桜島アミダ川流域における噴火活動の衰退に伴う植生回復および火山灰堆積の状況

Based on field surveys，we investigated the conditions of revegetation and the accumulation of volcanic ash，including their respective physical characteristics，caused by the ebb of volcanic activity at the Amida River basin of Sakurajima Volcano. The results were as follows: (1) The density of trees and the total volume of tree trunks tended to increase with decreasing volcanic activity effects and volcanic ash. (2) In a comparison of distance between two points located about 2.5 km and 3.5 km from the Minamidake crater of Sakurajima Volcano，the yearly depth of the volcanic ash layer during heightened volcanic activity (between 1972 and 1993) was，respectively，between about 3.0 and 6.0 times (average 4.6 times) greater than that measured during low activity (between 1994 and 2005) (3) In a comparison of distance between two points located about 2.5 km and 3.5 km from the Minamidake crater of Sakurajima Volcano，the dry density of the volcanic ash layer was，respectively，between 1.30 g/c㎥ and 1.45 g/c㎥ (average1.37 g/c㎥) during the period of heightened volcanic activity，and it was between 1.24 g/c㎥ and 1.33 g/c㎥ (average1.27 g/c㎥)，respectively，during the period of low activity. The void ratio of the volcanic ash layer during the periodof low activity was greater than that during the period of heightened volcanic activity. (4) In a comparison of distance between two points about 2.5 km and 3.5 km from the Minamidake crater of Sakurajima Volcano，the median diameter of solid particles in the volcanic ash layer was，respectively，between 0.14 mm and 0.23 mm ( average 0.18 mm) during the period of heightened volcanic activity，and it was between 0.12 mm and 0.20mm (average 0.15 mm)，respectively，during the period of low activity. The median diameter of solid particles in the volcanic ash layer during the period of low activity was smalier than that during the period of heightened volcanic activity.
本論では，桜島アミダ川流域で実施した調査結果に基づき，噴火活動の衰退に伴う植生回復状況，火山灰堆積の状況およびその土壌・土質と物理特性について検討した。得られた結果は次の通りである。 (1)木本植生の個体密度および蓄積量は，噴火活動および火山灰の影響が小さくなるほど増加する傾向にある。 (2)南岳火口から約2.5～3.5kmの範囲で比較すると，噴火活動が活発な期間(1972～1993年)における年平均での火山灰層厚の平均値は，穏やかな期間(1994～2005年)におけるそれの約3.0～6.0倍(平均4.6倍)である。 (3)南岳火口から約2.5～3.5kmの範囲で比較すると，噴火活動が活発な期間(1972～1993年)における火山灰層の乾燥密度は1.30～1.45g/c㎥ (平均1.37g/c㎥)，穏やかな期間(1994～2005年)におけるそれは1.24～1.33g/c㎥ (平均1.27g/c㎥) であり，活動が穏やかな時期における火山灰層の空隙率が大きい。 (4)南岳火口から約2.5～3.5kmの範囲で比較すると，噴火活動が活発な期間(1972～1993年)における火山灰層の中央粒径は0.14～0.23mm(平均0.18mm)，穏やかな期間(1994～2005年)におけるそれは0.12～0.20mm(平均0.15mm) であり，活動が穏やかな時期の方が小さい。