Crustal stress field in the focal area of MS7.0 Jiuzhaigou，Sichuan，earthquake by shear wave splitting
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摘要: 本文讨论了2017年8月8日九寨沟MS7.0 （MW6.5）地震之后震源区的地震活动性，并针对震后九寨沟地区架设的4个流动地震台站记录到的地震波形数据进行了剪切波分裂参数的计算，分析了震源区地壳应力场的特征性变化。九寨沟MS7.0地震的地震活动性表现为：8月8日主震后震源区地震活动明显增强，直至8月22日之后才有所下降，余震震级整体也呈降低趋势，震级多集中在MS2.0以下。震源区4个流动台站的剪切波分裂结果表明：快波偏振方向近似为东西向，揭示了九寨沟地区的区域应力场方向为近似东西向；区域内慢波延迟时间的平均值为5.04 ms/km，慢波延迟时间在九寨沟MS7.0地震之后降低，反映了随着余震的发生，地壳应力不断调整，应力持续释放，直至8月底地壳应力场逐渐趋于稳定状态。
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关键词:
- 地震活动性 /
- 剪切波分裂 /
- 地壳应力 /
- 九寨沟MS7.0地震
Abstract: This paper discussed the seismic activity in the focal area after Jiuzhaigou MS7.0 （MW6.5） earthquake on August 8, 2017, and calculated the shear wave splitting parameters by using the seismic waveform data from four temporary seismic stations in Jiuzhaigou area, then analyzed the characteristic change of crustal stress in the focal area. After the MS7.0 Jiuzhaigou earthquake, the seismic activity was significantly enhanced in the focal area on August 8, 2017. Until after 22 August 2017, the seismic activity dropped slightly; correspondingly, the magnitude of the aftershocks also became smaller, concentrating below MS2.0. The results of the polarization directions of fast shear wave at four seismic stations in the focal area show that the polarization directions are approximately east-west, which reveals the direction of the regional stress field in Jiuzhaigou area is approximately east-west. Meanwhile, the average value of the time delay of slow shear wave in the studied area is 5.04 ms/km, and after Jiuzhaigou earthquake the time delay of slow shear wave decreased, which reflects the release and adjustment of the crustal stress with the aftershocks occurrence. And thus the stress field in the crust tends to be stable at the end of August. -
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图 1 九寨沟地震震源区及邻区的构造背景（a）和震源区余震及台站分布图（b）
Figure 1. Tectonic settings of the source region of Jiuzhaigou earthquake and its vicinity （a） as well as distribution of the aftershocks and seismic stations in the source region （b）
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图 2 九寨沟震源区余震分布柱状图（a）和M-t图（b）
Figure 2. Histogram （a） and M-t （b） of the aftershocks in the source region of Jiuzhaigou earthquake
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图 3 L5112台站记录的地震数据剪切波分裂分析
所记录事件的发震时间为2017年8月10日20时12分59秒；震中位于（33.31°N，103.74°N），震源深度为7 km；震级为MS1.2；震中距为6.5 km。 （a） 三分量地震波行记录，两条红色竖虚线是截取的用于质点偏振图分析的剪切波段；（b） 质点偏振图，箭头标示了快慢波的质点运动方向，红点标记了快慢波的到达
Figure 3. The shear wave splitting analyses of the earthquakes recorded by station L5112
The earthquake with MS1.2 occurred at 20:12:59 on 10 August 2017，the epicenter is located at （33.31°N，103.74°E） with focal depth 7 km and epicentral distance 6.5 km。 （a） Three-component seismic waveforms，where the two red vertical dashed lines delineate the scope of shear wave shown in polarization figure；（b） Particle trail where the arrows are their particle motion directions of fast and slow waves，and red dots mark their arrivals
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图 4 研究区域内4个台站快波偏振方向等面积投影玫瑰图
Figure 4. The homolographic projection rose diagrams of the fast shear wave polarizations at four seismic stations
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图 5 九寨沟震源区各台站快波偏振方向等面积投影玫瑰图分布
Figure 5. The distribution of the homolographic projection rose diagrams at stations in Jiuzhaigou focal area
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图 6 L5111和L5112台站慢波延迟时间变化结果
黑点为慢波延迟时间计算值，红色曲线为7点滑动平均结果
Figure 6. The delay time changes of slow wave at the seismic stations L5111 and L5112
The black dots denote the time delay of slow shear wave，and red lines are the results of 7 points moving average
表 1 九寨沟地区4个流动台站的剪切波分裂参数
Table 1 Parameters of shear wave splitting at the four portable seismic stations in Jiuzhaigou area
台站 窗口内波形个数 平均偏振方向/° 偏振方向误差/° 平均慢波延时
/（ms·km−1）慢波延时误差
/（ms·km−1）L5110 4 104 6.9 4.23 1.63 L5111 217 97 26.7 5.35 2.35 L5112 304 99 23.8 5.08 2.18 L5113 58 82 19.5 4.76 2.14 
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