THE VITRINITE REFLECTANCE OF FAULT ROCKS FROM THE WENCHUAN EARTHQUAKE FAULT ZONE: CONSTRAINTS ON FRICTIONAL PROPERTIES OF THE FAULT DURING THE EARTHQUAKE

doi:10.3969/j.issn.0253-4967.2016.04.001

Abstract

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.

Key words: the Wenchuan earthquake fault zone, fault rocks, vitrinite reflectance, dynamic frictional strength, frictional heating

摘要：

地震断层滑移过程中的摩擦升温能够指示断层的动态摩擦性质，为我们认识地震的动力学过程及能量分配提供一种途径；断层带摩擦生热的残留标志也能作为识别地震活动的依据。镜质体反射率是煤和石油天然气领域广泛使用的地质温度计，在断层岩和断层力学研究中也具有潜在的应用价值。文中对采自汶川地震断裂带的多种断层岩样品开展了镜质体反射率研究，揭示了断层活动对断层岩镜质体反射率的影响；从断层角砾带向断层核部，镜质体反射率呈现出增大的趋势，指示着地震过程中断层的摩擦生热过程。八角庙露头样品的镜质体反射率明显高于深溪沟露头，可能暗示着2个露头上断层活动性质的差异。研究还表明，对断层剖面的镜质体反射率开展系统性研究有助于识别以往地震的滑动带以及限定滑动带的宽度。对深溪沟露头上断层滑动面样品开展的测试表明，2条宽度约2mm的局部黑色滑动带内存在镜质体反射率的异常高值；数值计算表明汶川地震过程中该露头附近断层滑动面上的宏观摩擦升温可能<162℃，同震阶段的动态摩擦系数较低，这些估计与高速摩擦实验的结果总体上是一致的。

关键词: 汶川地震断裂带, 断层岩, 镜质体反射率, 动态摩擦强度, 摩擦升温

CLC Number:

P315.2

YAO Lu, MA Sheng-li, WANG Yu, HE Hong-lin, CHEN Jian-ye, YANG Xiao-song, SHIMAMOTO Toshihiko. THE VITRINITE REFLECTANCE OF FAULT ROCKS FROM THE WENCHUAN EARTHQUAKE FAULT ZONE: CONSTRAINTS ON FRICTIONAL PROPERTIES OF THE FAULT DURING THE EARTHQUAKE[J]. SEISMOLOGY AND GEOLOGY, 2016, 38(4): 817-829.

姚路, 马胜利, 王羽, 何宏林, 陈建业, 杨晓松, 嶋本利彦. 汶川地震断层岩的镜质体反射率——对断层同震摩擦滑动性质的约束[J]. 地震地质, 2016, 38(4): 817-829.

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