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 )−206−Synthesiology - English edition Vol.4 No.4 (2012) conditions and reoccurrence intervals that are the basis of the long-term assessment is constant over super long-term is unknown, and we currently cannot conduct reliable assessment. Therefore, to handle the super long-term time span of a hundred thousand to million years, various geological surveys are necessary in addition to statistical inferences.In the forecast of geological and climatic phenomena for geological disposal, the foundation will be to clarify the trends of the geological changes for the phenomena that occurred in the assessment area, and to extrapolate this into the future. If the extrapolation is done for a hundred thousand to a million years into the future, it is necessary to track back the same number of years or more into the past. If the activity history of the phenomena with sufficient quantity and quality in terms of statistical inferences can be obtained, it may become possible to conduct probabilistic assessment, as in the long-term assessment of earthquakes. However, not all histories of activities are stored geologically, and there is overwhelmingly greater number of cases where sufficient history cannot be obtained. Therefore, forecast must be done from limited data, and it is necessary to consider the fact that quantitative handling cannot necessarily be done. For example, with the plate boundary earthquakes, the activity history depends on the historical records and tsunami deposits, and there is a limit in understanding the geological marks from surface survey over a long period. Also, for major earthquakes along the active fault, the activity history of only within about 10 thousand years can be obtained depending on the relationship of the fault and the cover stratum, and many cases are expected where the amount of information will be insufficient for the assessment period. In the case where the erosion history is studied by tectonic geomorphology, the geomorphic surface index with sufficient sequence appropriate for the assessment period may not necessarily be found in the assessed area or the surrounding area. For hydrogeological phenomena, only the current value that is the sum of all past changes can be observed, and in most cases it is difficult to separate the individual history of changes.In the case where it is difficult to extrapolate the super long-term history of change by statistical inferences, a different explanation is necessary to guarantee the stability of the geological environment of the assessed area over super long-term. For example, it is necessary to present a qualitative forecast by establishing the model of the structural development history of the assessed area, where the seismicity and uplift that accelerate erosions can be described. In hydrogeological change, qualitative forecasting will be possible only by establishing the mechanism for water quality formation that includes the chronology axis. Which kind of forecasting model is specifically necessary differs by the geological property of the area, and it is necessary to consider the forecasting theory according to the individual areas. 4.2 Example of analysis of long-term geological history for Japanese volcanic activitiesTo develop the analysis and assessment methods for the long-term geological history of the Japanese volcanic activities, as a typical cross-section model of the island arc, we conducted the research of the spatiotemporal distribution of the volcanic activities in the area from the Pacific Ocean side of southern Fig. 3 Index map in “Quaternary Volcanoes in Japan, RIO-DB, AIST”Red triangles are Quaternary volcanoes.Fig. 4 Cumulated erupted magma volumes of Adatara volcano1) and 2) diagrams are different in time scale. After Yamamoto and Sakaguchi[7]25°30°35°40°45°25°30°35°40°45°145°140°135°130°125°150°145°140°135°130°125°0100500 kmCumulated magma volume(km3DRE)Cumulated magma volume(km3DRE)Age(Ka)Age(Ma)02040608010012001.02.03.000.10.20.30.40510151)2)

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