Transition in the accumulation time and place of debris flow and flood deposits on the alluvial fans in the foot of Southern Rokko Mountains

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  • 六甲山地南麓扇状地での土石流・洪水堆積物の堆積時期・堆積場の変遷
  • ロッコウ サンチ ナンロク センジョウチ デ ノ ドセキリュウ コウズイ タイセキブツ ノ タイセキ ジキ タイセキバ ノ ヘンセン

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Abstract

Kobe and Ashiya Cities are situated on an alluvial fan and plain in the southern foot of the Rokko Mountains, western Japan. Talus debris from the mountains was produced due to collapse, which was triggered by the Hyogoken-Nambu earthquake. This area has experienced at least three strong earthquakes attacked, such as the ones in 1596, 5-6th century and 6300 years ago. We consider here whether there is an increase of debris flows after earthquake activity. The ages of debris flow and flood deposits were obtained from archeological data. Flood and debris flow deposits were recognized at archeological survey sites, and ancient sedimentary environment was estimated from outcrop investigations. Debris flow and flood deposits were found to often overlie earthquake induced sand dykes, indicating that flood events occur after earthquake. It is significant that debris flow and flood deposits show a tendency to increase after the earthquakes. Moreover, the main accumulation space shifted from alluvial fan to lowland about 1,000 years ago. Therefore, the talus debris of the Rokko Mountains is transported easily by debris flows due to heavy rains after the Hyogoken-Nambu earthquake, and accumulates on the alluvial plain.

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