5:15 PM - 6:45 PM
[HDS08-P03] Geological investigation of slope failures in northern Kyushu induced by heavy rainfall in June 2023
Keywords:slope failure, geological background, heavy rainfall in June 2023, northern Kyushu
From late June to middle July of 2023, many slope failures occurred across a wide area in northern Kyushu, induced by heavy rainfall accompanied by a stagnated seasonal rain front. The Geological Survey of Japan, together with the National Research Institute of Fire and Disaster and the National Agriculture and Food Research Organization, conducted field surveys of the slope failures in two periods as follows. From August 1st to 4th in 2023: Yufu and Hita (Oita Pref.), Toho (Fukuoka Pref.), Takachiho (Miyazaki Pref.), and Mashiki (Kumamoto Pref.). From August 27th to 29th in 2023: Kurume (Fukuoka Pref.) and Karatsu (Saga Pref.)
The investigated slope failures were classified into two types: A) failures occurred in the areas suffered from hydrothermal alteration or weak to middle weathering. This type includes failures in Yufu, Toho, Takachiho, Mashiki, Kurume and Karatsu. B) failures of surface covering debris other than the bedrocks. Failures of this type occurred in the Hita area.
There is a wide variety of collapsed rocks and formations among A-type failure sites. Details are provided as follows. A1) Kawanishi site, Yufu: Hydrothermal altered andesite pyroclastic rocks and lavas (Kawanishi andesite, Eary Pleistocene) collapsed. A2) Takeno site, Kurume: Pelitic schists (Suo metamorphic rocks, Late Triassic to Early Jurassic), subjected to contact metamorphism by Cretaceous granitic rocks, collapsed. A3) Hirabaru site, Karatsu: We observed collapses of corestones in weathered Cretaceous Itoshima granodiorite, and collapses and flows of decomposed granite soils originated from Cretaceous Fukae granite. A4) Sakanoshita site, Takachiho: Weathered non-welded Aso-4 pyroclastic flow deposits (Late Pleistocene) collapsed. A5) Akai site, Mashiki: Altered andesite scoria cone (Akai volcano, late of Middle Pleistocene) collapsed. A6) Koishiwaratsuzumi site, Toho: Decomposed granitic rocks (Cretaceous) collapsed.
At Ono and Tsurukawachi sites in Hita area B-type failures include a thick (approximately 5 to 10 m) debris deposit. This debris was derived from underlying andesite lavas and consolidated pyroclastic rocks (Yamakuni Formation, Late Miocene to Pliocene).
In conclusion, the 2023 slope failures in northern Kyushu can be characterized and categorized by their geological background. Specifically, changes in the rock properties due to contact metamorphism, hydrothermal alteration, weathering, and fault activity affects on triggering slope failures. Also, it was confirmed that thick surface covering sediments cause slope failures of considerable scale. This geological information should be taken into the risk evaluation of slope failures.
The investigated slope failures were classified into two types: A) failures occurred in the areas suffered from hydrothermal alteration or weak to middle weathering. This type includes failures in Yufu, Toho, Takachiho, Mashiki, Kurume and Karatsu. B) failures of surface covering debris other than the bedrocks. Failures of this type occurred in the Hita area.
There is a wide variety of collapsed rocks and formations among A-type failure sites. Details are provided as follows. A1) Kawanishi site, Yufu: Hydrothermal altered andesite pyroclastic rocks and lavas (Kawanishi andesite, Eary Pleistocene) collapsed. A2) Takeno site, Kurume: Pelitic schists (Suo metamorphic rocks, Late Triassic to Early Jurassic), subjected to contact metamorphism by Cretaceous granitic rocks, collapsed. A3) Hirabaru site, Karatsu: We observed collapses of corestones in weathered Cretaceous Itoshima granodiorite, and collapses and flows of decomposed granite soils originated from Cretaceous Fukae granite. A4) Sakanoshita site, Takachiho: Weathered non-welded Aso-4 pyroclastic flow deposits (Late Pleistocene) collapsed. A5) Akai site, Mashiki: Altered andesite scoria cone (Akai volcano, late of Middle Pleistocene) collapsed. A6) Koishiwaratsuzumi site, Toho: Decomposed granitic rocks (Cretaceous) collapsed.
At Ono and Tsurukawachi sites in Hita area B-type failures include a thick (approximately 5 to 10 m) debris deposit. This debris was derived from underlying andesite lavas and consolidated pyroclastic rocks (Yamakuni Formation, Late Miocene to Pliocene).
In conclusion, the 2023 slope failures in northern Kyushu can be characterized and categorized by their geological background. Specifically, changes in the rock properties due to contact metamorphism, hydrothermal alteration, weathering, and fault activity affects on triggering slope failures. Also, it was confirmed that thick surface covering sediments cause slope failures of considerable scale. This geological information should be taken into the risk evaluation of slope failures.