Catastrophic landslide induced by Typhoon Morakot, Shiaolin, Taiwan

Abstract

台湾南部の小林村は2009年8月9日に台風莫拉克(Morakot)によって深刻な崩壊がもたらされ，400人以上の死者が出た。三日間の累計降雨量は1676.5mmに達し，降雨強度のピークを迎えた翌日に崩壊が発生した。今回の崩壊は流れ盤に起こり，崩壊体積は25106m3に及んだ。その基盤は中新世後期から鮮新世前期にかけて形成した堆積岩であり，シルト頁岩，厚い泥岩と砂岩を含有する。大規模な斜面崩壊土が旗山川の谷を塞ぎ，向かい側の急斜面上に広がりながら，村の上流に高さ60mの天然ダムを形成した。天然ダムは形成後の1時間24分後に崩壊した。崩壊の移動速度は約20.4～33.7ms−1と推定され，また，見かけ摩擦角度は14°であった。崩壊源のディタッチメントは層理，節理及び断層であった。崩壊源の地層は重力変形を受けており，崩壊前には不規則な凹凸の地形を示していた。また，この地層の重力変形は崩壊後に座屈変形として確認された。崩壊の堆積物は泥岩，頁岩および砂岩などの岩屑を含有し，底部には広く粘度が認められ，それはイライト，緑泥石，石英，長石および方解石からなっていた。この粘度が高速かつ長距離の移動の大きな要因となったと推定される。

Typhoon Morakot induced the catastrophic and deadly Shiaolin landslide insouthern Taiwan on 9 August 2009, resulting in more than 400 casualties. We undertooka geological and geomorphological investigation with the aim of reconstructing theevents leading up to this landslide and to clarify factors that contributed to itsdevelopment. The research results have been published with the title “Catastrophiclandslide induced by Typhoon Morakot, Shiaolin, Taiwan” to the Journal ofGeomorphology, volume 127, in 2011, where we provided the same contents as thispaper. Cumulative rainfall reached up to 1676.5 mm in about three days under theinfluence of the typhoon, and the Shiaolin landslide, with a volume of 25  106 m3, occurred one day after the peak in rainfall intensity. The landslide occurred on a dipslope overlying late Miocene to early Pliocene sedimentary rocks consisting of siltyshale, massive mudstone, and sandstone. It started as a rockslide in the upper third ofthe landslide area and transformed into a rock avalanche that crossed a series of terracesand displaced or buried the village below. It buried the riverbed of the Chishan Riverand ran up the opposite slope, creating a landslide dam 60 m high, which was breachedabout 1 hour and 24 minutes later, flooding the village. The velocity of the landslide isestimated to have been 20.4 to 33.7 m s−1 and its apparent friction angle was 14°, whichindicates its high mobility. The detachments in the source area consist of combinationsof bedding planes and joints or faults. The landslide was preceded by gravitationaldeformation, which appeared as hummocky landforms before the landslide and asbuckle folds exposed after the event. The landslide deposits consist of fragments ofmudstone, shale, and sandstone, as well as clayey material at its base. This clayeymaterial, consisting of illite, chlorite, quartz, feldspar, and calcite, is assumed to havestrongly influenced the long, rapid runout.