鲜水河断裂带炉霍段的震后滑动与形变
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摘要
1973年2月在鲜水河断裂带炉霍段发生了M7.6地震破裂.自那以来,先后在炉霍县虾拉沱布设了若干横跨该地震断层(1973年破裂带)的地壳形变观测系统,包括断层近场的短基线、短水准、蠕变仪、人工构筑物等,以及断层近-远场的GPS观测站.利用这些观测系统的长期观测资料,本文分析了鲜水河断裂带炉霍段的震后滑动/变形及其时、空变化特征,并建立起解释这些特征的动力学模式.研究表明:(1)1973年地震后的头5年,地震断层在虾拉沱场地表现为开放性质,近场的断层震后滑动以无震左旋蠕滑为主,速率达到10.27 mm/a,且伴有微量的拉张性蠕动作用;1979年以来,左旋蠕滑速率由5.3 mm/a逐渐减小到2.27 mm/a,减小的过程呈对数函数型,反映此阶段断层面已逐渐重新耦合、正朝闭锁的方向发展,并伴有部分应变积累.(2)1999年以来,地震断层两侧远场的相对左旋位移/变形速率为10 mm/a,远大于同时期断层近场(跨距40～144 m)的左旋蠕滑速率0.66～2.52 mm/a;远-近场位移/形变速率的显著变化发生在地震断层两侧各宽约30 km的范围,显示出这是与大地震应力应变积累释放相关的断裂带宽度.(3)结合动力学背景与深部构造信息,本文对这里断层的震后位移/变形及其时、空变化的机理进行初步解释,要点是:震后约5年之后,由于逐渐增大的断层滑动/摩擦阻抗,上地壳脆性层中的断层面由震后初期的开放性质逐渐转向重新耦合、并朝闭锁的方向发展,但其两侧地块深部持续的延性相对运动拖拽着浅部脆性层发生相应的弹性位移/变形.(4)可估计再经历15～25年,研究断裂段将完全"闭锁",即进入积累下一次大地震应力应变的震间闭锁阶段.
An M=7.6 earthquake ruptured the Luhuo segment of the Xianshuihe fault zone, Sichuan, in Feb., 1973.Since then, several cross-fault (across the 1973 rupture) deformation observation systems have been set up at Xialatang site in Luhuo County, including a system of short baselines and short leveling, a set of creep-meters, an artificial construction, and some GPS geodetic stations near to or far from the fault.By using the observation data from these systems, this paper studies the characteristics of post-seismic slip/deformation and their temporal-spatial variations for the Luhuo segment of the Xianshuihe fault zone, and builds a geodynamic model to explain the characteristic features.Our study mainly shows that (1) in the first five years following the 1973 earthquake, the earthquake fault at Xialatang site behaved as an open one and the post-seismic slip was mainly aseismic left-lateral slip (creeping) at an average rate of 10.27 mm/a along with slight tensional creeping.From 1979, however, the creep rate has been slowing down gradually from 5.3 mm/a to 2.27 mm/a following a logarithmic function, suggesting that during this period the fault plane has been tending to re-couple and re-lock gradually with some strain having built-up.(2) Since 1999 the rate of relative left-lateral displacement/deformation at far-fields on both sides of the fault segment is estimated to be 10 mm/a, much greater than the near-fault (40 m to 144 m across the fault) left-lateral creep rates of 0.66 mm/a to 2.52 mm/a in the same stage.Also, such significant differences of the near-and far-fault displacement/ deformation rates occur along an about 2×30-km-wide zone centered along the fault segment, indicating the width of the seismogenic fault zone associated with the stress/strain build-up and release during major earthquake cycles here.(3) Combining with information of geodynamic background and deep crustal structure, the authors try to explain the mechanism of the post-seismic fault slip/deformation and its spatial-temporal variation for the studied fault segment. Key points of the explanation are as follows;Starting from the 5th year after the 1973 earthquake, the fault plane in the brittle upper crust, which was open in the earlier post-seismic stage, has been tending to re-couple and re-lock as a result of gradual increasing slip/friction resistance on the fault plane.In the deeper crust, however, the continuous ductile relative motion between the two sides of the fault keeps dragging the brittle upper crust to produce elastic displacement/deformation.(4) It can be estimated that the studied fault segment will entirely "re-lock", enter the stage of inter-seismic locking, and re-build up stress/strain for the next major event in the next 15 to 25 years.
An M=7.6 earthquake ruptured the Luhuo segment of the Xianshuihe fault zone, Sichuan, in Feb., 1973.Since then, several cross-fault (across the 1973 rupture) deformation observation systems have been set up at Xialatang site in Luhuo County, including a system of short baselines and short leveling, a set of creep-meters, an artificial construction, and some GPS geodetic stations near to or far from the fault.By using the observation data from these systems, this paper studies the characteristics of post-seismic slip/deformation and their temporal-spatial variations for the Luhuo segment of the Xianshuihe fault zone, and builds a geodynamic model to explain the characteristic features.Our study mainly shows that (1) in the first five years following the 1973 earthquake, the earthquake fault at Xialatang site behaved as an open one and the post-seismic slip was mainly aseismic left-lateral slip (creeping) at an average rate of 10.27 mm/a along with slight tensional creeping.From 1979, however, the creep rate has been slowing down gradually from 5.3 mm/a to 2.27 mm/a following a logarithmic function, suggesting that during this period the fault plane has been tending to re-couple and re-lock gradually with some strain having built-up.(2) Since 1999 the rate of relative left-lateral displacement/deformation at far-fields on both sides of the fault segment is estimated to be 10 mm/a, much greater than the near-fault (40 m to 144 m across the fault) left-lateral creep rates of 0.66 mm/a to 2.52 mm/a in the same stage.Also, such significant differences of the near-and far-fault displacement/ deformation rates occur along an about 2×30-km-wide zone centered along the fault segment, indicating the width of the seismogenic fault zone associated with the stress/strain build-up and release during major earthquake cycles here.(3) Combining with information of geodynamic background and deep crustal structure, the authors try to explain the mechanism of the post-seismic fault slip/deformation and its spatial-temporal variation for the studied fault segment. Key points of the explanation are as follows;Starting from the 5th year after the 1973 earthquake, the fault plane in the brittle upper crust, which was open in the earlier post-seismic stage, has been tending to re-couple and re-lock as a result of gradual increasing slip/friction resistance on the fault plane.In the deeper crust, however, the continuous ductile relative motion between the two sides of the fault keeps dragging the brittle upper crust to produce elastic displacement/deformation.(4) It can be estimated that the studied fault segment will entirely "re-lock", enter the stage of inter-seismic locking, and re-build up stress/strain for the next major event in the next 15 to 25 years.
引文
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[13] Shen Z K,Wang M,Li Y,et al.Crustal deformation associated with the Altyn Tagh fault system,Western China,from GPS.J.Geophys Res.,2001,106:30607～30621
[14] 王敏,沈正康,牛之俊等.现今中国大陆地壳运动与活动块体模型.中国科学(D辑),2003,33(增刊):21～32 Wang M,Shen Z K,Niu Z J,et al.Contemporary crustal deformation of the Chinese continent and tectonic block model.Science in China(Series D)(in Chinese),2003,33(Suppl.):25～40
[15] 刘本培.虾拉沱地震断层蠕动的观测与研究.地壳形变与地震,1985,5(4) :357～365 Liu B P.Observation and study of creeping for the Xialatuo seismic fault.Crustal Deformation and Earthquake(in Chinese),1985,5(4) .357～365
[16] 李延兴,杨国华,李智等.中国大陆活动地块与应变状态.中国科学(D辑),2003,33(增刊):65～81 Li Y X,Yang G H,Li Z,et al.Movement and strain conditions of active blocks in the Chinese mainland.Science in China(Series D),2003,46(Suppl.):82～117
[17] Scholz C H.The Mechanics of Earthquakes and Faulting,2nd ed.Cambridge,New York,2002． 471
[18] 李天袑,杜其方,游泽李等.鲜水河活动断裂带及强震危险性 评估.成都:成都地图出版社,1997． 1～230 Li T Z,Du Q F,Zhang Z L,et al.The Active Xianshuihe Fault Zone and Its Seismic Risk Assessment(in Chinese).Chengdu:Chengdu Cartographic Publishing House,1997． 230
[19] Savage J C,Prescott W H.Asthenospheric readjustment and the earthquake cycle.J Gephys Res,1978,83:3369～3376
[20] Thatcher W.The earthquake cycle and its role in the longterm deformation of the continental lithosphere.Annales di Geofisica,1993,36:13～24
[21] Kato N.Seismic cycle on a strike slip fault with rate-and state-dependent strength in an elastic layer overlying a viscoelastic half-space.Earth,Planets Space,2002,54(11) :1077～1083
[22] Tse S T,Rice J R.Crustal earthquake instability in relation to the depth variation of frictional slip properties.J.Geophys.Res.,1986,91(B9) .9452～9472
[23] Duba A G,Hugh Corey,Duba A G.The Brittle-Ductile Transition in Rocks:the Heard Volume.Washington,D.C.:American Geophysical Union,1990． 243
[24] 王椿镛,吴建平,楼海等.青藏高原东部壳幔速度结构和地幔变形场的研究.地学前缘,2006,13(5) :349～359 Wang C Y,Wu J P,Lou H,et al.Study of crustal and upper mantle's structure and mantle deformation field beneath the eastern Tibetan plateau.Earth Science Frontiers(in Chinese),2006,13(5) :349～359
[2] Takuya Nishimura,Satoshi Miura,Kenji Tachibana,et al.Distribution of seismic coupling on the subducting plate boundary,in northeastern Japan inferred from GPS observations.Tectonophysics,2000,323:218～238
[3] Reilinger R E,Ergintav S,B(u|¨)rgmann R,et al.Coseismic and postseismic fault slip for the 17 August,1999,M=7． 5,Izmit,Turkey Earthquake.Science,2000,289(5484) :1519～1524
[4] Yagi Y,Kikuchi M,Sagiya T.Co-seismic slip,post-seismic slip,and aftershocks associated with two large earthquakes in 1996 Hyuga-nada,Japan.Earth Planets Space,2001,53:793～803
[5] Ozawa S,Kaidzu M,Murakami M,et al.Coseismic and postseismic crustal deformation after the M_w8 Tokachi-oki earthquake in Japan.Earth Planets Space,2004,56:675～680
[6] Segall P,Matthews M.Time dependent inversion of geodetic data.J.Geophys.Res.,1997,102:22391～22409
[7] Naoyuki Kato,Xinglin Lei,Xueze Wen.A synthetic seismicity model for the Xianshuihe fault,southwestern China:simulation using a rate-and state dependent friction law.Geophys J.Int.,2007,169:286～300
[8] 唐荣昌,文德华,邓天岗等.1973年炉霍7． 9级地震的地裂缝特征及地震成因的初步探讨.地球物理学报,1976,19(1) :18～27 Tang R C,Wen D H,Deng T G,et al.A preliminary study on the characteristics of the ground fractures during the Luhuo M=7． 9 earthquake,1973 and the origin of the earthquake.Chinese J.Geophys.(in Chinese),1976,19(1) :18～27
[9] Allen C R,Lou Z L,Qian H,et al.Field study of a highly active fault zone:The Xianshuihe fault of southwestern China.GeolSocAm Bull.1991,103(9) :1178～1199
[10] 钱洪,C R艾伦,罗灼礼等.全新世以来鲜水河断裂的活动特征.中国地震,1988,4(2) :9～18 Qian H,Allen C R,Luo Z L,et al.The active characteristics of Xianshuihe fault in Holocene.Earthquake Research in China(in Chinese),1988,4(2) :9～18
[11] 闻学泽,Allen C R,罗灼礼等.鲜水河全新世断裂带的分段性、几何特征及其地震构造意义.地震学报,1989,11(4) :362～372 Wen X Z,Allen C R,Luo Z L,et al.Segmentation,geometric features,and their seismotectonic implications for the Holocene Xianshuihe fault zone.Acta Seismologica Sinica(in Chinese),1989,3(4) :437～452
[12] Shen Z K,Zhao C,Yin A,et al.Contemporary crustal deformation in east Asia constrained by Global Positioning System measurements.J.Geophys.Res.,2000,105:5721～5734
[13] Shen Z K,Wang M,Li Y,et al.Crustal deformation associated with the Altyn Tagh fault system,Western China,from GPS.J.Geophys Res.,2001,106:30607～30621
[14] 王敏,沈正康,牛之俊等.现今中国大陆地壳运动与活动块体模型.中国科学(D辑),2003,33(增刊):21～32 Wang M,Shen Z K,Niu Z J,et al.Contemporary crustal deformation of the Chinese continent and tectonic block model.Science in China(Series D)(in Chinese),2003,33(Suppl.):25～40
[15] 刘本培.虾拉沱地震断层蠕动的观测与研究.地壳形变与地震,1985,5(4) :357～365 Liu B P.Observation and study of creeping for the Xialatuo seismic fault.Crustal Deformation and Earthquake(in Chinese),1985,5(4) .357～365
[16] 李延兴,杨国华,李智等.中国大陆活动地块与应变状态.中国科学(D辑),2003,33(增刊):65～81 Li Y X,Yang G H,Li Z,et al.Movement and strain conditions of active blocks in the Chinese mainland.Science in China(Series D),2003,46(Suppl.):82～117
[17] Scholz C H.The Mechanics of Earthquakes and Faulting,2nd ed.Cambridge,New York,2002． 471
[18] 李天袑,杜其方,游泽李等.鲜水河活动断裂带及强震危险性 评估.成都:成都地图出版社,1997． 1～230 Li T Z,Du Q F,Zhang Z L,et al.The Active Xianshuihe Fault Zone and Its Seismic Risk Assessment(in Chinese).Chengdu:Chengdu Cartographic Publishing House,1997． 230
[19] Savage J C,Prescott W H.Asthenospheric readjustment and the earthquake cycle.J Gephys Res,1978,83:3369～3376
[20] Thatcher W.The earthquake cycle and its role in the longterm deformation of the continental lithosphere.Annales di Geofisica,1993,36:13～24
[21] Kato N.Seismic cycle on a strike slip fault with rate-and state-dependent strength in an elastic layer overlying a viscoelastic half-space.Earth,Planets Space,2002,54(11) :1077～1083
[22] Tse S T,Rice J R.Crustal earthquake instability in relation to the depth variation of frictional slip properties.J.Geophys.Res.,1986,91(B9) .9452～9472
[23] Duba A G,Hugh Corey,Duba A G.The Brittle-Ductile Transition in Rocks:the Heard Volume.Washington,D.C.:American Geophysical Union,1990． 243
[24] 王椿镛,吴建平,楼海等.青藏高原东部壳幔速度结构和地幔变形场的研究.地学前缘,2006,13(5) :349～359 Wang C Y,Wu J P,Lou H,et al.Study of crustal and upper mantle's structure and mantle deformation field beneath the eastern Tibetan plateau.Earth Science Frontiers(in Chinese),2006,13(5) :349～359
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