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Title:: Focal Mechanisms and Stress Field of Small Earthquakes of the M_S8.0 Wenchuan Earthquake Sequence

Author(s):: ZHANG Zhi-wei; ZHANG Yong-jiu; CHENG Wan-Zheng; WU Peng; LONG Feng; RUAN Xiang; (Earthquake Administration of Sichuan Province,Chengdu 610041,Sichuan,China)

Keywords:: M_S8.0 Wenchuan earthquake; aftershock sequence; focal-mechanism solutions; stress field

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M_L≥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。

Abstract:: 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 M_L≥3.5 of the M_S8.0 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.