Fault-rock analysis of spatial fixed initial break point, the case study in landslide area

2005 Fiscal Year Final Research Report Summary

• Project/Area Number: 15340173
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Geology
• Research Institution: Independent Administrative Institution, Japan Agency for Marine-Earth Sciece and Technology
• Principal Investigator: SAKAGUCHI Ario IFREE/JAMSTEC, Researcher, 地球内部変動研究センター, 研究員 (80304666)
• Co-Investigator(Kenkyū-buntansha): UJIIE Kohtaro IFREE/JAMSTEC, Researcher, 地球内部変動研究センター, 研究員 (40359188) ; KATO Aitaro Univ.Tokyo, Earthquake Research center, Assistant, 地震研究所, 助手 (20359201) ; YOKOYAMA Shunji Kochi Univ., Natural environmental Science, Professor, 理学部・自然環境科学科, 教授 (20325400) ; HASHIMOTO Yoshitaka Kochi Univ., Natural environmental Science, Assistant, 理学部・自然環境科学科, 助手 (40346698) ; SUEMUNE Akira Kyoto Univ., Disaster Prevention Research Institute, Assistant Professor, 防災研究所, 助教授 (00109092)
• Project Period (FY): 2003 – 2005
• Keywords: Fault-rock / initial break point / fluid / landslide

Research Abstract

The nucleation of rupture at a specific site has bee observed during rock frictional testing in the laboratory, and a spatially fixed initial break point has been recognized for large earthquakes by seismic observation, but the cause of this spatial fixation is still uncertain. A landslide is a midscale rock-friction event, in between a laboratory compressional experiment on a small test piece and a seismogenic fault. The landslide area studied is a pore-fluid-induced normal fault system that udergoes episodic slip during rainfall events. we monitored dynamic processes of his fault system in detain during slip and analyzed the fault rock at the initial break point immediately after slip events. Dilation and slip were initiated in an area with a high groundwater level, and the rupture propagated into the surrounding. A similar process was recognized repeatedly in 2003 and 2004, and the rupture initiated in the same area both years. The frictional heterogeneity seems to be spatially fixed and may depend on the hydrologic structure. At the initial break point, a high water content was found in a fracture zone under a low-permeable gouge layer. The fracture zone may act as a conduit for fluid, and the gouge, which originated from shear grinding, may have sealed in the pore fluid, causing high fluid pressure. Historical fault-rock development and the consequent hydrologic structure formed may reach a stationary state with maturation of the slip system, causing the initial break point of repeated slip events to be in the same area.