- 著者
- 石村 大輔 山田 圭太郎 馬場 俊孝 近貞 直孝 Ramos Noelynna 南舘 健太 Cantillep Ace Punzalan Clod
- 雑誌
- 日本地震学会2023年度秋季大会
- 巻号頁・発行日
- 2023-09-29
1 0 0 0 南シナ海の津波リスク評価に向けた海岸巨礫の分布把握と波源推定
人類の多くは沿岸低地に居住しており,津波リスクにさらされている.中でもフィリピンが面する南シナ海の周辺人口は数億人にのぼり,東南アジアの主要都市が立地している.しかし,南シナ海の津波リスク把握のための基礎的情報が圧倒的に不足しており,実証的なデータ(津波堆積物)に立脚した津波リスク評価は喫緊の課題である.そこで本研究では,南シナ海における津波リスク評価の高度化を目指して,ルソン島の海岸に分布する巨礫を対象にし,1)空撮画像による巨礫の大きさ・分布の把握,2)巨礫を運搬させうる津波の数値計算,を行う.そして,過去に南シナ海を襲った津波の規模と波源の推定を行い,津波リスクを評価する.
1 0 0 0 OA 長野県,青木湖の湖成堆積物中に新たに見出された十和田-中掫テフラ
1 0 0 0 OA 岩手県山田町小谷鳥における6000年前以降の津波堆積物の年代とその分布
- 著者
- 石村 大輔 山田 圭太郎
- 雑誌
- JpGU-AGU Joint Meeting 2020
- 巻号頁・発行日
- 2020-03-13
Long-term paleo-seismic history is significant for the understanding of earthquake mechanisms and the assessment of earthquake and related hazards (e.g., tsunami). Especially, tsunami deposits research progressed after the devastating large tsunamis (e.g., the 2004 Indian Ocean earthquake and 2011 Tohoku-oki earthquake) and paleo-tsunami deposits have been identified all over the world. In the terrestrial environment, it is generally difficult to find an appropriate site for revealing the long-term paleo-tsunami history and such appropriate sites were limited due to landform development and artificial modification. It is true along the Pacific coast area in the Tohoku region, northeast Japan. However, the authors found the most appropriate site for the reconstruction of long-term paleo-tsunami history on the Sanriku Coast. This is the Koyadori site, where organic fine sediments accumulated continuously since Towada-Chuseri (To-Cu) tephra (ca. 6 ka; Mclean et al., 2018) and sediments supply from surrounding are small except for tsunami deposits. In this study, we show the ages of paleo-tsunami deposits since 6 ka and their subsurface distribution in Koyadori lowland based on dense and many excavation and drilling surveys. From 2012 to 2015, we conducted trench excavation survey (Ishimura and Miyauchi, 2015), outcrop survey (Ishimura and Miyauchi, 2015), drilling survey (Ishimura et al., 2014), short Geoslicer survey (Ishimura et al., 2015), and long Geoslicer survey from 200 m to 400 m distance from the coastline and used these samples to this study. In the laboratory, we conducted the radiocarbon dating, tephra analysis, μXRF analysis, and gravel roundness analysis for lateral correlations of sediments. In this study, we mainly used five long Geoslicer samples. All samples reached To-Cu tephra and, that is, they record a continuous 6 thousand years history. From these samples, we confirmed that there are 14 tsunami deposits including the 2011 event after To-Cu tephra and the average recurrence interval is estimated to be 300-400 years. Subsurface distribution of them was revealed by sedimentary facies, geochemical signature, gravel roundness, and radiocarbon dates of long Geoslicer, short Geoslicer, and drilling cores. As a result, the most appropriate site in Koyadori for tsunami deposits research is limited only 300 m to 350 m area distance from the coastline. On the other hand, in the seaside area behind the 5-m-high beach ridge (200 m to 300 m distance from the coastline), large erosion occurred a few times after To-Cu tephra and these erosions were not expected from the present topography. This indicates that we need to care buried topography and to conduct a multi drilling survey. Takeda et al. (2018) pointed out a similar thing. This study gives us clues for tsunami deposits identification and accurate lateral correlation of sediments. Dense drilling surveys and/or continuous outcrops tell us an accurate number of tsunami deposits. Tephra layers give us robust ages and correlative layers in sediments. A geochemical signature can be used to correlate background sediments and gravel roundness is useful to identify tsunami deposits and correlate them. Radiocarbon dating gives us confirmation of the lateral correlation of sediments. We thought that this is an efficient procedure for paleo-tsunami deposits research.