霧島火山地域における土砂災害

抄録

Mount Kirishima is a cluster of volcanoes situated in the boundary between Kagoshima and Miyazaki Prefectures, Japan, and includes the Shinmoedake Volcano which erupted violently in January 2011, ejecting a large amount of volcanic ash and pumice onto surrounding land. In this paper, I describe the effects of volcanic activity on sediment transport phenomena and the potential for development of related hazards in relation to Mount Kirishima. First, I examine sediment transport phenomena that followed the volcanic eruption, such as covering by volcanic ash, lowering of infiltration capacity, increase in surface runoff, and rapid progress of erosion and related occurrence of debris flows. The thickness and grain size of the volcanic ash deposits, and the infiltration capacity on the hillside slope covered with volcanic ash, were investigated in terms of the potential for debris flow hazard posed to the area surrounding Shinmoedake. Second, I examine the bedrock aquifer that is widely distributed in the Mount Kirishima area and the deep-seated landslides which have been mobilized by the groundwater. Recently, such deep-seated landslides have caused serious damage, such as in Ebino in Miyazaki in 1972, Izumi in Kagoshima in 1997, and Minamata in Kumamoto in 2003. These landslides resulted from rising groundwater levels caused by heavy rainfalls and deeply weathered rocks. Some methods for onsite prediction and early-warning of potential deep-seated landslides are also examined, based on geomorphological, geological and hydrological surveys.