4:15 PM - 4:30 PM
[MIS11-10] Reexamination of ages of event deposits and their correlation to historical tsunamis in the southern Sanriku Coast
The Sanriku Coast is known for tsunami-prone area, and its southern portion is facing directly to the main rupture area of the 2011 Tohoku-oki earthquake. No paleotsunami record has so far been obtained beyond south of Onuma at Minamisanriku Town, Miyagi Prefecture (e.g. Ishimura and Miyauchi, 2017); this hinders estimation of sources of historical earthquakes and tsunamis such as the 869 Jogan, the 1454 Kyotoku and the 1611 Keicho earthquakes.
In this study, we conducted four field campaign from December 2022 to December 2024 at Kitakami Town in Ishinomaki City, which is located in the southern Sanriku Coast. Observations and laboratory analysis of 14 cores obtained by the year of 2023 revealed changes in background sedimentary environment from a silt layer containing shell fragment at a depth of approximately 3–5 meters to an organic silt layer at a depth of 1–3 meters. Multiple event sand layers and one event clay layer were intercalated within the background deposits. An upper sand layer (Ev1) was correlated across eight different sites and the clay layer (Ev2) was observed at four different sites. In the previous report, sedimentological features, laboratory data and sedimentary environment and topography suggested a tsunami origin of Ev1 and a flood origin of Ev2. Radiocarbon ages indicated that Ev1 was formed after 1730 cal AD, while Ev2 was formed after 1640 cal AD. However, different dating materials extracted from a same stratigraphic horizon returned varying ages with errors of several hundred years, highlighting the uncertainty of the dating results. On the other hand, cores collected from six new sites in December 2024 contained allochthonous rocks fragments in Ev1, suggesting that this sand layer is possibly of an anthropogenic origin. Additionally, an event sand layer was newly identified beneath Ev2.
In this presentation, we will discuss changes in the depositional environment and the presence or absence of allochthonous material influx by combining the previous results and new data on CNS elemental and biomarker analyses. Correlation between the sand and clay layers and historical tsunami events will be reexamined by using both existing and new age data.
In this study, we conducted four field campaign from December 2022 to December 2024 at Kitakami Town in Ishinomaki City, which is located in the southern Sanriku Coast. Observations and laboratory analysis of 14 cores obtained by the year of 2023 revealed changes in background sedimentary environment from a silt layer containing shell fragment at a depth of approximately 3–5 meters to an organic silt layer at a depth of 1–3 meters. Multiple event sand layers and one event clay layer were intercalated within the background deposits. An upper sand layer (Ev1) was correlated across eight different sites and the clay layer (Ev2) was observed at four different sites. In the previous report, sedimentological features, laboratory data and sedimentary environment and topography suggested a tsunami origin of Ev1 and a flood origin of Ev2. Radiocarbon ages indicated that Ev1 was formed after 1730 cal AD, while Ev2 was formed after 1640 cal AD. However, different dating materials extracted from a same stratigraphic horizon returned varying ages with errors of several hundred years, highlighting the uncertainty of the dating results. On the other hand, cores collected from six new sites in December 2024 contained allochthonous rocks fragments in Ev1, suggesting that this sand layer is possibly of an anthropogenic origin. Additionally, an event sand layer was newly identified beneath Ev2.
In this presentation, we will discuss changes in the depositional environment and the presence or absence of allochthonous material influx by combining the previous results and new data on CNS elemental and biomarker analyses. Correlation between the sand and clay layers and historical tsunami events will be reexamined by using both existing and new age data.