The spatial and temporal distribution of volcanoes in Japan

Japan is located on the western edge of the Pacific plate, in one of the most dynamic areas on Earth, with approximately 80 active volcanoes and considerable tectonic activity. The Japanese Nuclear Waste Management Organisation (NUMO) will construct a deep geological repository for high level nuclear waste somewhere in Japan. Potential hazard posed by a new volcanic event occurring and disrupting this facility during its operational lifetime need to be thoroughly assessed. The ITM (International Tectonics Meeting) group have assembled a methodology to assess such hazards, of which this study forms part. Significant differences in the volcanism of NE and SW Japan are explored here in two case studies; the Tohoku region of NE Japan, and Kyushu island in SW Japan. The study of the Tohoku region develops ideas of volcano classification, using a cladistic approach to classify volcanoes into coherent groups. Temporal examination of these groups elucidates differing volcanic histories along the volcanic arc of NE Japan. Probability analyses for new volcanoes in NE Japan suggest that missing data and poor definitions of a volcano may affect volcanic hazard analyses. Thus, ‘alternative datasets’ based of different definitions of ‘a volcano’ are tested to see how they affect hazard analyses. Spatial distributions of alternative datasets show how certain definitions of a volcano have a much higher tendency to spatially cluster. A comparative probability analysis showed also that probability varied significantly between datasets, suggesting careful examination of raw data used in probability and hazard analysis is required. The Kyushu case study revealed greater spatial and temporal variation of volcanism than Tohoku. Detailed study of the volcanism and tectonism led to temporal and spatial separation into similar distinct periods and discrete regions within Kyushu, hinting at strong volcanic-tectonic interactions. The Philippine Sea Plate drives subduction related volcanism, but small changes in its motion, features, and physical characteristics affect the character and timing of volcanism in Kyushu. Characterisation of the volcanism into coherent regions with similar volcanic histories allows hazard analyses to focus on these regions to create more detailed and informed analyses.