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Vol. 126 (2017) No. 5 p. 581-593




 Several volcaniclastic deposits discovered at the foot of the eastern wall of Aso caldera, central Kyushu, southwestern Japan, are divided into two types: lahar deposits (Lh1-Lh5 in descending order), which contain abundant subangular to subrounded lithic clasts ( 3.5 m in diameter) set in a sandy to silty matrix, and debris avalanche deposits (DA1 and DA2), which include numerous plastically deformed fragments of tephra (ash and scoria) and soil layers in a homogenous silty to clay matrix. DA2, which underlies a paleosol dated at 5.4 ka (calibrated 14C age), is the largest volcaniclastic deposit observed in the section (more than 2.5 m thick and about 70 m wide). Because the debris avalanche deposits display no evidence that they were transported by water, they are likely to have originated from landslides triggered by intense earthquakes. Tephra chronology and 14C-dating on paleosols along the succession suggest that lahars occurred once over 900 years (6.3-5.4 ka), three times over 1400 years (5.4-4 ka), once over 400 years (4-3.6 ka) and twice (including the 2012 lahar) in the last 3600 years. This evidence indicates that the lahars occurred at an interval of 400-1800 years. In contrast, two debris avalanche deposits exist in the same succession spanning the last 6300 years. Including landslides and associated debris avalanches triggered by the 2016 Kumamoto earthquake (Mj 7.3), landslides generating debris avalanches in the Aso caldera occurred at least three times in the last 6300 years. This may suggest the frequency of large earthquakes triggering debris avalanches in the central Kyushu region, which has many active faults.

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