5:15 PM - 6:30 PM
[MIS15-P02] Characteristics of event deposits and their distributions based on a drilling survey in Sekine-hama, the northern part of the Shimokita Peninsula, Aomori Prefecture
Keywords:tsunami deposits, Shimokita Peninsula, B-Tm tephra
This study reports the event deposits found at Sekinehama, Aomori Prefecture, which is the northernmost part of the Shimokita Peninsula. Tsunamis associated with earthquakes along the Japan and Kuril trenches and volcanic activities in the 17th-18th century possibly could reach this site. Therefore, tsunami deposit research at this site provides information for the assessment of tsunami hazards in this area and understanding of the mechanism of earthquakes and tsunamis along the subduction zone.
Our study site, Sekinehama, is located between Omazaki and Shiriyazaki, and the late Pleistocene marine terraces and sand dunes were distributed behind the coast. In the 2011 Tohoku-oki earthquake, tsunami runup height of 2 to 6 m was reported (The 2011 Tohoku Earthquake Tsunami Joint Survey (TTJS) Group (http://www.coastal.jp/ttjt/)). The outcrops are exposed along the coastline and its height is ca. 5 m. In 2020, we conducted the coring survey with the Handy-Geoslicer (GS) and obtained nine cores at the lowland in the incised valley of the late Pleistocene marine terrace. In this study, we conducted core descriptions, X-ray CT imaging, grain size analysis, tephra analysis, and radiocarbon dating.
At the outcrops, we found more than 10 sandy event deposits interbedded within the soil and marsh sediments. Additionally, we identified two tephras; one is the Baegdusan-Tomakomai (B-Tm) tephra (AD946; Oppenheimer et al., 2017), another is the Towada-Chuseri (To-Cu) tephra (ca. 6 ka; Mclean et al., 2018; Ishimura and Hiramine, 2020). We identified four event deposits above the B-Tm tephra. One of them after 530-490 calBP (1420-1460 calAD) showed an erosional feature at the base, indicating a large flow speed. Below the B-Tm tephra, we also identified some event deposits with grading and erosive contact at the base.
In the preliminary research, we found one sandy event deposit after 530-490 calBP (1420-1460 calAD) at the lowland in the incised valley of the late Pleistocene marine terrace. From the grain analysis, this sandy deposit has a similar grain size distribution with the modern beach sediments. In the GS cores, drilled at 200-400 m distance from the coast, we traced this sandy layer up to 400 m from the coast. From this drilled survey, we confirmed that this sandy layer was only one event deposit above the B-Tm tephra. The sandy layers above the B-Tm tephra show multiple units in the seaside cores and its thickness decreases toward inland. These characteristics indicate that the sand layer is related to paleo-tsunami events after the 15th century. On the other hand, several event deposits were also identified below the B-Tm tephra and radiocarbon dates revealed that the GS cores cover the recent six thousand years. We will show the grain size analysis and additional radiocarbon dates in the presentation.
This work was supported by the Sasakawa Scientific Research Grant from The Japan Science Society. This study was partially performed under the cooperative research program of the Center for Advanced Marine Core Research, Kochi University (Accept No. 20B052).
Our study site, Sekinehama, is located between Omazaki and Shiriyazaki, and the late Pleistocene marine terraces and sand dunes were distributed behind the coast. In the 2011 Tohoku-oki earthquake, tsunami runup height of 2 to 6 m was reported (The 2011 Tohoku Earthquake Tsunami Joint Survey (TTJS) Group (http://www.coastal.jp/ttjt/)). The outcrops are exposed along the coastline and its height is ca. 5 m. In 2020, we conducted the coring survey with the Handy-Geoslicer (GS) and obtained nine cores at the lowland in the incised valley of the late Pleistocene marine terrace. In this study, we conducted core descriptions, X-ray CT imaging, grain size analysis, tephra analysis, and radiocarbon dating.
At the outcrops, we found more than 10 sandy event deposits interbedded within the soil and marsh sediments. Additionally, we identified two tephras; one is the Baegdusan-Tomakomai (B-Tm) tephra (AD946; Oppenheimer et al., 2017), another is the Towada-Chuseri (To-Cu) tephra (ca. 6 ka; Mclean et al., 2018; Ishimura and Hiramine, 2020). We identified four event deposits above the B-Tm tephra. One of them after 530-490 calBP (1420-1460 calAD) showed an erosional feature at the base, indicating a large flow speed. Below the B-Tm tephra, we also identified some event deposits with grading and erosive contact at the base.
In the preliminary research, we found one sandy event deposit after 530-490 calBP (1420-1460 calAD) at the lowland in the incised valley of the late Pleistocene marine terrace. From the grain analysis, this sandy deposit has a similar grain size distribution with the modern beach sediments. In the GS cores, drilled at 200-400 m distance from the coast, we traced this sandy layer up to 400 m from the coast. From this drilled survey, we confirmed that this sandy layer was only one event deposit above the B-Tm tephra. The sandy layers above the B-Tm tephra show multiple units in the seaside cores and its thickness decreases toward inland. These characteristics indicate that the sand layer is related to paleo-tsunami events after the 15th century. On the other hand, several event deposits were also identified below the B-Tm tephra and radiocarbon dates revealed that the GS cores cover the recent six thousand years. We will show the grain size analysis and additional radiocarbon dates in the presentation.
This work was supported by the Sasakawa Scientific Research Grant from The Japan Science Society. This study was partially performed under the cooperative research program of the Center for Advanced Marine Core Research, Kochi University (Accept No. 20B052).