芦山地震前龙门山断裂带闭锁程度与变形动态特征研究
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摘要
利用1999—2007和2009—2011年中国大陆GPS水平速度场数据,采用DEFNODE(反演计算弹性岩石圈块体旋转、应变和块体边界断层闭锁或同震滑动的Fortran程序)负位错反演程序估算了芦山地震前龙门山断裂带的三维闭锁程度,并结合剖面结果分析了断层深浅部变形特征.GPS反演结果表明,1999—2007年,龙门山断裂中北段(闭锁比例为0.99)处于强闭锁(本文将闭锁比例大于0.97的称为强闭锁)状态;龙门山断裂南段地表以下深度16km内为强闭锁,深度16—21km处闭锁比例降低为0.62,深度21—24km处整条断裂逐渐转变为蠕滑状态.2009—2011年,即汶川地震后,龙门山断裂中北段处于震后蠕滑状态;龙门山断裂南段深度16—21km处闭锁比例降低为0.45,其它位置闭锁程度保持不变.GPS剖面结果显示,2009—2011年,即汶川地震后,龙门山断裂中北段为逆冲兼右旋走滑运动;而南段断层不能自由滑动、变形宽度较大.综合分析认为,汶川地震时,龙门山断裂南段并没有发生破裂,一直处于较强的闭锁状态,汶川地震的发生又加速了芦山地震的孕育进程;由于龙门山断裂带南段的闭锁深度较中北段浅,因此芦山地震较汶川地震强度低、震级小、破裂范围窄.
By using the negative dislocation model of DEFNODE,a Fortran program to model elastic lithospheric block rotations and strains,and locking or coseismic slip on block-bounding faults,we inverted the GPS horizontal velocity observations during 1999—2007 and 2009—2011for a 3-D fault locking model of the Longmenshan fault zone before the Lushan earthquake,and analyzed characteristics of fault deformation at different depth in combination with GPS profiles.The inversion results indicate that during 1999—2007 the middle-northern segment of the Longmenshan fault was tightly locked(locked fraction is 0.99).Meanwhile the southern segment of the Longmenshan fault was tightly locked from the surface to the depth of 16 km,and the locked fraction decreases to about 0.62 at the depth of 16—21 km.The whole fault turns to creeping gradually between the depth of 21 km and 24 km.During 2009—2011,i.e.,after the Wenchuan earthquake,the middle-northern segment of the Longmenshan fault was in a creeping state.Meanwhile the locked fraction of the southern segment of the Longmenshan fault decreased to about 0.45 at the depth of 16—21 km,and remained unchanged at other depths.In addition,the GPS profiles of 2009—2011 show that the middle-northern segment of the fault was thrust and dextral slip after the Wenchuan earthquake.The southern segment could not slip freely and the deformation width was large.Comprehensive analysis indicates that the southern segment of the Longmenshan fault has no rupture after the Wenchuan earthquake and is still tightly locked,and the Wenchuan earthquake speeds up the preparation process of the Lushan earthquake.Because the locked depth of the southern segment is lower than that of the middle-northern segment,therefore intensity,earthquake magnitude and rupture range of the Lushan earthquake are all less than those of the Wenchuan earthquake.
By using the negative dislocation model of DEFNODE,a Fortran program to model elastic lithospheric block rotations and strains,and locking or coseismic slip on block-bounding faults,we inverted the GPS horizontal velocity observations during 1999—2007 and 2009—2011for a 3-D fault locking model of the Longmenshan fault zone before the Lushan earthquake,and analyzed characteristics of fault deformation at different depth in combination with GPS profiles.The inversion results indicate that during 1999—2007 the middle-northern segment of the Longmenshan fault was tightly locked(locked fraction is 0.99).Meanwhile the southern segment of the Longmenshan fault was tightly locked from the surface to the depth of 16 km,and the locked fraction decreases to about 0.62 at the depth of 16—21 km.The whole fault turns to creeping gradually between the depth of 21 km and 24 km.During 2009—2011,i.e.,after the Wenchuan earthquake,the middle-northern segment of the Longmenshan fault was in a creeping state.Meanwhile the locked fraction of the southern segment of the Longmenshan fault decreased to about 0.45 at the depth of 16—21 km,and remained unchanged at other depths.In addition,the GPS profiles of 2009—2011 show that the middle-northern segment of the fault was thrust and dextral slip after the Wenchuan earthquake.The southern segment could not slip freely and the deformation width was large.Comprehensive analysis indicates that the southern segment of the Longmenshan fault has no rupture after the Wenchuan earthquake and is still tightly locked,and the Wenchuan earthquake speeds up the preparation process of the Lushan earthquake.Because the locked depth of the southern segment is lower than that of the middle-northern segment,therefore intensity,earthquake magnitude and rupture range of the Lushan earthquake are all less than those of the Wenchuan earthquake.
引文
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单斌,熊熊,郑勇,刁法启.2009.2008年5月12日MW7.9汶川地震导致的周围断层应力变化[J].中国科学:D辑,39(5):537--545.
邵志刚,周龙泉,蒋长胜,马宏生,张浪平.2010.2008年汶川MS8.0地震对周围断层地震活动的影响[J].地球物理学报,53(8):1784--1795,doi:10.3969/j.issn.0001-5733.2010.08.004.
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万永革,沈正康,盛书中,徐晓枫.2009.2008年汶川大地震对周围断层的影响[J].地震学报,31(2):128--139.
王敏.2009.基于GPS同震位移场约束反演2008年5·12汶川大地震破裂空间分布[J].地球物理学报,52(10):2519--2526,doi:10.3969/j.issn.0001-5733.2009.10.010.
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王卫民,赵连锋,李娟,姚振兴.2008.四川汶川8.0级地震震源过程[J].地球物理学报,51(5):1403--1410.
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闻学泽,杜方,张培震,龙锋.2011.巴颜喀拉块体北和东边界大地震序列的关联性与2008年汶川地震[J].地球物理学报,54(3):706--716,doi:10.3969/j.issn.0001-5733.2011.03.010.
武艳强,江在森,杨国华,刘晓霞,赵静.2011.汶川地震前GPS资料反映的应变率场演化特征[J].大地测量与地球动力学,31(5):20--25.
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许才军,刘洋,温扬茂.2009.利用GPS资料反演汶川MW7.9级地震滑动分布[J].测绘学报,38(3):195--201.
曾祥方,罗艳,韩立波,石耀霖.2013.2013年4月20日四川芦山MS7.0地震:一个高角度逆冲地震[J].地球物理学报,56(4):1418--1424,doi:10.6038/cjg20130437.
张培震,徐锡伟,闻学泽,冉勇康.2008.2008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[J].地球物理学报,51(4):1066--1073.
张培震,闻学泽,徐锡伟,甘卫军,王敏,沈正康,王庆良,黄媛,郑勇,李小军,张竹琪,马胜利,冉勇康,刘启元,丁志峰,吴建平.2009.2008年汶川8.0级特大地震孕育和发生的多单元组合模式[J].科学通报,54(7):944--953.
张勇,冯万鹏,许力生,周成虎,陈运泰.2008.2008年汶川大地震的时空破裂过程[J].中国科学:D辑,38(10):1186--1194.
张勇,许力生,陈运泰.2013.芦山4·20地震破裂过程及其致灾特征初步分析[J].地球物理学报,56(4):1408--1411,doi:10.6038/cjg20130435.
赵翠萍,陈章立,周连庆,李志雄,康英.2009.汶川MW8.0地震震源破裂过程研究:分段特征[J].科学通报,54(22):3475--3482.
赵静,江在森,武艳强,刘晓霞,魏文薪,李强.2012.汶川地震前龙门山断裂带闭锁程度和滑动亏损分布研究[J].地球物理学报,55(9):2963--2972,doi:10.6038/j.issn.0001-5733.2012.09.015.
赵静,江在森,武艳强,刘晓霞,魏文薪,王阅兵,李强,徐晶.2013.Defnode负位错模型反演结果的可靠性和稳定性分析[J].大地测量与地球动力学,33(1):21--24.
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