3:45 PM - 4:00 PM
[MIS11-08] Gravelly tsunami deposit formed by the 2011 Tohoku-oki tsunami in the Attari District, northeastern Japan
Keywords:Gravelly tsunami deposit, Roundness, The 2011 Tohoku-oki tsunami, LiDAR
The formation processes of sandy and boulder tsunami deposits have been well studied through numerous case studies as well as flume and numerical experiments. However, the formation processes of tsunami deposits consisting of intermediate-sized grains ranging from several millimeters to centimeters (i.e., gravelly tsunami deposits) remain poorly understood. This hinders the use of these deposits in evaluating paleotsunami size. In this study, we examine the formation process of a gravelly tsunami deposit formed by the 2011 Tohoku-oki tsunami in the Attari District, northeastern Japan. A gravelly deposit layer, possibly deposited by the 1896 Meiji-Sanriku tsunami, was previously reported at the same site (Yagishita, 2001). Recently, we discovered another gravelly layer immediately beneath the surface. This newly identified gravel layer can be traced up to approximately 100 m inland from the shore (at an elevation of 9.5 m) and contains numerous rounded gravels similar to the beach deposit. Therefore, we interpret this layer as a tsunami deposit. Also, this layer was not reported by Yagishita (2001), suggesting that this layer was formed by a very recent tsunami event, most likely the 2011 Tohoku-oki tsunami. The thickness of the layer exhibits a clear landward-thinning trend and reaches 20 cm maximum. Furthermore, we found sparsely scattered rounded granules both beneath and on the surface, further inland from the distribution limit of distinct gravel layer. The scattered granules were traced up to approximately 120 m inland (at an elevation of 12 m) where the floatable debris, such as vinyl fragments and cans, are also scattered.
We excavated 30 pits in the study area and collected samples from 23 pits that contain a distinct gravel layer. Topographic point cloud data was obtained by the mobile laser scanning using a backpack LiDAR system. The image-based grain size and shape analyses were performed to examine the landward trend of grain size and shape composition. The image analysis software Rgrains (Yamada, 2023) was utilized to calculate circle-equivalent diameter and Wadell's (1932) roundness of each particle. The result shows that the median grain size of 23 samples was mostly constant regardless of the distance from the shore, while the upper envelope of the maximum grain size exhibited an inland fining trend. The fraction of rounded beach gravel in the samples gradually decreased landward, while this reduction was compensated by an increase in angular slope gravels.
In this talk, we discuss the formation process of the 2011 gravelly tsunami deposit in the Attari District by integrating the sedimentological dataset obtained from the fieldwork and laboratory analyses.
We excavated 30 pits in the study area and collected samples from 23 pits that contain a distinct gravel layer. Topographic point cloud data was obtained by the mobile laser scanning using a backpack LiDAR system. The image-based grain size and shape analyses were performed to examine the landward trend of grain size and shape composition. The image analysis software Rgrains (Yamada, 2023) was utilized to calculate circle-equivalent diameter and Wadell's (1932) roundness of each particle. The result shows that the median grain size of 23 samples was mostly constant regardless of the distance from the shore, while the upper envelope of the maximum grain size exhibited an inland fining trend. The fraction of rounded beach gravel in the samples gradually decreased landward, while this reduction was compensated by an increase in angular slope gravels.
In this talk, we discuss the formation process of the 2011 gravelly tsunami deposit in the Attari District by integrating the sedimentological dataset obtained from the fieldwork and laboratory analyses.