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 )−207−Synthesiology - English edition Vol.4 No.4 (2012) Northeast Japan to the Japan Sea side from 2004. The example of the analysis is described below.In Japan, there are about 100 active volcanoes (those that erupted within the last 10 thousand years or volcanoes with vigorous fumaloric activity), and over 200 volcanoes that erupted in the Quaternary period (from 1.7 million years ago to present according to the old definition) (Fig. 3). However, the volcanoes are not distributed evenly throughout the Japan Islands, and their presences are determined by the plate arrangements. This means that the Quaternary volcanoes of the Japan Islands are most densely distributed on the volcanic front located 200~300 km away from the subduction boundary of the plates toward the plates on the landside, and there is no volcano between the volcanic front and the subduction boundary (fore-arc side). Also there is a significant tendency where the volcano distribution becomes sparse in the area distant from the back-arc side or area opposite the volcanic front. In soliciting the disposal site candidates, NUMO sets the condition “the area within the circle of 15 km radius with the center at a Quaternary volcano shall not be included” for the purpose of avoiding the effect of the volcanic activity[6]. However, in thinking about the volcanic activity several hundred thousand to million years into the future, is it possible to avoid the effect with this exclusion condition only? Next, we shall consider the specific analysis of the spatiotemporal distribution of the volcanic activities.When analyzing the eruption history of the volcanic activities, the so-called step diagram of eruptive volume is created, in which the horizontal axis represents time and the vertical axis is the cumulative eruptive magma volume. Figure 4 is an example created for the eruptive magma material of Adatara volcano, a representative active volcano in southern Northeast Japan[7]. Each magmatic eruption event is a geological instant shown as the vertical line, and during the non-eruptive state, it is shown as the horizontal line because there is no magmatic eruption. Figure 4.1 is a step diagram of the eruptive volume that goes back 120 thousand years, and as indicated by the average eruption rate shown by a dashed line, it can be seen from the step diagram that the eruptions occurred repeatedly at a certain frequency. However, when the history is extended to the time scale surpassing 120 thousand years for this volcano, as seen in Fig. 4.2, there is a major dormancy period 120 thousand to 200 thousand years ago, and the average eruption rate up to 120 thousand years ago cannot be extended into the past. There were separate magmatic activity periods 200~260 thousand year ago and 320~430 thousand years ago, but the individual average eruption rate differs by the activity period, and it is clear that the activity of Adatara volcano up to 120 thousand years ago, assuming a consistent rate, cannot be extrapolated to the past. In other words, it indicates that there is a lifespan on the magma supply that supports the individual activity periods, and for the forecast surpassing 100 thousand years, the assessment of the reactivation of the dormant volcanoes will become important rather than the currently active volcanoes.Figure 5 shows the spatiotemporal distribution change of the volcanoes in the southern Northeast Japan including Adatara volcano, from about 1.8 million years ago or the beginning of Early Pleistocene to present[8]. The important point in the distribution pattern of the volcanoes in this period is that there is almost no change at the volcanic front while there Fig. 5 Temporal change of the distribution of Quaternary volcanoes in the southern part of NE Japan arcAfter Yamamoto[8]1.78-0.78 Ma0.78-0.30 Ma0.30-0.00 Ma0.30 MaRhyolite-dacite caldera & lava domeAndesite(-basalt) stratovolcanoVolcanic front139°E140°E141°E0.20 MaSunagohara 0.29 MaNumazawa 0.11 Ma37°N050 km139°E140°E141°E37°N050 km139°E140°E141°E37°N050 km0.16 Ma

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