Near-field shear-wave splitting of 2014 Yingjiang MS6.1 earthquake sequence
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摘要: 使用横波分裂系统分析方法(SAM)， 对2014年5月30日盈江MS6.1地震震区内多个近场流动台站记录到的大量波形数据进行横波分裂研究． 研究结果表明， 盈江MS6.1地震序列的快S波偏振方向为近NS向， 与区域主压应力方向一致． 主震发生后， 由于震源区应力状态的调整， 卡场台(KAC)快S波偏振方向发生逆时针偏转， 勐弄台(MNO)快S波偏振方向离散度减小， 并且由于受到研究区内断裂的影响， MNO台偏振方向较KAC台偏振方向更加离散． KAC台和MNO台的慢S波时间延迟均表现出主震发生前短时间内突然减小， 震后逐渐增大的变化特征， 这意味着临震前震源区地壳应力的释放和震后地壳应力的增强， 预示了后续余震的持续发生． 地震序列时间延迟平均滑动曲线起伏振荡， 表明了余震的发生伴随着震源区地壳应力的不断调整．Abstract: SAM (systematic analysis method of shear-wave splitting) has been applied to numerous waveform data recorded by near-field mobile seismic stations of Yingjiang MS6.1 earthquake sequence occurred on May 30， 2014. The results showed that the polarization of fast shear-wave of the earthquake sequence was consistent with regional principal compressive stress of NS trending. The polarization of fast shear-wave has a counterclockwise deflection at the station KAC， and dispersion decrease at the station MNO due to the adjustment of stress state in seismic region after the main shock. Influenced by faults of the studied area， the polarization of fast shear-wave at the station MNO presents a more discrete state compared to that of KAC. The time-delays of slow shear-wave at the stations KAC and MNO both present the feature of sudden contraction before the main shock and increase after the event， which means release and enhancement of crustal stress before and after the earthquake， and also indicates constant occurrence of aftershocks. The oscillation of time-delay curves suggests that the occurrence of aftershocks is accompanied by continuous adjustment of crustal stress in seismic region.
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Keywords:
- Yingjiang MS6.1 earthquake sequence /
- shear-wave splitting /
- polarization /
- time-delay /
- fault
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图 1 2014年5月24日—6月30日盈江MS6.1地震序列重定位分布图
黑色箭头表示区域应力场方向(吴建平等，2004)； 左上角小图为青藏高原东南缘，蓝色矩形框标注范围为本文研究区域.
Figure 1. Relocation distribution map of Yingjiang MS6.1 earthquake sequence from May 24 to June 30，2014
Black arrows denote the regional stress field direction(after Wu et al，2004); insert on top-left corner shows the south-eastern Tibetan Plateau，in which blue box shows the studied area
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图 2 KAC台站记录的2014年5月31日ML3.2地震的横波分裂分析过程
(a)横波三分量波形；(b)横波质点运动轨迹，S1，S2分别表示快、 慢S波到时；(c)NS向和EW向波形记录；(d)快(F)、 慢(S)S波质点运动轨迹图，已消除时间延迟的影响；(e)快、 慢S波波形，两条竖线之间的时间差为慢S波的时间延迟． 波形采样率为100 sps(记为100 Hz)
Figure 2. Analysis process of SAM method of ML 3.2 earthquake on May 31，2014 recorded by station KAC
(a)Three-component waveforms of shear-wave;(b)Shear-wave particle moving track，where S1 and S2 represent the arrival of fast and slow shear-wave respectively;(c)NS and EW component waves；(d)Particle moving track of shear-wave with time delay removed;(e)Fast and slow shear-waves，where two vertical lines delineates the time-delay of slow shear-wave，sampling rate is 100 sps(recorded as 100 Hz)
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图 3 盈江MS6.1地震序列快S波偏振方向的等面积极射投影图和等面积投影玫瑰图
(a)KAC台;(b)MNO台;(c)XIM台;(d)ZHX台左下角数字为各台站用于分析的有效地震事件次数，短线方向为各台站记录的每个地震事件的快S波偏振方向，下同
Figure 3. Equal-area polar projection diagram and projection rose diagram of fast shear-wave polarization of Yingjiang MS6.1 earthquake sequence
(a)Station KAC;(b)Station MNO;(c)Station XIM;(d)Station ZHX The bold figures on lower-left indicate the earthquake number used in study，and directions of short bars indicate the fast shear-wave polarization of earthquakes recorded by each station，the same below
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图 4 盈江MS6.1地震主震发生前、 后地震序列快S波偏振方向的等面积极射投影图和等面积投影玫瑰图
(a)KAC台主震前序列；(b)KAC台余震序列；(c)MNO台主震前序列；(d)MNO台余震序列
Figure 4. Equal-area polar projection diagram and projection rose diagram of fast shear-wave polarization of Yingjiang MS6.1 earthquake sequence before and after the main shock
(a)Earthquake sequence before the main shock recorded by the station KAC；(b)Aftershock sequence recorded by the station KAC；(c)Earthquake sequence before the main shock recorded by the station MNO；(d)Aftershock sequence recorded by the station MNO
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图 5 KAC台(a)和MNO台(b)记录的盈江MS6.1地震余震序列3个时段快S波偏振方向等面积极射投影图和等面积投影玫瑰图
Figure 5. Equal-area polar projection diagram and projection rose diagram of fast shear wave polarization of Yingjiang MS 6.1 aftershock sequence recorded at the stations KAC(a)and MNO(b)in time periods a1，a2 and a3
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图 6 KAC台(a)和MNO台(b)记录的地震事件的慢S波时间延迟变化及盈江
MS6.1地震序列M-t图(c). 倒三角表示余震前曲线下降段，下同
Figure 6. Slow shear-wave time-delay changes of Yingjiang MS6.1 earthquake sequence at the stations KAC(a)and MNO(b)and M-t map(c)
Inverse triangle denotes the decreasing stage before aftershocks，the same below
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图 7 2014年5月24日至6月1日KAC台(a)和MNO台(b)记录的地震事件的慢S波时间延迟变化与盈江MS6.1地震序列M-t图(c). 倒三角表示时间延迟开始下降的时刻
Figure 7. Slow shear-wave time-delay changes at the stations KAC(a)and MNO(b)from May 24 to June 1，2014 and M-t map(c)of Yingjiang MS6.1 earthquake sequence Inverse triangle denotes the decreasing moment of the curve
表 1 本文所用的5个流动台站的有效数据
Table 1 Valid data from five stations used in this study
台站 代码 震中距
/km有效数
据个数卡场台 KAC 9.0 664 勐弄台 MNO 10.5 304 昔马台 XIM 28.0 2 新城台 XIC 31.5 0 盏西台 ZHX 32.5 1 
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表 2 KAC台和MNO台记录到的盈江MS6.1地震序列在不同时段的快S波偏振方向
Table 2 The fast shear-wave polarization of Yingjiang MS6.1 earthquake sequence recorded by stations KAC and MNO in different time periods
地震序列 KAC台 MNO台 平均偏振方向/° 数据条数 平均偏振方向/° 数据条数 主震前序列 10.6±26.6 121 -13.5±38.6 63 整个盈江地震序列 -0.9±28.0 664 -15.0±34.8 304 余震序列 -3.5±27.7 543 -15.0±33.8 241 5月30—31日余震序列(a1) -2.6±28.8 180 -16.9±33.3 111 6月1—9日余震序列(a2) 0.8±27.1 186 -13.4±31.9 72 6月10—30日余震序列(a3) -8.8±26.4 177 -16.5±37.7 58
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