THE VITRINITE REFLECTANCE OF FAULT ROCKS FROM THE WENCHUAN EARTHQUAKE FAULT ZONE: CONSTRAINTS ON FRICTIONAL PROPERTIES OF THE FAULT DURING THE EARTHQUAKE |
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| Authors: | YAO Lu MA Sheng-li WANG Yu HE Hong-lin CHEN Jian-ye YANG Xiao-song SHIMAMOTO Toshihiko |
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| Institution: | 1. State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China;
2. Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China;
3. Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing 100029, China |
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| Abstract: | The temperature rise caused by frictional heating during seismic slip is able to indicate dynamic frictional properties of the seismic fault,which provides an approach to understand the dynamic process and energy budget of an earthquake.The residual indicators of frictional heating within the fault zone also can be taken as an evidence for seismic events.The vitrinite reflectance is a commonly-used geothermometer in the coal,oil and gas industries.It also has some potential applications in the studies of fault rock and fault mechanics.We studied vitrinite reflectance (VR) of fault rocks collected from surface outcrops of the Wenchuan earthquake fault zone in this paper.The measured data reveal that the VR of fault rocks are affected by fault motion,and there is a trend that the VR increases towards the fault core,which indicates the effects of frictional heating.The VR of fault rocks from the Bajiaomiao outcrop is much higher than those from the Shenxigou outcrop,which probably suggests the difference in fault activity at the two outcrops.Our study also suggests that systematic measurement of VR across the fault zone is helpful in identifying slip zones and determining their widths.From the VR measurement on an oriented specimen containing the slip surface of the Wenchuan earthquake from the Shenxigou outcrop,we observed anomalous high VR values in two black-colored slip zones of about 2mm in width near the slip surface.The numerical calculation shows that the maximum temperature rise on the fault plane near Shenxigou was probably less than 162℃ during the Wenchuan earthquake,which means the dynamic fault strength was quite low.These estimations are roughly in accord with the results from the high-velocity frictional experiments. |
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| Keywords: | the Wenchuan earthquake fault zone fault rocks vitrinite reflectance dynamic frictional strength frictional heating |
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