2005 Fiscal Year Final Research Report Summary
Direct measurements of frictional heating by earthquake faulting
| Project/Area Number |
15340143
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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 | 一般 |
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| Research Field |
Solid earth and planetary physics
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| Research Institution | The University of Tokyo |
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Principal Investigator |
NAKATANI Masao The Univ. of Tokyo, Earthquake Research Institute, Research Associate, 地震研究所, 助手 (90345174)
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| Co-Investigator(Kenkyū-buntansha) |
IIO Yoshihisa Kyoto University, Disaster Prevention Res. Inst., Associate Professor, 防災研究所, 助教授 (50159547)
OGASAWARA Hiroshi Ritsumeikan Univ., Faculty of Science and Engineering, Professor, 理工学部, 教授 (40213996)
SANO Osamu The University of Tokyo, Earthquake Research Institute, Professor, 地震研究所, 教授 (20127765)
YAMAGUCHI Tsuneo Nagoya University, Grad. Sch. of Environmental Sci., Associate Professor, 環境科学研究科, 助教授 (80022713)
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| Project Period (FY) |
2003 – 2005
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| Keywords | fault / mine / earthquke / friction / thermometer / strength / slip / heating |
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| Research Abstract |
An array of precision thermometers has been deployed very close to the fault of impending earthquakes. The site was at a 3 km depth in a deep gold mine in South Africa, where mining is taking place near a major geologic fault. We drilled many holes of about a 30 m length to find out a three dimensional structure of complex fault zone of over a 20 m thickness. Extensive analysis of core and borehole video images was carried out. The reuslt has shown that there is a distinct plane of weakness ; only of an about 10 cm thickness part at one on the boundary between the host rock and fault zone is severely damaged. This feature was continuous and quite planar over the whole area (10 x 10 m) surveyed with the intersecting boreholes. We could install many thermometers in a 1 m distance from this plane. The fault zone consisted of the lithified attrition material deriving from the host rocks. However, the plane-like structures were very limited. Besides the boundary weak plane mentioned above,
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there were only a few planes showing a foliated structures indicateve of localized shear. However, it was only the boundary part mentioend above that was so friable that all the boreholes intersections on this plane has a ring of lost material from the borehole wall. Such a damage was observed only at the intersection with this boundary in each borehole. The temperature data were continuously recorded, which showed a good stability of the ground temperature. The quality of data was good enough to resolve the frictional heating by a sliding at as low as one tenth of the frictional resistance measured in laboratory experiments, if an earthquake of M2-3, which is expected in this mine, occurred on the plane of weakness we identified above. This means that the resolution of our measurements covers the possible range of strength argued in the so-called crustal stress paradox. The present observation watches the temporal changes of rock temperature around the fault, which is caused by the frictional heating of the fault plane. This requires data for about a month following the eartquake. The observation has to be done very close to the fault, which is likely to suffer the damage from the targeted earthquake. It is quite possible that access to the observational instruments is lost by the earthquake damage. Therefore, number of measures, including the use of wireless data transfer, were taken to fail-safe the observational system, in terms of data recording, telemetry, and power supply. Less
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