汶川M_s8.0级地震发生背景与过程的研究
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摘要
本文首先阐明汶川M_S8.0级地震发生在由区域布格重力异常和地震震中分布所确定的武都——松潘—茂汶—汶川—沪定地震带上.汶川地震所在地段是地震前兆和中小地震(M≤7.0)的空白区,震前出现明显的孕震空区,M_S8.0级地震发生在空区周围区域中小地震活动峰值之后的减少段里.地震的破裂超出孕震空区范围,空区内、外余震活动呈现出不同的衰减特征,依此将余震活动分为WS和NE两个区段.地震破裂过程、4级以上余震矩张量及震区应力场反演和余震应力降的测定结果表明.两个区域的位错、余震机制解和应力降及最大主应力的方向等明显有别.根据这些特征和地震应力触发的研究,推测NE段地震的发生可能是由WS段主破裂的发生所触发.
In this paper, we firstly demonstrated that the M_S 8. 0 earthquake occurred on the Wudu-Songpan-Maowen-Wenchuan-Luding seismic zone,which is delineated by the regional gravity anomalies and seismic activity. The main ruptured region of the earthquake was located on a segment that lacked strong earthquakes during the active seismic period, i. e. seismic gap. The seismic gap is very remarkable before the M_S 8. 0 event and the main event occurred in the period of decreased seismicity outside the gap area. The rupture process extended towards NE direction and exceeded the seismic gap region. Seismicity in and out the gap show different active features, therefore the aftershock seismicity is divided into NE and WS segments by the author. Researches on source rupture process, moment tensor inversion of M≥4.0 aftershocks, stress field inversion, as well as the stress drops of aftershocks reveal that there exists significant difference between the two segments in slip distribution on the fault, focal mechanisms of aftershocks, stress drops, as well as direction of the tress field. According to those characteristics and study on the stress triggering, it is suggested that the seismicity on the NE segment may be triggered by the main rupture starting on WS segment.
In this paper, we firstly demonstrated that the M_S 8. 0 earthquake occurred on the Wudu-Songpan-Maowen-Wenchuan-Luding seismic zone,which is delineated by the regional gravity anomalies and seismic activity. The main ruptured region of the earthquake was located on a segment that lacked strong earthquakes during the active seismic period, i. e. seismic gap. The seismic gap is very remarkable before the M_S 8. 0 event and the main event occurred in the period of decreased seismicity outside the gap area. The rupture process extended towards NE direction and exceeded the seismic gap region. Seismicity in and out the gap show different active features, therefore the aftershock seismicity is divided into NE and WS segments by the author. Researches on source rupture process, moment tensor inversion of M≥4.0 aftershocks, stress field inversion, as well as the stress drops of aftershocks reveal that there exists significant difference between the two segments in slip distribution on the fault, focal mechanisms of aftershocks, stress drops, as well as direction of the tress field. According to those characteristics and study on the stress triggering, it is suggested that the seismicity on the NE segment may be triggered by the main rupture starting on WS segment.
引文
[1] 唐荣昌,文德华,黄祖智等.松潘龙门山地区地震活动断裂带第四纪活动特征.中国地震,1991,7(3) :64-71 Tang R C, Wen D H, Huang Z Z, et al. The quaternary activity characteristics of several major active faults in the Songpan-Longmenshan region. Earthquake Reaserach in China (in Chinese), 1991, 7(3) : 64-71
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[7] 张培震,徐锡伟,闻学泽等.2008年汶川8． 0级地震发震断裂的滑动速率、复发周期和构造成因.地球物理学报,2008,5(4) :1066-1073 Zhang P Z,Xu X W,Wen X Z,et al. Slip rates and recurrence intervals of the Longmen Shan active fault zone, and tectonic implications for the mechanism of the May 12 Wenchuan earthquake, 2008, Sichuan, China. Chinese J. Geophys. (in Chinese), 2008, 51(4) :1066-1073
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[9] 梅世蓉.中国的地震活动性.地球物理学报,1960,9(1) :15-19 Mei S R. Seismicity in China, Chinese J, Geophys. (in Chinese), 1960,9(1) :15-19
[10] 梅世蓉.从华北地震活动的规则性看地震危险区划分的一个途径.学术讨论专集.地震战线,1970． 1-11 Mei S R. One way to delineate the earthquake risking area from the regular seismic activity in north-eastern China. Earthquake Science (in Chinese) , 1970． 1-11
[11] 赵翠萍,陈章立,周连庆等.汶川M_w8． 0级地震震源破裂过程研究分段特征.科学通报(已投稿) Zhao C P, Chen Z L, Zhou L Q, et al. Research on the rupture process of Mw8． 0 Wenchuan earthquake-the segmentation feature. Submitted to Chinese Science Bulletin
[12] 王勤彩,陈章立,郑斯华.汶川地震余震序列震源机制与应力方向的空间分段特征.科学通报(已接收) Wang Q C, Chen Z L, Zheng S H. The segmentation characteristics of focal mechanism and the stress field of the Wenchuan 8． 0 earthquake sequence. Chinese Science Bulletin (in press)
[13] Wang C Y, Herrman R B. A numerical study of P, SV, SHwave generation in a plane layered medium. Bull. Seism. Soc. Am. ,1980, 70:1015-1036
[14] Herrman R B, Wang C Y. A comparison of synthetic seismogram. Bull. Seism. Soc. Am. , 1985, 75:41-56
[15] Gephart J W, Forsyth D W. An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to San Fernando earthquake. J.Geophys. Res. , 1984, 89:9305-9320
[16] Gephart J W. Stress and the direction of slip on fault planes. Tectonics, 1994 , (4) :845-858
[17] 华卫,陈章立,郑斯华.2008年汶川8． 0级地震序列震源参数分段特征的研究.地球物理学报,2009,52(2) :365-371 Hua W, Chen Z L, Zheng S H. A study on segmentation characteristics of aftershock source parameters of Wenchuan M8． 0 earthquake in 2008． Chinese J. Geophys. (in Chinese), 2009,52(2) :365-371
[18] 华卫,陈章立,李志雄等.汶川8． 0级地震触发与余震活动空间分布研究.地震,2009,待发表 Hua W, Chen Z L, Li Z X, et al. Study on the seismic triggering and the distribution of aftershocks for Wenchuan M8． 0． Earthquake, 2009,(in press)
[19] Robinson R,Mcginty P J. The enigma of the Authur's Pass, New Zealand, earthquake 2． The aftershock distribution and its relation to regional and induce stress field. J. Geophys. Res. , 2000, 105:16139-16150
[20] Reasenberg P A, Simpson R W. Response of regional seismicity to the static stress change produced by the Loma Prieta earthquake. Science, 1992, 255:1687-1690
[21] King G C P, Stein R S, Lin J. Static stress changes and the triggering of earthquakes. Bull. Seism. Soc. Am. , 1994, 84:935-953
[22] HarrisR A, Introduction to special section:Stress triggering, Stress shadow, and implications for seismic hazard. J. Geophys. Res. , 1998, 103:24347-24358
[23] Stein R S. The role of stress transfer in earthquake occurrence. Nature, 1999, 402:605-609
[24] 华卫,刘杰,郑斯华等.2003年云南大姚6． 2级、6． 1级地震序列特征分析及地震触发研究.中国地震,2006,(1) :10-23 Hua W, Liu J, Zheng S H, et al. Earthquake sequence analysis of Dayao M_s6． 2 and M_s6． 1 events, 2003, and earthquake trigging analysis. Chinese Earthquake Research (in Chinese), 2006, (1) : 10-23
[2] 邓起东,陈社发,赵小麟等.龙门山及其邻近的构造和地震活动性及动力学.地震地质,1994,16(4) :389-402 Deng Q D, Chen S F, Zhao X L, et al. Tectonics, seismisity and dynamics of Longmenshan mountains and its adjacent regions. Seismology and Geology (in Chinese),1994,16 (4) :389-403
[3] 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).地震地质,1994,16(4) :404-412 Chen S F, Deng Q D, Zhao X L, et al. Deformational characteristics, evolutionary history, and deformation mechanism of the middle Longmenshan thrust-nappes and related tectonics. Seismology and Geology (in Chinese), 1994, 16(4) : 404-412
[4] 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).地震地质,1994,16(4) :413-421 Chen S F, Deng Q D, Zhao X L, et al. Deformational characteristics, evolutionary history, and deformation mechanism of the middle Longmenshan thrust-nappes and related tectonics. Seismology and Geology (in Chinese), 1994 ,16(4) :413-421
[5] 马起保,苏刚,侯治华等.利用岷江阶地的变形估计龙门山断裂带中段晚第四纪滑动速率.地震地质,2005,27(2) :234-242 Ma B Q, Su G, Hou Z H, et al. Late Quaternary slip rate in the central part of the Longmenshan fault zone from terrace deformation along the Minjiang river. Seismology and Geology (in Chinese) ,2005,27(2) :234-242
[6] 陈国光,计凤桔,周荣军等.龙门山断裂带晚第四纪活动性分段的初步研究.地震地质,2007,29(3) :657-673 Chen G G ,Ji F J ,Zhou R J ,et al. Primary research of activity segmentation of Longmenshan fault zone since Late-Quaternary. Seismology and Geology (in Chinese), 2007,29 (3) :657-673
[7] 张培震,徐锡伟,闻学泽等.2008年汶川8． 0级地震发震断裂的滑动速率、复发周期和构造成因.地球物理学报,2008,5(4) :1066-1073 Zhang P Z,Xu X W,Wen X Z,et al. Slip rates and recurrence intervals of the Longmen Shan active fault zone, and tectonic implications for the mechanism of the May 12 Wenchuan earthquake, 2008, Sichuan, China. Chinese J. Geophys. (in Chinese), 2008, 51(4) :1066-1073
[8] 陈章立.浅论地震预报地震学方法基础.北京:地震出版社,2004 Chen Z L. Introduction to the Foundation of Seismological Method in Earthquake Prediction (in Chinese). Beijing: Seismological Press, 2004
[9] 梅世蓉.中国的地震活动性.地球物理学报,1960,9(1) :15-19 Mei S R. Seismicity in China, Chinese J, Geophys. (in Chinese), 1960,9(1) :15-19
[10] 梅世蓉.从华北地震活动的规则性看地震危险区划分的一个途径.学术讨论专集.地震战线,1970． 1-11 Mei S R. One way to delineate the earthquake risking area from the regular seismic activity in north-eastern China. Earthquake Science (in Chinese) , 1970． 1-11
[11] 赵翠萍,陈章立,周连庆等.汶川M_w8． 0级地震震源破裂过程研究分段特征.科学通报(已投稿) Zhao C P, Chen Z L, Zhou L Q, et al. Research on the rupture process of Mw8． 0 Wenchuan earthquake-the segmentation feature. Submitted to Chinese Science Bulletin
[12] 王勤彩,陈章立,郑斯华.汶川地震余震序列震源机制与应力方向的空间分段特征.科学通报(已接收) Wang Q C, Chen Z L, Zheng S H. The segmentation characteristics of focal mechanism and the stress field of the Wenchuan 8． 0 earthquake sequence. Chinese Science Bulletin (in press)
[13] Wang C Y, Herrman R B. A numerical study of P, SV, SHwave generation in a plane layered medium. Bull. Seism. Soc. Am. ,1980, 70:1015-1036
[14] Herrman R B, Wang C Y. A comparison of synthetic seismogram. Bull. Seism. Soc. Am. , 1985, 75:41-56
[15] Gephart J W, Forsyth D W. An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to San Fernando earthquake. J.Geophys. Res. , 1984, 89:9305-9320
[16] Gephart J W. Stress and the direction of slip on fault planes. Tectonics, 1994 , (4) :845-858
[17] 华卫,陈章立,郑斯华.2008年汶川8． 0级地震序列震源参数分段特征的研究.地球物理学报,2009,52(2) :365-371 Hua W, Chen Z L, Zheng S H. A study on segmentation characteristics of aftershock source parameters of Wenchuan M8． 0 earthquake in 2008． Chinese J. Geophys. (in Chinese), 2009,52(2) :365-371
[18] 华卫,陈章立,李志雄等.汶川8． 0级地震触发与余震活动空间分布研究.地震,2009,待发表 Hua W, Chen Z L, Li Z X, et al. Study on the seismic triggering and the distribution of aftershocks for Wenchuan M8． 0． Earthquake, 2009,(in press)
[19] Robinson R,Mcginty P J. The enigma of the Authur's Pass, New Zealand, earthquake 2． The aftershock distribution and its relation to regional and induce stress field. J. Geophys. Res. , 2000, 105:16139-16150
[20] Reasenberg P A, Simpson R W. Response of regional seismicity to the static stress change produced by the Loma Prieta earthquake. Science, 1992, 255:1687-1690
[21] King G C P, Stein R S, Lin J. Static stress changes and the triggering of earthquakes. Bull. Seism. Soc. Am. , 1994, 84:935-953
[22] HarrisR A, Introduction to special section:Stress triggering, Stress shadow, and implications for seismic hazard. J. Geophys. Res. , 1998, 103:24347-24358
[23] Stein R S. The role of stress transfer in earthquake occurrence. Nature, 1999, 402:605-609
[24] 华卫,刘杰,郑斯华等.2003年云南大姚6． 2级、6． 1级地震序列特征分析及地震触发研究.中国地震,2006,(1) :10-23 Hua W, Liu J, Zheng S H, et al. Earthquake sequence analysis of Dayao M_s6． 2 and M_s6． 1 events, 2003, and earthquake trigging analysis. Chinese Earthquake Research (in Chinese), 2006, (1) : 10-23
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