Focal mechanism and seismogenic structure of the Yongning MS4.6 earthquake on 20 November 2012
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摘要: 2012年11月20日在宁夏银川市永宁县与兴庆区交界处发生MS4.6地震，为了更好地了解此次地震的发震构造，首先采用Hypo2000绝对定位方法得到该地震的震中位置及余震分布；然后采用CAP方法反演了此次地震的震源机制解和震源深度. 反演结果表明，永宁MS4.6地震是一个带有少量逆冲分量的右旋走滑地震.该地震矩震级为MW4.3，最佳双力偶解为：节面Ⅰ走向11°，倾角74°，滑动角171°；节面Ⅱ走向103°，倾角81°，滑动角16°.最佳震源深度为8km左右.从该地震震中和震源机制解以及震源深度剖面分布来看，这次地震很可能发生在银川隐伏主断层西侧的次级断层上.Abstract: The Yongning MS4.6 earthquake on November 20, 2012 is relocated by using the absolute positioning method (Hypo2000), and distribution of aftershocks is also given. In order to investigate the seismogenic structure of the earthquake, focal mechanism and depth of the earthquake are inverted by using CAP method. The inversion result shows that the Yongning earthquake is of dextral strike-slip faulting with a small amount of thrust component, and the moment magnitude is MW4.3. The best double couple solution is 11°/74°/171° for the nodal plane Ⅰ,and 103°/81°/16° for the nodal plane Ⅱ. The estimated focal depth is about 8 km. The earthquake epicenter, focal mechanism solution and cross sections along the focal depth suggest that the earthquake is likely to occur on the secondary fault in the western Yinchuan buried fault zone.
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Keywords:
- relocation /
- CAP method /
- focal mechanism solution /
- seismogenic structure
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图 1 2012年11月20日永宁MS4.6地震震中及研究区活动断裂分布右下角小图给出了研究区域的位置及本文所用的台站分布
Figure 1. The epicenter of Yongning MS4.6 earthquake on 20 November 2012 and distribution of active faults in the studied area.The inset in the lower right corner gives the location of the studied area and the distribution of stations used in this study
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图 2 宁夏部分台站下方地壳速度结构. 虚线和实线分别表示S波、P波速度
Figure 2. Crustal velocity structures beneath eight seismic stations of Ningxia. The dash lines and the solid lines represent S wave velocity and P wave velocity，respectively
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图 3 (a)沿断层走向的AA′剖面位置；(a′)沿断层走向的震源深度剖面分布；(b)垂直于断层走向的BB′剖面位置；(b′)垂直于断层走向的震源深度剖面分布
Figure 3. (a)Location of the profile AA′ along the strike of Yinchuan buried fault;(a′)Distribution of focal depths along the strike of Yinchuan buried fault;(b)Location of the profile BB′ perpendicular to the strike of Yinchuan buried fault;(b′)Distribution of focal depths perpendicular to the strike of Yinchuan buried fault
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图 4 震源机制解反演误差随深度变化图
Figure 4. Misfit error as a function of depth in the focal mechanism inversion of the Yongning MS4.6 earthquake
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图 5 永宁MS4.6地震震源机制解及部分台站波形拟合图图中第一行显示震源机制沙滩球的形状以及发震日期、波形最佳拟合深度、节面Ⅰ的机制解、矩震级和拟合误差；沙滩球采用下半球投影方式绘制，图上的小圆圈为台站位置；黑色和红色曲线分别表示理论地震图与观测地震图；波形下面的第一行和第二行数字分别表示理论地震图相对于观测地震图的移动时间及二者的相关系数；波形左侧自上至下分别表示震中距、台站名称和方位角；右侧3列曲线上面的V，R，T分别表示S波垂向、径向和切向
Figure 5. The focal mechanism solution of Yongning MS4.6 earthquake and waveform fittings between synthetic(red) and observed(black)seismograms The first row in the figure shows beach-ball of focal mechanism, occurrence date, the best focal depth, focal mechanism solution of nodal plane I, moment magnitude and fitting error. The beach ball is drawn by lower hemisphere projection method, in which the small open circles st and for station locations. The numbers in the first and second rows below the waveform are the time shifts of theoretical seismograms relative to observation seismograms, and correlation coefficients of the two types of seismograms. The texts on the left side of the waveform are epicenter distance, station code and azimuth angle
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图 6 永宁MS4.6地震震源机制解不同方法的结果对比
Figure 6. Comparison of focal mechanisms of Yongning MS4.6 earthquake using P wave initial motion，amplitude ratio and CAP method
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