1:45 PM - 2:00 PM
[MIS16-01] Gravelly event deposit research using particle shape parameter
★Invited Papers
Keywords:Gravelly tsunami deposits, Particle shape parameter, Gravel, Roundness, Image analysis
In tsunami deposit research, generally, sedimentological description and analysis, paleontological analysis, and geochemical analysis have been conducted for event deposits to identify tsunami deposits (Sawai, 2012; Goto and Sugawara, 2021; Shinozaki, 2021). However, analysis applied to gravelly event deposits recognized interbedded within sediments is limited. Therefore, in the case of gravelly event deposits, information to discuss whether they are tsunami deposits or not is often less available than that obtained for fine-grained event deposits. In Japan, tsunami deposits found in many locations are often sandy, and except in special settings, analysis and characterization focusing on the individual grains that compose tsunami deposits are not usually conducted. While gravels can provide visibility of individual particles, they are often not suitable for the analysis described above, and unless they contain gravel-sized remains of marine organisms, they are often not analyzed in detail, but only described sedimentologically. In this presentation, we focus on the particle shape parameter of gravel particles, especially roundness. We will introduce a study by Ishimura and Yamada (2019) that obtained roundness information from gravelly tsunami deposits, clarified their sources, and estimated the size of paleo-tsunamis from their roundness distributions. Additionally, we discuss the information on the particle shape parameter and its applicability to other event sediments.
In this study, the particle shape parameter used is the roundness (hereafter “R”) defined by Wadell (1932), and the MATLAB code (Zheng and Hryciw, 2015) for the calculation of R is used. Ishimura and Yamada (2019) measured R for gravels in tsunami deposits as well as for gravels in beach and river sediments as sources and calculated the ratio of sources in tsunami deposits. Traditionally, R has been measured and estimated manually or visually (Nakayama, 1954; Utsugawa and Shirai, 2016), but in this study, image analysis enabled us to obtain roundness data 10-100 times more than in the past.
In the case of Koyadori in Iwate Prefecture (Ishimura and Yamada, 2019), the ratio of each source was calculated for each tsunami deposit, and changes in mixing ratios were observed, such as an increase in the ratio of river gravel from the sea to the inland. These changes could not be recognized by conventional naked-eye observation. Mixing ratios in the tsunami deposits correlated to the historical tsunamis with known inundation distances, the 2011 Tohoku-oki earthquake tsunami, 1896 Meiji Sanriku tsunami, and 1611 Keicho tsunami, show the changes at about 40% of the inundation distance from the coast (the mixing ratio of the gravels of river origin increases rapidly) despite different tsunami sizes (inundation distances). The size of the paleo-tsunami that hit this area about 1,000 years ago is unknown, but the change in the mixing ratio suggests that the tsunami was almost the same size (in terms of inundation distance) as the 1896 Meiji Sanriku tsunami. In general, the topography of the Sanriku Coast abruptly increases in elevation with distance from the coast, making it difficult to estimate the size of paleo-tsunamis from tsunami deposit distributions, as is the case in the Sendai Plain. However, in this study, we were able to estimate the size of paleo-tsunamis using a completely different approach from conventional methods by using and interpreting the shape parameters of gravels in tsunami deposits.
Although the trend described above is likely to be specific to Koyadori, the information contained in the gravels of tsunami deposits may help us understand not only the source but also the transport process by interpreting their characteristics. It is difficult to notice such a change of gravel shape by naked eye observation. Therefore, we expect that the introduction of shape parameters like this study will allow us to further extract the information recorded in gravelly event deposits. In my presentation, I will introduce our current procedures from sampling to analysis, as well as examples of fundamental research and applications to other event deposits.
In this study, the particle shape parameter used is the roundness (hereafter “R”) defined by Wadell (1932), and the MATLAB code (Zheng and Hryciw, 2015) for the calculation of R is used. Ishimura and Yamada (2019) measured R for gravels in tsunami deposits as well as for gravels in beach and river sediments as sources and calculated the ratio of sources in tsunami deposits. Traditionally, R has been measured and estimated manually or visually (Nakayama, 1954; Utsugawa and Shirai, 2016), but in this study, image analysis enabled us to obtain roundness data 10-100 times more than in the past.
In the case of Koyadori in Iwate Prefecture (Ishimura and Yamada, 2019), the ratio of each source was calculated for each tsunami deposit, and changes in mixing ratios were observed, such as an increase in the ratio of river gravel from the sea to the inland. These changes could not be recognized by conventional naked-eye observation. Mixing ratios in the tsunami deposits correlated to the historical tsunamis with known inundation distances, the 2011 Tohoku-oki earthquake tsunami, 1896 Meiji Sanriku tsunami, and 1611 Keicho tsunami, show the changes at about 40% of the inundation distance from the coast (the mixing ratio of the gravels of river origin increases rapidly) despite different tsunami sizes (inundation distances). The size of the paleo-tsunami that hit this area about 1,000 years ago is unknown, but the change in the mixing ratio suggests that the tsunami was almost the same size (in terms of inundation distance) as the 1896 Meiji Sanriku tsunami. In general, the topography of the Sanriku Coast abruptly increases in elevation with distance from the coast, making it difficult to estimate the size of paleo-tsunamis from tsunami deposit distributions, as is the case in the Sendai Plain. However, in this study, we were able to estimate the size of paleo-tsunamis using a completely different approach from conventional methods by using and interpreting the shape parameters of gravels in tsunami deposits.
Although the trend described above is likely to be specific to Koyadori, the information contained in the gravels of tsunami deposits may help us understand not only the source but also the transport process by interpreting their characteristics. It is difficult to notice such a change of gravel shape by naked eye observation. Therefore, we expect that the introduction of shape parameters like this study will allow us to further extract the information recorded in gravelly event deposits. In my presentation, I will introduce our current procedures from sampling to analysis, as well as examples of fundamental research and applications to other event deposits.