Geomorphological setting of shallow landslides by heavy rainfall on tephra-covered slopes of Aso Volcano, southwest Japan

Abstract

  Shallow landslides repeatedly occurred on the slopes of Aso Volcano, Central Kyushu, southwest Japan, during heavy rainfall in 1990, 2001, and 2012. This study aimed to identify the geomorphological causes of these intensive landslides covered with fall-out tephra layers via geographic information system analysis and geomorphological classification of their distribution in the Takadake and Saishigahana areas. Source areas of the abovementioned landslides were mapped on Red Relief Image Maps processed from light detection and ranging digital elevation model data (1m resolution). The fact that they were mostly distributed on slopes with gradients of more than25° and relative heights of more than 20 m in an area of a 50×50 m window indicates the minimum gradient of a rainfall induced landslide occurrence and the fall-down possibility of unstable material, respectively. More than half of the landslide locations in 2012 were different from those in 1990 and 2001 ; this was also found for landslides in 2001 and 1990 in both study areas. The slopes in contact with the rear and sides of the1990 and 2001 landslides collapsed in 2012 at rates of 32.7％ and 21.2％ in Takadake and Saishigahana, respectively, and those of the 1990 landslides collapsed in 2001 at rates of 19.1％ and 28.4％ in the respective study areas. In contrast, the occurrence in 2001 and 2012 at the inside of the former landslide slope was 3―4％ in Takadake and 1―7％ in Saishigahana. Approximately 50％ aerial occupancy of remnants of past landslides (RPL) revealed that shallow landslides are geomorphologically common phenomena on the slopes of Aso Volcano. The spatial relation of landslide occurrence area in 2012 to RPL was classified into seven types based on geomorphological mapping, namely 1) in contact with rear of RPL, 2) in contact with side of RPL, 3) inside of RPL without deposition of collapsed material, 4) below RPL, 5) in and around deposition of collapsed material in RPL, 6) on valley side slope below slope break, and 7) independent. Type 1 was dominant, and types 2 and 3 were subsequent. Deposition of tephra or transported materials from the head scrap on the collapsed material of a past landslide which increases the soil depth above the rupture surface might be the cause of the difference between fewer repeated landslide occurrences on former slopes of the 1990, 2001, 2012 landslides and type 3. Types 1 and 2 are concordant with frequent occurrences of landslides behind and at the side of former landslides that are caused by the instability of the slope toe or side where its counterbalance has decreased. Based on these findings, viewpoints for the detection of landslide-susceptible slopes caused by rainfall is also proposed.