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Landslides and debris flows in volcanic rocks triggered by the 2017 Northern Kyushu heavy rain
Ohta, Takehiro Eguchi, Seiya
Ohta, Takehiro
Eguchi, Seiya
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2019
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Abstract
About 200 landslides and debris flows occurred in Northern Kyushu during heavy rain at Asakura City, Toho Village, Hita City on July 5th to 6th, 2017. At Hita City, the total precipitation during this two-day event was 402.5 mm. At Asakura city, underlain by granitic rocks and schist, shallow landslides dominated. Whereas, at Toho Village and Hita City, which is underlain by volcanic rocks, the number of landslides and debris flows are fewer, are larger and deeper, than those at Asakura City. We examined the geomorphology and geology at 19 landslides in volcanic rocks, 11 of which mobilized as debris flows. We studied the initiation mechanism of landslides underlain by volcanic rocks. The geology consists of pyroclastic rocks and lava flows in ascending order. The lava flows are distributed at ridges and contain vertical cooling joints. Scarps of landslides caused by the 2017 rain are located near the boundary of pyroclastic rocks and lava flows. The sliding surfaces of these landslides are at the contact between the lava flows and the pyroclastic rocks. We consider, therefore, that the trigger for these landslides was a decrease of strength at the contact caused by an increase in groundwater pressure caused by infiltration of rain water through the cooling joints in lava flows. Therefore, we conclude that the landslides caused by the 2017 heavy rain at volcanic rock fields are cap rock type landslide. Furthermore, the curvature of the hillsides downslope from landslides is concave, which may be a required condition for debris-flow mobilization.
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