4:45 PM - 5:00 PM
[MIS15-08] Identification of invisible tsunami deposits formed by the 2011 Tohoku-oki tsunami
Keywords:Invisible tsunami deposits, 2011 Tohoku-oki tsunami, Aomori, Grain-size distribution, Organic geochemistry
We conducted a field survey at Oirase town, Aomori Prefecture, to examine the identification of invisible tsunami deposit. Evidence of paleo-tsunamis is not always preserved as a visible deposit. Near the inundation limit, tsunami deposits tend to be thin and fine-grained because the weakened tsunami flow cannot transport coarser particles and large amounts of sand; thus, tsunami deposits can be difficult to identify from underlying and overlying sediment by eye observation. Identification of invisible tsunami evidence as well as visible tsunami deposits is necessary for more accurate estimation of tsunami recurrence and its magnitude.
To examine the identification of invisible tsunami deposits, we focused on the 2011 Tohoku-oki tsunami because we have information on inundation area and sediment distribution. In October 2020, we set two transects that were nearly perpendicular to the shoreline. About 20 cm deep sediment samples were directly collected from walls of excavated pits at 12 locations. Based on the eye observation, sediments consist of an upper soil layer and a lower thick fine to medium sand layer. At the relatively seaward locations, 1.5 to 4 cm-thick fine to medium sand layer, which is considered as the 2011 tsunami deposit, was intercalated within the soil layer. On the other hand, no sand layer within the soil was found by eye observation at the relatively landward locations. As well as the eye observation, we could not identify event deposits by CT images and the depth profile of sand contents. In the study sites, wind may constantly transport sand particles from the beach to the landward sampling locations around the inundation limit (about 300 m from the shoreline and about 150 m from the beach). This setting made it difficult to identify evidence of tsunami inundation based on CT images and sand contents. In the presentation, we discuss about invisible tsunami deposits based on the data of CT, sand contents, and depth profile of grain size distribution and organic geochemistry.
To examine the identification of invisible tsunami deposits, we focused on the 2011 Tohoku-oki tsunami because we have information on inundation area and sediment distribution. In October 2020, we set two transects that were nearly perpendicular to the shoreline. About 20 cm deep sediment samples were directly collected from walls of excavated pits at 12 locations. Based on the eye observation, sediments consist of an upper soil layer and a lower thick fine to medium sand layer. At the relatively seaward locations, 1.5 to 4 cm-thick fine to medium sand layer, which is considered as the 2011 tsunami deposit, was intercalated within the soil layer. On the other hand, no sand layer within the soil was found by eye observation at the relatively landward locations. As well as the eye observation, we could not identify event deposits by CT images and the depth profile of sand contents. In the study sites, wind may constantly transport sand particles from the beach to the landward sampling locations around the inundation limit (about 300 m from the shoreline and about 150 m from the beach). This setting made it difficult to identify evidence of tsunami inundation based on CT images and sand contents. In the presentation, we discuss about invisible tsunami deposits based on the data of CT, sand contents, and depth profile of grain size distribution and organic geochemistry.