Geological study on the Abu Monogenetic Volcano Group, Southwest Japan -Characterization of temporal-spatial distribution in volcanic activity and prediction of the future_fs activity-

Safety assessment for the long-term stability of the geological environment, over 100,000 years in the future, is required for a siting of high-level radioactive waste repository in Japan. The case study was carried out in the Abu Monogenetic Volcano Group distributed in the northern part of Yamaguchi Prefecture, Southwest Japan, not only to clarify the temporal-spatial distribution of Monogenetic Volcano Group but also to predict the future_fs activity. Late Pliocene and Quaternary monogenetic volcanoes are distributed over an area of ca. 30km×40km in the Abu Monogenetic Volcano Group. 54 volcanic centers are decided by means of field geological survey and topographic analysis, and they are intensively distributed in the elongated areas in NW-SE and NE-SW direction. The regional stress field around this area (σ1 = N70~90W) is not a major factor to decide the trend of these elongated areas. These trends are concordant with the strike of faults and lineaments around this region. Therefore, it is possible that the magma is ascending through the existing fractures in the upper crust. The analogue experiments using gelatin block were carried out to verify this magma injection model. An air crack, which is injected from the bottom of a gelatin block, ascends and arrives at a pre-existing vertical fracture in the top of the block. If the fracture is not closed hardly under low normal stress, the air crack can ascend through it, and this supports the proposed magma injection model for the Abu Monogenetic Volcano Group. The temporal-spatial distribution of the Abu Monogenetic Volcano Group was investigated based on the newly determined location of volcanic centers and existing ages. As a result, the locations of volcanic centers shift from NE-SW trend to E-W trend with time. The change of volcanic activity may be due to the cooling of the mantle diapir or the concentration of magma in the crust. The temporal-spatial distribution of Monogenetic Volcano Group reflects the existing structure and the processes of magma forming and ascending. Therefore, it is possible to predict the future_fs activity of a Monogenetic Volcano Group by considering the existing geological structures and temporal-spatial distribution of volcanoes. In case of the Abu Monogenetic Volcano Group, the future_fs activity is thought to be occurred along the NE-SW and NW-SE existing fractures in the E-W elongated area by the extrapolation of the past temporal-spatial distribution characteristics.