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High-velocity frictional behavior of Longmenshan fault gouge from Hongkou outcrop and its implications for dynamic weakening of fault during the 2008 Wenchuan earthquake

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Earthquake Science 2011年第3期24卷 267-281页

作者： Tetsuhiro Togo Toshihiko Shimamoto Shengli Ma Takehiro Hirose Department of Earth and Planetary Systems Science Graduate School of Science Hiroshima University Higashi-Hiroshima 739-8526 Japan State Key Laboratory of Earthquake Dynamics Institute of Geology China Earthquake Administration Beijing 100029 China Kochi Institute for Core Sample Research Japan Agency for Marine-Earth Science and Technology ( JAMSTEC) 200 Monobe-otsu Kochi 783-8502 Japan

High-velocity friction experiments were conducted on clayey fault gouge collected from Hongkou outcrop of beichuan fault, located at the southwestern part of Longmenshan fault system that caused the disastrous 2008 Wenchuan earthquake. The ultimate purpose of this study is to reproduce this earthquake by modeling based on measured frictional properties. Dry gouge of about 1 mm in thickness was deformed dry at slip rates of 0.01 to 1.3 m/s and at normal stresses of 0.61 to 3.04 MPa, using a rotary-shear high-velocity frictional testing machine. The gouge displays slip weakening behavior as initial peak friction decays towards steady-state values after a given displacement. Both peak friction and steady-state friction remain high at slow slip rates are exam- ined and gouge only exhibits dramatic weakening at high slip rates, with steady-state friction coefficient values of about 0.1 to 0.2. Specific fracture energy ranges from 1 to 4 MN/m in our results and this is of the same order as seismically determined values. Low friction coefficients measured on experimental faults are in broad agree- ment with lack of thermal anomaly observed from temperature measurements in WFSD-1 drill hole （Wenchuan Earthquake fault Scientific Drilling Project）, which can be explained by even smaller friction coefficient for the Wenchuan earthquake fault. High-velocity friction experiments with pore water needs to be done to see if even smaller friction is attained or not. Shiny slickenside surfaces form at high slip rates, but not at slow slip rates. Slip zone with slickenside surface changes its color to dark brown and forms duplex-like microstructures, which are similar to those microstructures found in the fault gouges from the Hongkou outcrop. Detailed comparisons between experimentally deformed gouge samples and WFSD drill cores in the future will reveal how much we could reproduce the dynamic weakening processes in operation in fault zones during Wenchuan earthquake at present.

关键词： Wenchuan earthquake high-velocity friction fault mechanics Longmenshan fault system beichuan fault

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Internal structure of Longmenshan fault zone at Hongkou outcrop,Sichuan,China,that caused the 2008 Wenchuan earthquake

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Earthquake Science 2011年第3期24卷 249-265页

作者： Tetsuhiro Togo Toshihiko Shimamoto Shengli Ma Xueze Wen Honglin He Department of Earth and Planetary Systems Science Graduate School of Science Hiroshima University Higashi-Hiroshima 739-8526 Japan State Key Laboratory of Earthquake Dynamics Institute of Geology China Earthquake Administration Beijing 100029 China Earthquake Administration of Sichuan Province Chengdu 610041 China

This paper reports the internal structures of the beichuan fault zone of Longmenshan fault system that caused the 2008 Wenchuan earthquake, at an outcrop in Hongkou, Sichuan province, China. Present work is a part of comprehensive project of Institute of Geology, China Earthquake Administration, trying to understand deformation processes in Longmenshan fault zones and eventually to reproduce Wenchuan earthquake by modeling based on measured mechanical and transport properties. Outcrop studies could be integrated with those performed on samples recovered from fault zone drilling, during the Wenchuan Earthquake fault Scientific Drilling （WFSD） Project, to understand along-fault and depth variation of fault zone properties. The hanging wall side of the fault zone consists of weakly-foliated, clayey fault gouge of about 1 m in width and of several fault breccia zones of 30-40 m in total width. We could not find any pseudotachylite at this outcrop. Displacement during the Wenchuan earthquake is highly localized within the fault gouge layer along narrower slipping-zones of about 10 to 20 mm in width. This is an important constraint for analyzing thermal pressurization, an important dynamic weakening mechanism of faults. Overlapping patterns of striations on slickenside surface suggest that seismic slip at a given time occurred in even narrower zone of a few to several millimeters, so that localization of deformation must have occurred within a slipping zone during coseismic fault motion. fault breccia zones are bounded by thin black gouge layers containing amorphous carbon. fault gouge contains illite and chlorite minerals, but not smectite. Clayey fault gouge next to coseismic slipping zone also contains amorphous carbon and small amounts of graphite. The structural observations and mineralogical data obtained from outcrop exposures of the fault zone of the Wenchuan earthquake can be compared with those obtained from the WFSD-1 and WFSD-2 boreholes, which have been drill

关键词： Wenchuan earthquake Longmenshan fault system beichuan fault fault rock fault mechanics fault-zone structure amorphous carbon graphite

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