The spatial variability of landslide occurrences and transported sediments induced by the 2018 Hokkaido Eastern Iburi earthquake, Japan

The spatial variability of landslides and associated sediment deposits induced by earthquakes alters both short- and long-term sediment dynamics in watersheds. Linkages between landslide occurrence and sediment accumulations within channels are important for evaluating spatial and temporal dynamics of sediment from headwaters to downstream. To evaluate spatial variability of landslides, we examined landslide-area density (LAD: landslide area divided by watershed area) in different sub-watersheds (areas 0.01 to 4.4 km²) of Habiugawa watershed (40 km²), which was affected by the 2018 Hokkaido Eastern Iburi earthquake, Japan. The watershed is located 13 km north of the epicenter and is covered by secondary conifer and deciduous forest. The topography is hilly associated with long-term landform development by paleo-glacial erosion. Altitude ranges from 30 to 440 m; mean hillslope and channel gradients are 30° and 10°, respectively. Landslides mostly occurred at depths from 1 to 2 m below pumice layers formed by the Mt. Tarumae eruption 9000 yr ago (Ta-d), with total soil depths from 2 to 3 m. The 0.5 m LiDAR-based DEM and 0.2 m post-earthquake orthophotos were used to calculate LAD by GIS analysis. To examine spatial variability of in-channel sediment deposited by landslides, we used deposit-length ratio (DLR: total length of sediment accumulations within channels divided by total channel length within sub-watersheds). Sediment deposition in channels was assessed as rough surface topography by DEM and orthophotos.

We identified 2941 landslides: mean area=1620 m²; range from 20 to 34710 m². LAD in the entire Habiugawa watershed was 0.12 km² km^-2, which is high compared to the other earthquake-induced landslides (e.g., Wenchuan earthquake: 0.03 km² km^-2). Sub-watersheds < 0.1 km² had wide ranges in LADs (0.0 to 0.8 km² km^-2), while sub-watersheds from 0.1 to 0.5 km² ranged from 0.2 to 0.5. Sub-watersheds > 0.5 km² had LADs from 0.1 to 0.3. Seventy-four percent of small watersheds (< 0.5 km²) with high LADs (> 0.3) also had high sediment accumulations within gentle channels (DLR ≥ 0.8). This suggests that poorly mobilized sediments that initiate in headwaters rapidly deposit in channels. Conversely, the other small watersheds (26%) had lower sediment accumulation within steeper channels (DLR < 0.8), suggesting that these high-mobilized sediments traveled longer and were evacuated from watersheds to some extent. Such differences in sediment mobility in small sub-watersheds (< 0.5 km²) may cause sporadic sediment accumulations within channels of larger watersheds (> 0.5 km²). Our findings suggest that geomorphic features of watersheds associated with long-term legacies of geomorphic evolution possibly affect the spatial variability of landslide occurrence and the associated in-channel sediment accumulation induced by the earthquake.