July 2024 in the areas of water depths of 5.1, 10, 14.2, 18, 22, 23.5, and 32.4 m, along the line of tsunami invasion from the river mouth. The lowest 1 cm thick sediments in each core were used.
Sand bottom dweller and phytal ostracod species were identified from all samples. Total 36 terrestrial samples and four sea bottom samples containing more than 50 ostracod specimens were used for Q-mode cluster analysis. As a results, five biofacies were recognized. Thirty-six terrestrial samples were found in biofacies 2 and 3. Genus Aurila, Cythere, Loxoconcha and Neonesidea show relative high abundance in both biofacies. Four sea bottom samples were found in biofacies 1, 3, 4, and 5, respectively. Similarities in ostracod species compositions between terrestrial sediments and sea bottom sediments suggest that terrestrial tsunami sediments were supplied from sea bottom in the water depth of 5–15 m located 0.7–2 km far from river mouth. This water depth is consistent with estimated water depth from tsunami deposits of 2011 off Tohoku earthquake tsunami.
Tsunami deposits in the terrestrial sites are composed of laminated sand with 8–16 cm thickness. The 1 to 4 cm thick layers consist by conglomerate and plant materials were found in the lower part of the tsunami deposits. Further 2 cm thick mud layer is recognized in the upper most part from the site at the left bank of the river. The number of ostracod individuals per 1g sediments increased upward from the bottom, then decreased once and increased again in all terrestrial sites. The decreasing horizons correspond to slight decrease in mud content, and lower boundary of the plant fragment layer. Few ostracods occurred from mud layer in the top of the tsunami deposits at the left bank. For the terrestrial tsunami deposits, Biofacies 2 was recognized above the biofacies 3 at the sites excepting for one site that no biofacies 2 was found. This observation suggests that tsunami transported sea bottom sediments from shallower area first, then from deeper area. Biofacies 3 was found in all sites and in most horizons of the tsunami deposits in 3 sites at the right bank. However, biofacies 2 was identified dominantly in the site at the left bank. We may discuss the behavior of the tsunami flood and depositional process by tsunami in this site, comparing our ostracod records to sedimentary knowledge and estimates from model simulations.