Research plan on geosphere stability for long-term isolation of radioactive waste (Scientific program for fiscal year 2023)

Niwa, Masakazu; Shimada, Koji; Sueoka, Shigeru; Fujita, Natsuko; Yokoyama, Tatsunori; Ogita, Yasuhiro; Fukuda, Shoma; Nakajima, Toru; Kagami, Saya; Ogata, Manabu; et al.

JAEA-Review 2023-017, 27 Pages, 2023/10

JAEA-Review-2023-017.pdf:0.94MB

This report is a plan of research and development (R&D) on geosphere stability for long-term isolation of high-level radioactive waste (HLW) in Japan Atomic Energy Agency (JAEA), in fiscal year 2023. The objectives and contents in fiscal year 2023 are described in detail based on the JAEA 4th Medium- and Long-term Plan (fiscal years 2022-2028). In addition, the background of this research is described from the necessity and the significance for site investigation and safety assessment, and the past progress. The plan framework is structured into the following categories: (1) Development and systematization of investigation techniques, (2) Development of models for long-term estimation and effective assessment, (3) Development of dating techniques.

Stress tensor inversion method for minor faults in areas of unclear active fault topography; Example in area of around source of the 1984 Western Nagano Earthquake

Nishiyama, Nariaki; Nakajima, Toru; Goto, Akira*; Hakoiwa, Hiroaki; Sueoka, Shigeru; Shimada, Koji; Niwa, Masakazu

no journal, , 

For some earthquakes of magnitude 6 to 7, the surface earthquake fault is unclear, and it is necessary to establish an evaluation method to understand the active faults that cause such earthquakes. In this study, in the source area of the 1984 western Nagano Prefecture earthquake, where no distinct surface earthquake faults appeared, we estimated the stress field that may have affected the activity of small faults by stress tensor inversion method using striation data of small faults collected by surface exploration, and extracted areas with many small faults consistent with the regional stress field. As a result of the surface survey, we collected data on 321 small faults. Considering the location and number of these small fault data, the study area was divided into 13 areas. As a result of stress tensor inversion method in each area, stress with a maximum compressive stress axis in the NW-SE direction, which is harmonic to the regional stress field in this area, was detected in the area near the upper end of the hidden fault. In these regions, there are relatively many small faults that were displaced by the activity of the hidden fault, and their distribution area is expected to correspond to the damage zone of the hidden fault or a deformation zone that extends over a wider area. Therefore, even when no clear fault displacement topography is recognized, we expect to be able to estimate its distribution and extent of influence by surface exploration. This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan as part of its R&D supporting program titled "Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (JPJ007597) (Fiscal Years 2020-2022)".

Application of multiple inverse method to minor faults around the concealed active fault; An Example in the 1984 western Nagano earthquake region

Nishiyama, Nariaki; Nakajima, Toru; Goto, Akira*; Hakoiwa, Hiroaki; Nagata, Mitsuhiro; Shimada, Koji; Niwa, Masakazu

no journal, , 

Earthquakes of magnitude 6-7 class may occur in areas where active faults have not been detected. To reduce earthquake damage, it is important to understand the concealed active fault that generates such earthquakes, but there is little evidence to provide clues to this understanding. In this study, we collected data on striations observed in fractures by geological survey around the source area of the 1984 Western Nagano Earthquake, where the existence of a concealed active fault is known and conducted multiple inverse method. As a result, stress fields similar to the present stress field were detected in minor faults around the known concealed active fault. Some of the minor faults were recognized in Quaternary volcanic rocks, confirming that the minor faults were very recently active. This indicates that these minor faults may be part of the damage zone that develops around the concealed active fault and is expected to provide a clue to understanding a concealed active fault.