汶川8.0级地震序列的小震震源机制及应力场特征
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摘要
利用区域地震台网的数字地震波记录资料,由垂直向记录P和S振幅比值,结合部分清晰的P波初动记录资料,反演得到了2008年5月12日至2009年4月12日汶川8.0级地震序列中829个ML≥3.5的小震震源机制解。采用统计和力轴张量计算方法,分析了震源机制解参数并求取了余震区平均应力场。结果表明:用余震区北段小震震源机制解求得的节面为直立或倾斜,走向为NNE-SSW向,主压应力P轴方位为SWW-NEE方向,计算得到的平均应力张量σ1方向为77.1°;用余震区南段小震震源机制解求得的节面倾角较陡,在50°～90°之间,走向相对较分散,平均应力张量σ1方向为92.4°,呈EW向。从余震区南、北段的平均应力场方位随时间演化过程可以看出,余震区在2008年8月、9月、12月和2009年1月处于应力场调整阶段。最后研究了余震区南、北段的震源机制一致性参数θ及逆冲型地震类型随时间的变化,得到了一些有意义的结果。
Based on the data of digital seismic wave from May 12,2008 to April 12,2009 recorded by regional seismic network,the focal-mechanism solutions of 829 small events with ML≥3.5 of the MS8.0 Wenchuan earthquake sequence were determined by the amplitude ratio of PG and SG in vertical component records with clear initial motion.Using the method of statistical analysis and stress axis tensor,the parameters of focal-mechanism solutions are analyzed and the average stress-field azimuth of aftershock area is calculated.The results show that the source-fracture plane of small earthquakes presents the strike of NNE direction,and the dip angles are near vertical or inclined at northern segment of the Longmenshan fault.The principal direction of compressive stress P-axis is SWW-NEE,the azimuth of maximum principal stress axis is 77.1°.At northern segment of aftershock area,the nodal plane strikes are relatively dispersed.The dip angles are steep,dispersed between 50° and 90°.The azimuth of maximum principal stress axis is EW,that is 92.4°.It can be seen from the evolution process of average stress-field with time at northern and southern segment of aftershock area,that the stress adjustment stages of aftershock area are August,September,December in 2008 and January in 2009.The consistency parameter curve and the proportional change of thrust-type earthquakes with time in the aftershock area are also studied,and then some significant results are given.
Based on the data of digital seismic wave from May 12,2008 to April 12,2009 recorded by regional seismic network,the focal-mechanism solutions of 829 small events with ML≥3.5 of the MS8.0 Wenchuan earthquake sequence were determined by the amplitude ratio of PG and SG in vertical component records with clear initial motion.Using the method of statistical analysis and stress axis tensor,the parameters of focal-mechanism solutions are analyzed and the average stress-field azimuth of aftershock area is calculated.The results show that the source-fracture plane of small earthquakes presents the strike of NNE direction,and the dip angles are near vertical or inclined at northern segment of the Longmenshan fault.The principal direction of compressive stress P-axis is SWW-NEE,the azimuth of maximum principal stress axis is 77.1°.At northern segment of aftershock area,the nodal plane strikes are relatively dispersed.The dip angles are steep,dispersed between 50° and 90°.The azimuth of maximum principal stress axis is EW,that is 92.4°.It can be seen from the evolution process of average stress-field with time at northern and southern segment of aftershock area,that the stress adjustment stages of aftershock area are August,September,December in 2008 and January in 2009.The consistency parameter curve and the proportional change of thrust-type earthquakes with time in the aftershock area are also studied,and then some significant results are given.
引文
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梁尚鸿,李幼铭,束沛镒,等.1984.利用区域地震台网P、S振幅比资料测定小震震源参数[J].地球物理学报,27(3):247-257.
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王曰风,刁桂苓,张秀萍,等.2009.2001年云南永胜60级地震余震序列震源机制解与震源区应力场分析[J].地震研究,31(2):119-123.
许忠淮,戈澍谟.1984.用滑动方向拟合法反演富蕴地震断裂带应力场[J].地震学报,6(4):395-404.
许忠淮.1985.用滑动方向拟合法反演唐山余震区的平均应力场[J].地震学报,7(4):349～361.
张勇,许力生,陈运泰.2009.2008年汶川大地震震源机制的时空变化[J].地球物理学报,52(2):379-389.
钟继茂,程万正.2006.由多个地震震源机制解求川滇地区平均应力场方向[J].地震学报,28(4):337-346.
C h eng W Z,Zhang Z W,Ruan X.2009.Spatio-temporal variation and fo-cal mechanismof WenchuanMS8.0earthquake sequence[J].Earth-quake Science,22(2):107-117.
E ll sworth W L,Xu Z H.1980.Determination of the stress tensor from fo-cal mechanism data[J].Eos Trans,AGU,61:1117.
G e phart J W,Forsyth D W.1984.An improved method for determining theregional stress tensor using earthquake focal mechanism data:appli-cation to the San Fernando earthquake sequence[J].JGR,89:9305-9320.
陈运泰,许力生,张勇,等.2008.2008年5月12日汶川特大地震震源特性分析报告[R].中国地震局地球物理所http://www.csi.ac.cn/sichuan/chenyuntai.pdf.
刁桂苓,王绍晋,龙晓帆,等.1996.1976年龙陵震群的小震震源机制[J].地震研究,19(4):331-339.
刁桂苓,于利民.1992.震源机制解的系统聚类分析:以海城地震序列为例[J].中国地震,8(3):86-92.
刁桂苓,于利明,宁杰远,等.1993.1989年大同震群的破裂特征[J].地球物理学报,36(3):360-368.
胡幸平,俞春泉,陶开,等.2008.利用P波初动资料求解汶川地震及其强余震震源机制解[J].地球物理学报,51(6):1711-1718.
华卫,陈章立,郑斯华.2009.2008年汶川8.0级地震震源参数分段特征的研究[J].地球物理学报,52(2):365-371.
梁尚鸿,李幼铭,束沛镒,等.1984.利用区域地震台网P、S振幅比资料测定小震震源参数[J].地球物理学报,27(3):247-257.
万永革.2008.美国Landers地震和Hector Mine地震前震源机制与主震机制一致性现象的研究[J].中国地震,24(3)216-225.
王卫民,赵连锋,李娟,等.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.
王曰风,刁桂苓,张秀萍,等.2009.2001年云南永胜60级地震余震序列震源机制解与震源区应力场分析[J].地震研究,31(2):119-123.
许忠淮,戈澍谟.1984.用滑动方向拟合法反演富蕴地震断裂带应力场[J].地震学报,6(4):395-404.
许忠淮.1985.用滑动方向拟合法反演唐山余震区的平均应力场[J].地震学报,7(4):349～361.
张勇,许力生,陈运泰.2009.2008年汶川大地震震源机制的时空变化[J].地球物理学报,52(2):379-389.
钟继茂,程万正.2006.由多个地震震源机制解求川滇地区平均应力场方向[J].地震学报,28(4):337-346.
C h eng W Z,Zhang Z W,Ruan X.2009.Spatio-temporal variation and fo-cal mechanismof WenchuanMS8.0earthquake sequence[J].Earth-quake Science,22(2):107-117.
E ll sworth W L,Xu Z H.1980.Determination of the stress tensor from fo-cal mechanism data[J].Eos Trans,AGU,61:1117.
G e phart J W,Forsyth D W.1984.An improved method for determining theregional stress tensor using earthquake focal mechanism data:appli-cation to the San Fernando earthquake sequence[J].JGR,89:9305-9320.
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