Focal mechanism of the Sheyang MS4.4 earthquake on October 20, 2016 and the spatio-temporal distribution of the earthquake sequence
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摘要: 基于江苏地区测震台网记录，采用CAP方法反演了2016年10月20日射阳MS4.4地震的震源机制解和震源深度；利用HypoDD方法对射阳地震序列中ML≥1.5地震进行了重新定位.结果显示：射阳MS4.4地震的震源机制参数分别为节面Ⅰ:走向304°，倾角53°，滑动角0°；节面Ⅱ：走向214°，倾角90°，滑动角143°，震源深度约为14 km.双差定位结果显示：此次射阳地震序列分布于洪泽—沟墩断裂与盐城—南洋岸断裂之间，在水平空间内，其震中分布的优势方向为NW60°，由SE向NW迁移; 地震序列深度分布在6—23 km范围内.根据所反演的震源机制参数和地震序列精定位结果，本文推测射阳MS4.4地震的断层面解为震源机制解的节面Ⅰ, 该地震可能是在区域背景应力场的作用下，沿NW向剪切破裂产生的左旋走滑地震事件.Abstract: Based on the digital waveforms of Jiangsu seismic network, the focal mechanism of the Sheyang MS4.4 earthquake was determined by using the CAP (cut and paste) method, and the ML≥1.5 events of Sheyang earthquake sequence were relocated by using the HypoDD method. The result shows that the strike, dip and rake of the MS4.4 event are 304°, 53°, 0° for the nodal plane Ⅰ, and 214°, 90°, 143° for the nodal plane Ⅱ, respectively, and the centroid depth is about 14 km. The relocation for the Sheyang earthquake sequence shows that the earthquake sequence was located between Hongze-Goudun fault and Yancheng-Nanyang'an fault with the predominant direction NW60° in horizontal space, and the sequence propagated from southeast to northwest with the focal depth of 6--23 km. According to the focal mechanism solutions and the relocation results, we deduce that the Sheyang MS4.4 earthquake is a left-lateral slip event induced by shear rupture northwestwards under regional stress field, and its fault plane is nodal plane Ⅰ.
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Keywords:
- Sheyang MS4.4 earthquake /
- focal mechanism /
- hypoDD /
- Mohr-Coulomb failure criterion
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江苏省地震局监测中心提供了波形数据，熊振研究员核查了地震构造信息，审稿专家提出了宝贵意见，作者在此一并表示衷心的感谢.
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图 1 射阳地区地震震中及周围台站分布
F1：淮阴—响水断裂；F2：洪泽—沟墩断裂；F3：盐城—南洋岸断裂；F4：陈家堡—小海断裂；F5：泾口—沙沟断裂；阴影区域为苏北盆地
Figure 1. Distributions of earthquake epicenters and seismic stations in Sheyang area
F1: Huaiyin-Xiangshui fault; F2: Hongze-Goudun fault; F3: Yancheng-Nanyang'an fault; F4: Chenjiabao-Xiaohai fault; F5: Jingkou-Shagou fault. The shadow region denotes Subei basin
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图 2 射阳MS4.4地震的矩张量反演中波形拟合误差随震源深度的变化
Figure 2. Misfit variation with focal depth of the Sheyang MS4.4 earthquake
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图 3 射阳MS4.4地震计算地震波形(红色)与实际观测地震波形(黑色)的比较
波形左侧为地震观测台站和震中距(单位：km)，波形下面依次为计算波形相对观测波形的到时差(单位：s)和计算波形与观测波形的相关系数
Figure 3. Comparison between calculated seismograms (red) and observed ones (black) of the Sheyang MS4.4 earthquake
The focal mechanism solution of the Sheyang earthquake is also given, the texts on the left of waveform denote the station and epicentral distance (unit in km), the numbers below the seismic wave show the time difference (unit in s) between the calculated and observed seismograms and their correlation coefficient, respectively
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图 4 射阳地震序列重新定位后的地震震中分布图
AA′ and BB′分别为地震序列分布的长轴和短轴
Figure 4. Relocation of event epicenters for the Sheyang earthquake sequence
AA′ and BB′ are the major axis and minor axis of the earthquake sequence, respectively
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表 1 射阳地区地壳速度模型参数
Table 1 The parameters of crustal velocity model for Sheyang area
地层 模型分层厚度/km ρ/(kg·m-3) vP/(km·s-1) vS/(km·s-1) 软沉积层 0.5 2.10 2.50 1.44 硬沉积层 1.5 2.50 4.40 2.54 3.0 2.75 5.22 3.01 上地壳 5.0 2.80 5.59 3.23 5.0 2.80 6.17 3.56 中地壳 5.0 2.90 6.49 3.75 下地壳 5.0 2.90 6.43 3.71 7.0 3.30 7.03 4.08 上地幔 300 3.30 8.00 4.62 
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