Estimation of Sediment Transportation by the 2016 Kumamoto Earthquake and Subsequent Rainfalls in the Aso Caldera

Yasuyuki HIRAKAWA, Hisayoshi TAKEISHI, Kazuya FUNAKOSHI and Hidenori EGUCHI


A large number of slope failure and landslide events occurred inside the Aso caldera following the Kumamoto Earthquake (Mj 7.3) in April 2016. Subsequent rainfalls also caused new slope failures as well as slope failure expansion and promoted secondary sediment movement. This study quantitatively analyzed and summarized the condition and characteristics of these sediment movements. LiDAR data used for analysis are those of three periods―before the earthquake, immediately after the earthquake, and after the subsequent rainfalls. Deviation of coordinates attributable to ground displacement by the earthquake was corrected using Classification and Combined Iterative Closest Point (CCICP) method. After that, altitude difference analysis was performed. The results show that (1) the amount of sediment load from a stream catchment basin by post‐earthquake rainfalls was roughly from 10,000 to 200,000 m3/km2. (2) In addition, the sediment load was greater at the central cone than at the caldera wall for both the earthquake and rainfall events. (3) However, this trend is more apparent during the rainfall event than at the time of the earthquake. The results also show the sediment load by the rainfalls was 5.9 times greater at the central cone than at the caldera wall. This is possibly attributed to the difference in grain size of sediments deposited by the earthquake. (4) Taking into consideration the sediment balance of the entire caldera, about half of the sediments deposited by the earthquake was underwent secondary erosion during the rainfalls. Furthermore, failure expansion and re‐erosion inside the failure site led to new generation of the sediments on the same scale as secondary erosion. As a result, it was considered that the amount of sediments close to the sediment yield by the earthquake was produced by the rainfalls. While the sediment delivery ratio by the earthquake was about 10 %, it increased significantly to about 60% due to subsequent heavy rainfalls. Approximately 11.04 million m3 were produced jointly by the earthquake and rainfalls, of which about 6.64 million m3 were deposited inside the caldera.

Key words

sediment transportation, earthquake, rainfalls, sediment balance, Aso caldera