[S-GD03] Crustal Deformation

[SGD03-P12] Estimation of seafloor topography displacement using airborne laser bathymetry (ALB) data before and after the 2024 Noto Peninsula earthquake

*Hisashi Sasaki¹, Yukihiro Kato², Katsuhiro Kusunoki², Shino Kato (Naruke)¹, Shigenobu Takayanagi¹, Kazuo Goto¹ (1.Asia Air Survey Co., Ltd., 2.Japan Hydrographic Association)

Keywords:the 2024 Noto Peninsula Earthquake, airborne laser bathymetry (ALB), topographic change, ICP

The 2024 Noto Peninsula Earthquake caused various topographic changes, including slope failures, landslides, and coastal uplift. Although land surface surveys have mostly been conducted using surface exploration, aircraft, and satellites, no arial surveys of the shallow sea bottom topography have been conducted to understand the landform changes. Therefore, we investigated the topographic changes in the shallow sea bottom around the Noto Peninsula caused by the 2024 earthquake using airborne laser bathymetry (ALB). This survey was conducted by the Nippon Foundation and the Japan Hydrographic Association as part of the Umi-no-Chizu Project (Map of the Sea Project), which started in 2022 to survey and map shallow sea bottoms along the coast of Japan. We surveyed the shallow sea bottom (water depths from 0 to 20 m) along a total coastal distance of approximately 225 km in the outer coastal area of the Noto Peninsula (Shiga Town to Suzu City). Detailed bathymetry data were obtained using an ALB for the shallow sea bottom before and after the 2024 Noto Peninsula Earthquake that displayed significant upheavals. Prior to 2024 Noto Peninsula earthquake (September to October 2022), data were acquired using Chiroptera4X on a rotary-wing aircraft, while post-earthquake data (April to May 2024) were acquired using Chiroptera-5 and Hawkeye-5 on a fixed-wing aircraft. Elevation data were generated by noise processing and bathymetry data filtering, followed by creation of a digital elevation model (DEM) at 0.5-meter intervals. To understand the topographic changes caused by the earthquake, we performed a difference analysis of the DEM before and after the earthquake and calculated the amount of displacement. The amount of vertical and horizontal displacement was estimated using a topographic change analysis method (Patent No. 6817721), which calculates the amount of displacement by applying an Iterative Closest Point (ICP), a 3D point cloud alignment technique. The amount of displacement was calculated in map information level 500 units (300 m north-south, 400 m east-west) on the basic national land map, and the amounts of vertical and horizontal displacement were calculated for each tile. Using a difference analysis, we calculated the area of land affected by the 2024 Noto Peninsula Earthquake. The total land area of the Noto Peninsula is 4.56 km², and the land extends up to 250 m offshore around Monzen Kuroshima Island, where the uplift is large. Figures 1 and 2 show the results of the vertical and horizontal displacement obtained by applying the ICP. Vertical displacement peaked at two locations in the east and west, with a maximum of ~5.2 m in the western part around Cape Saruyama to Saruyama-zaki, and a maximum of ~2.7 m in the eastern part around Nakadahama. Similar analyses using aerial laser survey data of land areas were done by Sasaki et al. (2024) and Sasaki et al. (submitted). The maximum vertical displacement of the land area of the Noto Peninsula is ~4.6 m in the mountains around Cape Saruyama and Saruyama-zaki. The maximum shallow sea bottom displacement was adjacent to the western side of the maximum land displacement (Figure 3). The horizontal displacement averaged ~2.2 m from west to west-southwest around Cape Saruyama to Shikaiso Fishing Port and ~2.5 m from west-northwest to north-northwest around the Sosogi Coast to Nakadahama (Figure 2). Compared to the land analysis data, the displacement tended to increase gradually from the land area to the shallow sea bottom. This method is considered safe and reliable for assessing topographic changes in shallow sea bottom as aftershocks of an earthquake increases the possibility of tsunamis.

References
Sasaki, H., Funakoshi, K. and Chiba, T. (2024) Ground deformation caused by the 2024 Noto Peninsula earthquake and its characteristics: A case study of analysis using airborne laser data before and after the earthquake. Asia Air Survey Technical Journal For the Future 2025, 22-23.
Sasaki, H., Funakoshi, K., Chiba, T., Fujita, H., and Misono, T. (submitted) Estimation of ground surface displacement caused by the 2024 Noto Peninsula earthquake using airborne LiDAR survey data.