[H-GM02] Geomorphology

[HGM02-03] Morphological changes and sediment connectivity following coseismic landslides: Case study on the 2018 Hokkaido Eastern Iburi Earthquake.

*Tennyson Lap Wing Lo¹, Yuichi S. Hayakawa², Yasutaka Nakata³, Masato Hayamizu⁴, Takuro Ogura⁵ (1.Graduate School of Environmental Science, Hokkaido University, Japan, 2.Faculty of Environmental Earth Science, Hokkaido University, Japan, 3.Graduate School of Life and Environmental Sciences Division of Environmental Sciences, Kyoto Prefectural University, Japan, 4.Forestry Research Institute, Hokkaido Research Organization, Japan, 5.Faculty of Life and Environmental Science, University of Tsukuba, Japan)

Keywords:2018 Hokkaido Eastern Iburi Earthquake, Coseismic landslides, Drainage basin, Morphological changes, Drainage network development, Sediment connectivity

On 6th September, 2018, a strong earthquake with Magnitude 6.7 happened in Hokkaido Eastern Iburi region. The epicenter was located at 42.690^oN and 142.007^oE near Karumai Fault and Atsuma Fault, with a focus depth of 37 km. Nearly 700 km2 of areas were affected by the coseismic landslides. Morphological changes within watersheds are expected, such as a decrease in elevation along slid slopes and an increase in elevation at the lower part of the fluvial channel due to the accumulation of collapsed materials. Slid slopes with bare soil may expose to rain splash after the earthquake. The investigation of stream network development during the post-seismic periods may be valuable for knowing the effects of hte earthquake on fluvial landscape evolution. This study focuses on the post-seismic changes in drainage basin morphology, drainage network development, and sediment connectivity within watersheds, and assesses the long-term effects of landslides in the areas. The study sites are five catchments along the Atsuma River with minor artificial modification after the earthquake. Digital elevation models (DEMs) with a 0.5-m resolution, generated from airborne laser scanning (ALS) data of October 5 in 2012 and September 11 in 2018, were used to represent the topography of pre- and post-earthquake. Field surveys were conducted from April 2020 to October 2021 for collecting topographic data during post-seismic periods by the use of Unmanned Aviation Vehicles (UAVs), including DJI Phantom 4RTK and Matrice 300 RTK with L1 sensor. Structure-from-Motion Multi-View Stereo (SfM-MVS) photogrammetry were performed using the UAV-based aerial images to generate point cloud, DEMs, and orthorectified mosaic images. Twenty quantitative morphometric parameters were selected to indicate the pattern and predominant processes within the drainage basin relating to mass movements and fluvial erosion with drainage network development. Landslide area density, deposit length ratio, and landslide mobility were used to analyze the characteristics of sediment connectivity and mobility of coseismic landslides. By comparing the changes between pre- and post-seismic periods, with ALS DEMs (October 2012 and September 2018) and UAV-SfM photogrammetry DEMs (April 2020 to October 2021), the areas of cosismic landslides, sediment depositon, and changes in drainage basin characteristics were identified. The pattern changes in the study sites showed that channel developments on slid slope surfaces increased progressively with the increase in stream length and bifurcation ratio. Moreover, potentially higher surface runoff, gully development on the slid surfaces, and further slope deformations may be expected based on the increase in drainage intensity. The results of the quantitative analysis also provide evidence on drainage network development during post-seismic periods. Since vegetation recovery has been observed in some portions of the slopes, the correspondence between vegetation cover, topographical changes, and drainage network formation processes would also be essential for further investigation. Also, further assessments of factors on the near future morphological changes, including freeze and thaw action and channel initiation, will be necessary.