Numerous slope failures occurred along the Futagawa fault zone in the central part of Kumamoto Prefecture due to the Kumamoto Earthquake on April 16, 2016. In Minamiaso Village, situated on the western region of Aso caldera, collapse of the surface layer, composed mainly of volcanic ash and soil, generated earthflow ( Fig.1) and caused severe damage. Earthflow from the northern slope of Koeboshi-dake, a post-caldera volcano, flowed down and spread around the Sannoudani River (Fig.2 ) within a

more »

... w hours after the main shock, despite the lack of rainfall around the catchment area of the river. This sediment has poor water saturation (Figs.3, 4), and mechanism underlying its movement is still unclear. Mudflows were also generated by stream water (Fig.5) . Furthermore, lahar deposits due to heavy rainfall (total amount of rainfall; 453.5 mm, June 19-21, 2016) caused further damage (Fig.6) . On the cover:The earthflow overflowed from the Sannoudani River and a crushed greenhouse. There were few traces of dehydration in the cultivated land. These observations suggested that the sediment was highly viscous fluid. 南阿蘇村山王谷川で発生した 3 つのタイプの土砂移動(鳥井真之・奥野 充・西山賢一・北園芳人・長谷中利昭・横田修一郎) 2016 年 4 月 16 日,熊本地震の本震により熊本県中部の布田川断層帯に沿って多くの斜面崩壊が発生した.阿蘇カルデラ内の南阿 蘇村では主に降下火山灰層から構成される表層部の崩壊・流動により甚大な被害をもたらした(Fig.1) .後カルデラ火山の小烏帽 子岳を源とする山王谷川では,本震後数時間以内に斜面を構成する火山灰質土がアースフローとして流下した (Fig.2).この堆積 物はあまり含水していないという特徴があるが(Figs.3, 4) ,その流下メカニズムは明らかでない.また,その直後には泥流も発 生している(Fig.5) .同年 6 月 19 日-21 日の豪雨(総雨量 453.5mm)でもラハールが発生し,新たに被害が生じた(Fig.6) . 謝辞:航空写真撮影には国土交通省九州地方整備局にご協力頂いた.この場を借りて深謝いたします. 表紙:山王谷川から溢れたアースフロー.ビニールハウスの一部がアコーディオンのように押しつぶされ,耕地上への脱水の痕跡 がほとんどないことは,この堆積物が高粘性であったことを示す. Fig.1:Index map. A topographic map issued by the Geospatial Information Authority of Japan was used. Arrows indicate the approximate camera directions. Fig.2:Oblique aerial view of the Eboshi-dake and Koeboshi-dake volcanoes and a large amount of volcanic ash and soil flowing down the Sannoudani River. Fig.3:Earthflow overflowing a river levee of the river, consisting of volcanic ash, driftwood (due to slope collapse), and a few andesite lava blocks. Fig.4:A bridge blocking the earthflow. Overflowed deposits caused severe damage to nearby houses. Fig.5:Mudflow deposits with driftwood around the downstream region. Fig.6:Lahar deposits caused by a heavy rainfall in June. The deposits are mainly on an andesite block and sand.