2004 Fiscal Year Final Research Report Summary
Direct, underground observation of hypocenters
| Project/Area Number |
14403005
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 海外学術 |
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| Research Field |
固体地球物理学
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| Research Institution | Ritsumeikan University |
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Principal Investigator |
OGASAWARA Hiroshi Ritsumeikan Univ., Faculty of Science and Engineering, Professor, 理工学部, 教授 (40213996)
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| Co-Investigator(Kenkyū-buntansha) |
ISHII Hiroshi Tono Res. Inst. Earth Science, Vice President, 東濃地震科学研究所, 副主席・主任研究員
SATOH Takashi Advanced Institute of Science and Technology, Research Group Leader, 地球科学情報研究部門, 研究グループ長
KAWAKATA Hironori Kyoto Univ., Disaster Prevention Research Institute, Assistant professor, 防災研究所, 助手 (80346056)
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| Project Period (FY) |
2002 – 2004
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| Keywords | South Africa / Gold mine / Hypocenter / Fault / Stress / Rock Mechanics / Earthquake / Fault rock |
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| Research Abstract |
As it is very hard to reach the hypocenters of natural earthquakes, direct information on the environment and property of hypocenters is very valuable. In South African gold mines, seismic faults of earthquakes (2<M<5) are often well exhumed at tunnels and stopes intersecting the faults. So, it is possible to directly investigate the environment and property of hypocenters. We planned to investigate the fault underground and carry out stress measurements at the proximity of the fault of the events (2<M<5).
In a 3-year period from 2002 to 2004, the hypocenters of events with 2<M<5 were investigaed underground at five gold mines in South Africa. At a hypocentral area 〜3 km deep, actually with M2 events during our investigation period, we observed rock mass behaviour a few times a week for couples of months. The rock mass quickly yielded within couples of days once it started yielding. Although we attempted an overcoring stress measurement, the associated serious borehole breakout and tunnel deformation did not allow us to make the overcoring. We also attemped the ASR and DSA stress measurement with the drilled core. Little ASR was observed and the stress-strain profile was almost straight. This suggested that the intact rock mass was almost elastic.
Fault rock was investigated at other two mines where two M4 events caused the fault slip of several cm. However, no evidence of melting was found. It is also confirmed that the fault rock that the mine geological enginners and rock engineers refer to as "pseudotachilyte" was the solidified cataclasite.
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