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Triggering mechanisms and rainfall threshold of shallow landslides were discussed based on field and laboratory surveys for the disaster by heavy rainfall on 20 Aug 2014 at Hiroshima, southwest Japan. Shallow landslides occurred in areas of maximum 3 h-rainfall >160 mm/3h with hourly rainfall intensity >70 mm/h. Hillslopes with bedrock of granite and hornfels slid severely, while areas underlain by rhyolite suffered only sparse landsliding. Three types of landslide were identified in field observation: 1) planer translational landslide, 2) gash-out failure, and 3) wedge sliding. Lithological and subsurface structural factors seem to affect the occurrence of these different types of landslide. Majority of landslides in granite hillslopes and about half in hornfels were planer translational landslides. This type forms a sliding surface within soil layer along mechanical or hydraulic discontinuity at a depth of ~1 m. Other half of landslides in hornfels area was gash-out failure with scars of 1–3 m deep, outcropping an openwork gravel layer at the bottom. Water supply exceeding drainage capacity of this conduit may generate excess pore-water pressure, leading to fluidization of overburden. Wedge sliding was a minor type, with a shallow gullying of soil or a deeper slip surface in bedrock (~5 m), occurring along geologic discontinuity such as joints, faults and dykes. Prediction of potential sites, timings, and magnitude of such landslides requires detailed investigations of thickness and mechanical strength of potential sliding material, observation and modeling of subsurface hydrological processes.

台風12号は，2011年9月2日から5日にかけて西日本を横断し，特に紀伊山地に2000mmを超える降雨をもたらし，主に四万十帯で50以上の深層崩壊を発生した。発生したものの内大規模な14の深層崩壊について，発生前後の1m－DEM解析および空中写真観察，さらに地質調査を行った結果，いずれも発生前に将来冠頂となる位置に重力変形による小崖または線状凹地を伴っていたことがわかった。これらの小崖は，比高2mから50mで，空中写真では極めて注意深く観察して見出されるものが多い。斜面傾斜方向断面で考えると，その水平長と崩壊斜面水平長との比は5～21％であり，これは発生前の斜面変形程度が小さかったことを示している。これらの斜面変形の多くは，楔形不連続面に沿うすべりによるものである。発生時の特定された19の深層崩壊は，レーダーアメダス解析雨量によれば，3日間での累積雨量が約700mmを超えるあたりから発生した。, Typhoon 1112 crossed the Japanese Islands from 2 to 5 September in 2011, brought more than 2000 mm rainfall, and induced more than 50 deep-seated catastrophic landslides of Jurassic to Palaeogene sedimentary rocks. We studied fourteen landslides by using 1m-DEMs or aerial photographs and field survey to clarify that all of these occurred on slopes with scarplets or a linear depression along their future crowns. These scarplets and a linear depression were 2 m to 50 m high, and are scarcely observable on the aerial photographs without particular attention. Horizontal length ratio between the scarplets and slopes along slope lines were 5 to 21 %, suggesting that "strains" before the failure were relatively small. Gravitational deformations were mainly due to sliding along wedge-shaped discontinuities. Nineteen landslides that have been specified for their occurrence time, were preceded by rainfalls exceeding 700 mm on the basis of amount of precipitation analyzed by rader-AMeDAS.

第28回宇宙利用シンポジウム (2012年1月23-24日. 日本学術会議), 港区, 東京都, Space Utilization Research (January 23-24, 2012. Science Council of Japan), Minato-ku, Tokyo Japan, 資料番号: SA6000032078, レポート番号: L38