深層崩壊発生危険地におけるテフロクロノロジーによる斜面変動履歴の解明 ―宮崎県鰐塚山の2005年崩壊地周辺―

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  • Tephrochronological investigation of the long-term landslide history in an area prone to deep-seated landslides, Mount Wanitsukayama, Miyazaki, Japan
  • シンソウ ホウカイ ハッセイ キケンチ ニ オケル テフロクロノロジー ニ ヨル シャメン ヘンドウ リレキ ノ カイメイ ミヤザキケン ワニツカヤマ ノ 2005ネン ホウカイチ シュウヘン

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It is necessary to examine data from a long-term perspective in order to understand the history of large deep-seated landslides because they occur infrequently. Using Kikai-Akahoya tephra (ejected in 7300 cal years BP) as the key bed, we investigated the landslide history of Mount Wanitsukayama, Miyazaki, Japan, an area that has old landslide scars. We dug 42 sites and determined whether the tephra layer was observable in the soil profiles. Three types of soil profiles were observed: type A contained air-carried tephra, demonstrating that neither landslides nor surface erosion had occurred in the past 7300 years; type B contained a mixture of tephra and other soil types, showing that the tephra has been reworked by small shallow landslides in the past 7300 years; and type C lacked tephra, indicating slope denudation in the past 7300 years. Most of the ridges are considered to be long-term stable slopes because type A is widely distributed over them; but some narrow parts of the ridges have been sharpened by old landslides, which destabilized and disturbed regolith of the sharpened ridges, because type B was found on the narrow ridges. Some landslide scars were found on the slopes according to the detailed maps derived from a LiDAR survey. Mosaic distributions of types A, B, and C over the middle area of the landslide were probably attributable to non-uniform movements of slopes in the past 7300 years. Reactivation of this landslide caused a debris-flow-triggering landslide and slope cracking during a rainstorm in the year 2005.

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