2011年霧島新燃岳噴火後の谷壁斜面における降下テフラ移動と谷床地形変化

抄録

The 2011 Kirishima Shinmoe-dake eruption brought pumice fall deposits to the south-eastern flank of the Kirishima Volcanoes. This study aims to confirm the processes of tephra deposits movements which followed to the eruption and to catch the trend of the valley bottom topographic change after the eruption. The method of this work is to observe the topographic change at the same point repeatedly to clarify the changing pattern of micro-topography on the valley slope, and to measure elevation of the valley floor repeatedly to clarify the volumetric and topographic change of the valley floor deposits using GIS. Study area is located at the south-eastern foot of Takachihonominepeak, where the 2011 air-fall pumice deposits about 10cm thick mantled the valley slope and the valley floor. These observation and measurements lead to the conclusions as follows. Sheet wash eroded surface tephra particles on the upper part of valley slope and transport them downslope. This process formed lobe-like micro-topography on the original air-fall tephra surface. Another process is gully erosion, which started from the middle part of valley slope and washed away tephra layer linearly, but never eroded subsurface of buried soil. These two erosional processes started in the first summer rainy season after the eruption. On the valley floor, rapid deposition appeared in the first summer rainy season. Depositional tendency on the valley floor continued in August 2012, then the valley floor changed to erosional condition to form two steps of terrace-like topography. In 2015, the valley floor condition changed to equilibrium of erosion/deposition. The secondary movement of tephra deposits occurred rapidly in the first rainy season but reached the equilibrium in several years.