著者

宮縁 育夫 大丸 裕武 小松 陽一

出版者

日本地形学連合

雑誌

地形 (ISSN:03891755)

巻号頁・発行日

vol.25, no.1, pp.23-43, 2004-01-25

被引用文献数

3

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The 29 June 2001 rainstorm (total 238 mm; maximum hourly rainfall 98 mm) triggered more than 700 landslides and associated lahars at Aso Volcano, central Kyushu, SW Japan. Most of the landslides were concentrated in a 20-km^2 area that includes the northeastern slope of post-caldera central cones, the northern slope of Nekodake Volcano and the eastern caldera wall. We mapped landslides triggered by the storm using field observations and 1 : 20,000-scale aerial photography provided by the Kumamoto District Forest Office taken in December 2001. Most of the triggered landslides were shallow (about 1 m thick) soil slips in unconsolidated airfall tephra layers overlying lava flows and pyroclastic rocks. Average volume of the landslides was 550 m^3, but some had volumes exceeding 10,000 m^3. The rupture surfaces of most landslides were formed near the boundary between upper blackish and lower brownish tephra layers. These layers have differences in permeability, grain size and soil hardness. Most landslides mobilized completely into lahars, traveling a few kilometers along stream channels. Although some lahars eroded channel side slopes and transported boulders (2-3 m in diameter), most contained up to about 80 percent silt-to-clay-size particles, similar in composition to the original landslide debris. Sediment discharge volumes by the tephra-slip-induced lahars were estimated at an order of 10^3-10^4 m^3/km^2 using sediment volumes trapped by check dams. These volumes are similar or one-order of magnitude smaller than the volumes of the 1953 and 1990 landslide disasters in the same region. The characteristics of landslides and lahars and their estimated volumes provide important information about landslide and lahar hazards at Aso Volcano.