5:15 PM - 7:15 PM
[HTT14-P01] Reexamination of the Late Holocene Crustal Deformation in Yaeyama Islands using Notches
Coastal notches are useful to estimate the amount of crustal deformation. Notches are depressions formed in sea cliffs by physical or chemical action or both (Takenaga, 1968), and elevations of retreat points, which is the most depressed point, is considered to represent the mean sea level at the time of formation (Kawana, 1987). In Yaeyama Islands, Kawana (1987) examined elevations of retreat points of notches. According to Kawana (1987, 1989), elevations of retreat points are higher to the southeast and lower to the northwest, suggesting that tectonic uplift with tilting has occurred in the past. However, few papers have fully examined the basis on which notches represent the history of crustal deformation. The purpose of this study is to reexamine the uplift with tilt that is believed to have occurred in Yaeyama Islands 2000 years ago as mentioned in Kawana (1989), and to evaluate the usefulness of notches as a marker of crustal deformation. In this study, elevations of retreat points of notches were measured by RTK-LiDAR technology and compared with Kawana (1987), and parameters other than retreat points of notches are also discussed to examine the formation process of notches.
Yaeyama Islands are located about 400 km from the main island of Okinawa, and the survey was conducted on Ishigaki, Iriomote, Kohama, Taketomi, and Kuroshima Islands. The Ryukyu Trench is located approximately 150 km to the south and northeast of Yaeyama Islands, and it is pointed out that Yaeyama Islands may have been strongly influenced by the Ryukyu Trench (Kawana, 1987). According to Hongo and Kayanne (2009), winds throughout the year tend to come from the northeast and south. Because of this, the areas on the Pacific Ocean side that tend to be upwind have higher wave energy than those on the East China Sea side.
Measurements of notches were conducted by using ViDOC, a machine that can obtain position and elevation information of a point cloud of subject, the accuracy of measurements made by ViDOC is in millimeters for position information (Aksoy, 2025) and in centimeters for elevation information (Suleymanoglu et al., 2023). The notches at 46 sites were measured. Measurements of notches included not only elevations of retreat points, but also depth and elevations of roof points. The notches were reconstructed as a 3D model by point cloud using Cloud Compare. The rock strength of some of the notches was measured with a Schmidt hammer (KS type).
The distribution of elevations of retreat points of the notches measured in this study follows the southeast-northwest trend described by Kawana (1987), but some of the notches show different trends. Correlations were made for each parameter of the notches. The most characteristic feature is the relationship between elevations of retreat points and depth, which shows a positive correlation on the Pacific side while a negative correlation on the East China Sea side. This means that as elevations of retreat points increases, the notches become deeper on the Pacific Ocean side and shallower on the East China Sea side. Analysis of the shape of notches indicates that it is unlikely that a large tectonic uplift occurred at about 2000 years ago. We found that waves contribute significantly to the formation of the notch, it is possible to explain the relationship among the parameters. Therefore, a notch alone is unlikely to be useful as a marker of history of crustal deformation, and careful consideration of factors other than crustal deformation that affect parameters of notches, such as wave conditions and rock hardness at the location where a notch is formed, is necessary.
Yaeyama Islands are located about 400 km from the main island of Okinawa, and the survey was conducted on Ishigaki, Iriomote, Kohama, Taketomi, and Kuroshima Islands. The Ryukyu Trench is located approximately 150 km to the south and northeast of Yaeyama Islands, and it is pointed out that Yaeyama Islands may have been strongly influenced by the Ryukyu Trench (Kawana, 1987). According to Hongo and Kayanne (2009), winds throughout the year tend to come from the northeast and south. Because of this, the areas on the Pacific Ocean side that tend to be upwind have higher wave energy than those on the East China Sea side.
Measurements of notches were conducted by using ViDOC, a machine that can obtain position and elevation information of a point cloud of subject, the accuracy of measurements made by ViDOC is in millimeters for position information (Aksoy, 2025) and in centimeters for elevation information (Suleymanoglu et al., 2023). The notches at 46 sites were measured. Measurements of notches included not only elevations of retreat points, but also depth and elevations of roof points. The notches were reconstructed as a 3D model by point cloud using Cloud Compare. The rock strength of some of the notches was measured with a Schmidt hammer (KS type).
The distribution of elevations of retreat points of the notches measured in this study follows the southeast-northwest trend described by Kawana (1987), but some of the notches show different trends. Correlations were made for each parameter of the notches. The most characteristic feature is the relationship between elevations of retreat points and depth, which shows a positive correlation on the Pacific side while a negative correlation on the East China Sea side. This means that as elevations of retreat points increases, the notches become deeper on the Pacific Ocean side and shallower on the East China Sea side. Analysis of the shape of notches indicates that it is unlikely that a large tectonic uplift occurred at about 2000 years ago. We found that waves contribute significantly to the formation of the notch, it is possible to explain the relationship among the parameters. Therefore, a notch alone is unlikely to be useful as a marker of history of crustal deformation, and careful consideration of factors other than crustal deformation that affect parameters of notches, such as wave conditions and rock hardness at the location where a notch is formed, is necessary.