Frequencies of Deep Catastrophic Landslide during the Past 10000 Years in Hokkaido, Japan.

Abstract:

We investigated the frequencies of occurrence of deep catastrophic landslides (DCL) using Tephrochronology and analyzed frequency distributions of slope failures from shallow rapid failures to DCL. DCL is defined as failure depth > 10 m including bedrock with mass volume > 100000 m³. This study was conducted in the upper area (252 km²) of Saru river system (1350 km²) in the west side of Hidaka Mountain region, Hokkaido, Japan. Altitude of the area ranged from 200 to 1500 m. Recent DCL occurred in 2003 at middle stream area of Saru river system. We firstly investigated topographic features of DCL using aerial photographs and airborne LiDAR for identifying locations of old DCL scars and deposits. We also identified knick lines for topographic evolutions which typically occurred in altitude around 600 m. Extracted topographic features were archived into ArcGIS to analyze area, volume, height, altitude and slope gradient. Areas of extracted DCL were ranged from 1.6 x 10⁴ to 2.8 x 10⁵ m² which agreed to the contemporary DCL occurred in Japan in past 120 yrs. We used six index tephra deposited in AD 1739, AD 1694, AD 1667, 2500 yrs BP, 9000 yrs BP, 16,000 to 19,000 yrs BP which were came from various volcanos located east side of our study site. Field investigation of soil profile in 15 DCL scars among 32 extracted DCL scars from LiDAR showed that 10 DCLs occurred before 9000 yrs BP, the one DCL occurred from 9000 to 2500 yrs BP, three DCLs occurred 2500 to 300 yrs ago, and the other DCL occurred after 300 yrs ago. Our findings showed that topographic feature formed by DCL potentially remained more than 9000 years in the areas and high frequencies of DCL occurred from last glacial period. We further examine relationship probability density from small to large scale landslide occurred in this region.