5:15 PM - 7:15 PM
[MIS11-P03] Reconsideration of Grain Size Distribution and Formation Processes of the 2011 Tsunami Deposits in the Sendai Plain
Keywords:Tsunami Deposits, Grain Size, Sedimentary Structures
Grain size distribution in tsunami deposits is crucial in understanding sediment sources, depositional processes, and the relationship between tsunami deposits and tsunami-induced flows (Minoura et al., 1996). Tsunami deposits generally show inland fining features and often exhibit inverse or normal grading structures. These characteristics are known to be useful for the identification of tsunami deposits (e.g., Naruse et al., 2012).
The 2011 Tohoku-oki tsunami inundated the Sendai Plain in Miyagi Prefecture, reaching more than 4 km inland from the coastline. As a result, tsunami deposits were widely distributed. Sandy deposits extended up to approximately 2.8 km inland, while mud-dominated deposits were found further inland (Goto et al., 2011; Richmond et al., 2012). Many sedimentological studies, including grain size analysis along transects perpendicular to the coastline, have been conducted on tsunami deposits in the Sendai Plain (Jaffe et al., 2012; Szczuciński et al., 2012; Matsumoto et al., 2023). These studies revealed variations in median grain size depending on the distance from the coastline and the location of the transects; however, the grain size distribution of tsunami deposits generally exhibited unimodal characteristics.
In this study, we conducted new grain size analyses on tsunami deposits formed by the 2011 Tohoku-oki tsunami at the Takaose site in Iwanuma City, Miyagi Prefecture. It is located approximately 1 km from the coastline of the Sendai Plain and sandy tsunami deposits were found.
Grain size analysis using the settling tube method revealed that the tsunami deposits at the Takaose site exhibited a bimodal grain size distribution. The deposits comprised fine grains with a narrow peak around 2φ and coarser grains forming a broad peak around 0–0.5φ.
On the other hand, grain size analysis conducted by Jaffe et al. (2012) at a site located in the Sendai Plain approximately 1 km from the coastline showed a unimodal grain size distribution with grains around 2φ. This site is at a similar distance from the coastline as the Takaose site, and the tsunami characteristics are presumed to be similar.
The factors causing a bimodal grain size distribution in tsunami deposits may include differences in transport mechanisms as suggested by Minoura et al. (1996), or differences in sediment sources. This study examines these factors to understand the formation processes of bimodal tsunami deposits.
The 2011 Tohoku-oki tsunami inundated the Sendai Plain in Miyagi Prefecture, reaching more than 4 km inland from the coastline. As a result, tsunami deposits were widely distributed. Sandy deposits extended up to approximately 2.8 km inland, while mud-dominated deposits were found further inland (Goto et al., 2011; Richmond et al., 2012). Many sedimentological studies, including grain size analysis along transects perpendicular to the coastline, have been conducted on tsunami deposits in the Sendai Plain (Jaffe et al., 2012; Szczuciński et al., 2012; Matsumoto et al., 2023). These studies revealed variations in median grain size depending on the distance from the coastline and the location of the transects; however, the grain size distribution of tsunami deposits generally exhibited unimodal characteristics.
In this study, we conducted new grain size analyses on tsunami deposits formed by the 2011 Tohoku-oki tsunami at the Takaose site in Iwanuma City, Miyagi Prefecture. It is located approximately 1 km from the coastline of the Sendai Plain and sandy tsunami deposits were found.
Grain size analysis using the settling tube method revealed that the tsunami deposits at the Takaose site exhibited a bimodal grain size distribution. The deposits comprised fine grains with a narrow peak around 2φ and coarser grains forming a broad peak around 0–0.5φ.
On the other hand, grain size analysis conducted by Jaffe et al. (2012) at a site located in the Sendai Plain approximately 1 km from the coastline showed a unimodal grain size distribution with grains around 2φ. This site is at a similar distance from the coastline as the Takaose site, and the tsunami characteristics are presumed to be similar.
The factors causing a bimodal grain size distribution in tsunami deposits may include differences in transport mechanisms as suggested by Minoura et al. (1996), or differences in sediment sources. This study examines these factors to understand the formation processes of bimodal tsunami deposits.