Vol.4 No.4 2012

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Research paper : Safety assessment of high-level nuclear waste disposal in Japan from the standpoint of geology (T. Yamamoto )−208−Synthesiology - English edition Vol.4 No.4 (2012) are always volcanoes along the front. On the other hand, the distribution of the volcanoes in the back-arc region differs greatly by intervals, and the volcano that was active in the back-arc region during the Early Pleistocene became dormant in the first half of the Middle Pleistocene. 0.3 million years ago and after or the latter half of Middle Pleistocene, the new back-arc volcanic activities (Numazawa volcano, Sunagohara caldera) began, and it should be noted that they are located in the areas of volcanic activity gap. This means that if the time is set back 0.3 million years, Numazawa volcano and Sunagohara caldera appeared outside the “range within the circle of 15 km radius with the center being an existing Quaternary volcano”, and the volcanic activities cannot be eliminated based on location under this condition. To understand the spatiotemporal change of the volcanoes in southern Northeast Japan shown in Fig. 5, it is necessary to extend the time scale further and to look at the tendency of the magmatic eruption rate for the entire area. Figure 6 is the result, and the time scale of the horizontal axis is in the unit of million years, and the vertical axis is the sum total of the magmatic eruptive volume from all volcanoes in the Aizu region, from the Nasu volcano group along the volcanic front to the Numazawa volcano on the back-arc side[9]. It is clear from the ladder diagram of eruptive volume of the entire region in Fig. 6, that the long-term magmatic eruption rate shown in a dashed line for this area is dominated by the mega-eruptions where the single eruptive volume surpasses 100 km3 and which occur at 1~2 million year intervals. The eruption site is covered with overlapping large collapse structures (caldera, shown in pink in the diagram) with diameters of over 10 km in the range of 60 km × 50 km, and the activities can be traced back to 10 million years ago[10]. The appearance of Numazawa volcano and Sunagohara caldera occurred in this volcanic activity region, and they cannot be fully positioned only by the investigation of the activity history limited to Quaternary volcanoes. The existence of the hierarchical structure of the phenomena, as presented here, shows the difficulty of forecast by simple extrapolation of the history of change. To reduce the uncertainty of the forecast for several 100 thousand to a million years into the future, it is necessary to understand the structural development history of the subject region, by tracing way beyond the assessment period, and effort must be made to minimize any “unexpected phenomenon”.4.3 Limit of probabilistic assessment: need for a forecast that steps into the geological understanding of the phenomenaIn the United States, Yucca Mountain in Nevada was determined as the HLR waste disposal site in 2002, and the safety assessment for the approval of construction was started from September 2008. However, President Obama decided to terminate the Yucca Mountain project, and the appeal to withdraw the application for approval was submitted in Fig. 6 Cumulative erupted magma volumes from the Aizu regions in the southern part of NE Japan arcThe average eruption rate (dotted line) is controlled by huge eruptions from caldera volcanoes (red solid lines). After Yamamoto[9][10]Nasu volcanoesTonohetsuriNariokaHiwadaTakagawaIriyamazawaSunagoharaUwaigusaNumazawaVolcanic frontAizu Basin01.02.03.04.00100200300400500600700Age(Ma)Cumulated magma volume(km3 DRE)37°40’N37°30’N37°20’N37°10’N10 km0139°45’E140°0’E

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