龙门山北端青川断层附近应力测量与断层稳定性
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摘要
汶川8.0级地震后,余震是否沿青川断裂向汉中方向发展,是否还会发生强烈余震成为社会关注的问题。为了查明汶川8.0级地震后青川断层库仑应力的变化,判明余震的发展趋势,2008年6月2日~28日我们在青川断裂中段阳平关一带开展了断层附近原地应力测量研究,在3个钻孔中分别用水压致裂法和解除法得到了断层上下盘构造应力的大小、方向和分布特征。根据实测的地应力资料,断层附近最大水平主压应力方向为NE-NEE向,与区域构造应力SN-NNE向主压应力方向有一些差异,说明受断层活动影响,断层附近的应力状态与区域应力场已经发生了改变。青川断层是一条逆冲兼右行走滑的活动断裂,在汶川大地震中虽未见其活动,但其阻隔了余震向北发展,表明该断层是一条重要的边界断层,既是区域构造分区的界线,也是区域应力场的分区标志线。利用地应力实测资料研究了断层的活动性,在测量深度域内水平和铅直应力的关系为σH>σh>σV,该应力状态有利于逆断层活动,与地质资料反映的以逆断层活动及近期在断层附近发生的地质异常现象一致。利用库仑摩擦滑动准则,摩擦强度取0.6~1.0进行分析,断层附近的现今地应力状态接近产生逆断层摩擦滑动的临界值,其中断裂东段发生逆断层活动的可能性较小,相比而言,青川断裂带西段危险性更大。
After the M8.0 Wenchuan earthquake,it becomes an important concern that whether aftershocks direct to Hanzhong along the Qingchuan fault and a strong aftershock also occur.The study on in situ stress measurements was carried out at Ningqiang region in the northeastern section of the fault in order to master the changes of the coulomb stress in the Qingchuan fault after the earthquake and to identify the development trend of aftershocks,using the hydraulic fracturing in situ stress measurement techniques on June 2 to 28,2008.The size,orientation and distribution of the tectonic stress in the fault were obtained respectively by the hydraulic fracturing and stress relieving in three boreholes.According to the measured stress data,the direction of the maximum level principal stress is NE-NEE near the fault,and does not accord with the SN-NNE direction of the regional tectonic stress.It is shown that the stress state near the fault changes obviously compared to the regional stress field,affected by the impact of fault activity.The relationship between horizontal and vertical stress is σ_H>σ_h>σ_V at the measurement depth.The study reveals that the stress state is conducive to the activities of reverse fault and is basically the same as the geological data.By using sliding friction Coulomb Guidelines and friction strength from 0.6 to 1.0,it can be seen that present stress state near the fault has reached or exceeded the sliding friction threshold of the thrust fault,indicating that present tectonic activity is mainly thrust movement and it is impossible for the strike-slip movement to occur in the measured area.The thrust fault activity is less likely in the eastern section of the fault but much more likely in western part of the Qingchuan fault.
After the M8.0 Wenchuan earthquake,it becomes an important concern that whether aftershocks direct to Hanzhong along the Qingchuan fault and a strong aftershock also occur.The study on in situ stress measurements was carried out at Ningqiang region in the northeastern section of the fault in order to master the changes of the coulomb stress in the Qingchuan fault after the earthquake and to identify the development trend of aftershocks,using the hydraulic fracturing in situ stress measurement techniques on June 2 to 28,2008.The size,orientation and distribution of the tectonic stress in the fault were obtained respectively by the hydraulic fracturing and stress relieving in three boreholes.According to the measured stress data,the direction of the maximum level principal stress is NE-NEE near the fault,and does not accord with the SN-NNE direction of the regional tectonic stress.It is shown that the stress state near the fault changes obviously compared to the regional stress field,affected by the impact of fault activity.The relationship between horizontal and vertical stress is σ_H>σ_h>σ_V at the measurement depth.The study reveals that the stress state is conducive to the activities of reverse fault and is basically the same as the geological data.By using sliding friction Coulomb Guidelines and friction strength from 0.6 to 1.0,it can be seen that present stress state near the fault has reached or exceeded the sliding friction threshold of the thrust fault,indicating that present tectonic activity is mainly thrust movement and it is impossible for the strike-slip movement to occur in the measured area.The thrust fault activity is less likely in the eastern section of the fault but much more likely in western part of the Qingchuan fault.
引文
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[3]罗志立,龙学明.龙门山造山带崛起和川西前陆盆地沉降[J].四川地质学报,1992,12(1):1~17.LUO Zhi-li,LONG Xue-ming.The uplifting of the Longmenshan orogenic zone and the subsidence of the western Sichuan forelandbasin[J].Acta Geologica Sichuan,1992,12(1):1~17.
[4]刘树根.龙门山冲断带与川西前陆盆地形成演化[M].成都:成都科技大学出版社,1993.17~117.LIU Shu-gen.The formation and evolution of Longmenshan thrust zone and western Sichuan,China[M].Chengdu:Press ofChengdu University of Science and Technology,1993.17~117.
[5]陈社发,邓起东,赵小麟,等.龙门山中段推覆构造带及相关构造的演化历史和变形机制(一)[J].地震地质,1994,16(4):404~412,421.CHEN She-fa,DENG Qi-dong,ZHAO Xiao-lin,et al.Deformational characteristics,evolutionary history,and deformationmechanism of the middle Longmenshan thrust-nappes and related tectonics(I)[J].Seismology and Geology,1994,16(4):404~412,421.
[6]陈社发,邓起东,赵小麟,等.龙门山中段推覆构造带及相关构造的演化历史和变形机制(二)[J].地震地质,1994,16(4):413~421.CHEN She-fa,DENG Qi-dong,ZHAO Xiao-lin,et al.Deformational characteristics,evolutionary history,and deformationmechanism of the middle Longmenshan thrust-nappes and related tectonics(II)[J].Seismology and Geology,1994,16(4):413~421.
[7]彭华,马秀敏,白嘉启,等.甘孜玉树断裂带第四纪活动特征[J].地质力学学报,2006,12(3):295~304.PENG Hua,MA Xiu-min,BAI Jia-qi,et al.Characteristics of Quaternary activities of the Garz-êYushu fault zone[J].Journalof Geomechanics,2006,12(3):295~304.
[8]彭华,马秀敏.南水北调西线工程区地震危险性分析及预测[J].地质力学学报,2007,13(1):15~24.PENG Hua,MA Xiu-min.Risk analysis and prediction of earthquakes in the construction area of the west line of the south to northwater diversion project[J].Journal of Geomechanics,2007,13(1):15~24.
[9]Tapponnier P,Peltzer G,Armijo R.On the mechanics of the collision between India and Asia[J].Geological Society,London,Special Publications,1986,19:113~157.
[10]李勇,周荣军,Densmore A L,等.青藏高原东缘龙门山晚新生代走滑-逆冲作用的地貌标志[J].第四纪研究,2006,26(1):40~51.LI Yong,ZHOU Rong-jun,Densmore A L,et al.Geomorphic evidence for the Late Cenozoic strike-slipping and thrusting inLongmen Mountain at the eastern margin of the Tibetan Plateau[J].Quaternary Sciences,2006,26(1):40~51.
[11]李勇,周荣军,Densmore A L,等.青藏高原东缘龙门山晚新生代走滑挤压作用的沉积响应[J].沉积学报,2006,24(2):153~164.LI Yong,ZHOU Rong-jun,Densmore A L,et al.Sedimentary responses to Late Cenozoic thrusting and strike-slipping ofLongmenshan along eastern margin of Tibetan Plateau[J].Acta Sedimentologica Sinica,2006,24(2):153~164.
[12]姚鑫,张永双.基于差分干涉雷达的汶川地震同震形变特点[J].地质力学学报,2009,15(2):151~161.YAO Xin,ZHANG Yong-shuang.Co-seismic deformation of 5.12 Wenchuan earthquake based on D-InSAR[J].Journal ofGeomechanics,2009,15(2):151~161.
[13]张永双,雷伟志,石菊松,等.四川5.12地震次生地质灾害的基本特征初析[J].地质力学学报,2008,14(2):109~116.ZHANG Yong-shuang,LEI Wei-zhi,SHI Ju-song,et al.General characteristics of 5.12 earthquake-induced geo-hazards inSichuan[J].Journal of Geomechanics,2008,14(2):109~116.
[14]张永双,石菊松,孙萍,等.汶川地震内外动力耦合及灾害实例[J].地质力学学报,2009,15(2):131~141.ZHANG Yong-shuang,SHI Ju-song,SUN Ping,et al.General manifestation of coupling processes between endogenic andexogenic geological processes in 5.12 Wenchuan earthquake[J].Journal of Geomechanics,2009,15(2):131~141.
[15]谭成轩,孙叶,吴树仁,等.5.12汶川MS8.0大地震后关于我国区域地壳稳定性评价的思考[J].地质力学学报,2009,15(2):142~150.TAN Cheng-xuan,SUN Ye,WU Shu-ren,et al.A consideration on regional crustal stability assessment after 5.12 WenchuanMS8.0 strong earthquake in China[J].Journal of Geomechanics,2009,15(2):142~150.
[16]王连捷,崔军文,周春景,等.汶川5.12地震发震机理的数值模拟[J].地质力学学报,2009,15(2):105~113.WANG Lian-jie,CUI Jun-wen,ZHOU Chun-jing,et al.Numerical modeling of Wenchuan 5.12 earthquake mechanism[J].Journal of Geomechanics,2009,15(2):105~113.
[17]王连捷,周春景,孙东生,等.汶川5.12地震引起的库仑应力变化及其对周边地震活动的影响[J].地质力学学报,2008,14(3):193~200.WANG Lian-jie,ZHOU Chun-jing,SUN Dong-sheng,et al.Coulomb stress changes caused by Wenchuan earthquake and itsinfluence on seismic activity in the adjacent area[J].Journal of Geomechanics,2008,14(3):193~200.
[18]邓起东,陈社发,赵小麟.龙门山及其邻区的构造和地震活动及动力学[J].地震地质,1994,16(4):389~403.DENG Qi-dong,CHEN She-fa,ZHAO Xiao-lin.Tectonics,seismicity and dynamics of Longmenshan Mountains and its adjacentregions[J].Seismology and Geology,1994,16(4):389~403.
[19]王全伟,梁斌,谢启兴,等.青川地区青川断裂带的显微构造及其变形条件研究[J].矿物岩石,2000,20(1):87~90.WANG Quan-wei,LIANG Bin,XIE Qi-xing,et al.Research on micro-structures and deformation conditions of the Qingchuanfault zone[J].Journal of Mineralogy and Petrology,2000,20(1):87~90.
[20]樊春,王二七,王刚,等.龙门山断裂带北段晚新近纪以来的右行走滑运动及其构造变换研究——以青川断裂为例[J].地质科学,2008,43(3):417~433.FAN Chun,WANG Er-qi,WANG Gang,et al.Dextral strike-slip and tectonic transformation of the northern Longmenshan faultbelt from Late Neocene:A case study from the Qingchuan fault[J].Chinese Journal of Geology,2008,43(3):417~433.
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