10:00 AM - 10:15 AM
[MIS13-05] Facies of Tsunami deposits from the 1804 Kisakata Earthquake
Keywords:Tsunami deposit, thickness, grain-size, Akita prefecture
A strong earthquake with a magnitude of 7.3 hit the Japan Sea coast in the Tohoku district on July 10, 1804. The vertical deformation produced by the earthquake in the Kisakata area probably amounted to 180 cm uplift (Hirano et al., 1979). Tsunamis caused by the earthquake flooded areas along the coast between Noshiro and Sanze. Maximum wave height varied considerably along the coast, reaching 5 m in the Kisakata area (Hatori, 1986). Here, we present the first stratigraphic evidence of 1804 tsunami inundation in the paleo Kisakata lagoon.
The survey sites were set up at two survey lines. First, a 500 m survey line is vertical to the coastline in a paddy field about 700 m from the shoreline (Line 1). Second, a 300 m survey line is parallel to the coastline in a paddy field about 600 m from the shoreline (Line 2). A 2 m long Geoslicer and a 1 m long Hand Corer were used to drill the core. Two 8 m boring core (KK1 and KK2 core) drilled by Japan Sea Earthquake Tsunami Research Project (https://www.eri.u-tokyo.ac.jp/project/Japan_Sea/gaiyou.html) were also used. We investigated the core samples by stratified phase observation, description, grain sizes analysis and the principal component element analysis of volcanic glass in the tsunami deposits measured by EPMA (Electron Probe Micro Analyzer) in order to compare with the widespread tephras that may have fallen in this area.
According to stratified phase observation, the thickness of the tsunami deposits showed the thinning trend for the coast to the inland. The maximum thickness of the sand layer at Line 1 is 23 cm, and it is disappeared around 1,300 m from the seaside. Line 2 is 29 cm, and it becomes thinner from south to north. In addition, the tsunami deposits layers contained on KK2 and Line 1 (eight cores) were analyzed by grain-size analysis three times every 1~2 cm. The grain-size of the tsunami deposits showed the fining trend for the coast to the inland. The mean median of the clastic particles varied greatly from 1.91 φ to 4.06 φ. This change ranged from the medium sand size to the silt size.
The survey sites were set up at two survey lines. First, a 500 m survey line is vertical to the coastline in a paddy field about 700 m from the shoreline (Line 1). Second, a 300 m survey line is parallel to the coastline in a paddy field about 600 m from the shoreline (Line 2). A 2 m long Geoslicer and a 1 m long Hand Corer were used to drill the core. Two 8 m boring core (KK1 and KK2 core) drilled by Japan Sea Earthquake Tsunami Research Project (https://www.eri.u-tokyo.ac.jp/project/Japan_Sea/gaiyou.html) were also used. We investigated the core samples by stratified phase observation, description, grain sizes analysis and the principal component element analysis of volcanic glass in the tsunami deposits measured by EPMA (Electron Probe Micro Analyzer) in order to compare with the widespread tephras that may have fallen in this area.
According to stratified phase observation, the thickness of the tsunami deposits showed the thinning trend for the coast to the inland. The maximum thickness of the sand layer at Line 1 is 23 cm, and it is disappeared around 1,300 m from the seaside. Line 2 is 29 cm, and it becomes thinner from south to north. In addition, the tsunami deposits layers contained on KK2 and Line 1 (eight cores) were analyzed by grain-size analysis three times every 1~2 cm. The grain-size of the tsunami deposits showed the fining trend for the coast to the inland. The mean median of the clastic particles varied greatly from 1.91 φ to 4.06 φ. This change ranged from the medium sand size to the silt size.