Effect of Forest Management on Earthquake-Induced Landslide by the 2018 Mw6.7 Eastern Iburi Earthquake, Hokkaido

Abstract:

On September 6th, 2018, more than 6000 landslides that occurred within an area of 400 km² were induced by the M_w6.7 Eastern Iburi earthquake. Landslides occurred on the forested areas with a high density of multiple orders of forest road networks on a hillslope. We investigated the effect of forest management related to forest road networks (including skid trails) on the occurrence of EIL using geo-statistical analysis. A total of 428 landslide scars was identified in the total study area of 4.4 km². Landslide density became 0.22 km²/km², which was 10 times higher to EIL in Kumamoto. The individual landslide area ranged from 63 to 18360 m² with a mean of 2260 m² and a median of 1637 m². The total road network in 4.4 km² was 78.6 km with 17.9 km/km² of road density. The road was typically installed with 5 m width, 25° of road surface gradient, and 2 m in a road cut depth. Most of the roads were only used during forest operation. Road and skid trail density in our study site was 4.5 times higher than the road density reported in Sicily and Marche in Italy. We found 56% of landslides (239 landslides) initiated within 5 m of forest roads, while forest road locations varied from lower and middle of hillslope to near ridgeline. Among the road-related landslides, 42% of landslides occurred in recently logged areas. For identifying the effects of road location along hillslope transects on the occurrence of landslides, we classified forest road networks into the nearest road to the ridge (1^st order) to a road located near stream channel (4^th order). The mean altitude of the 1^st to 4^th order was 57 to 227.6 m. We also found 37% and 27 % of total road-related landslides occurred in the 1^st and 2^nd order roads, while that in 3^rd order and 4^th order occurred 17% and 19%. Landslides initiated by upper road tended to be large areas (1^st order mean: 2502 m²; 2^nd order mean: 2085 m²) than landslide occurred in lower hillslope (3^rd order mean: 1686 m²; 4^th order mean: 1189 m²). Further, we are going to analyze the possibility of the direct impact of forest road networks on EIL by comparing landslides initiated from forest road networks and those were not, including in forested and logged areas.