Characteristics of shear-wave splitting for the 2013 Lushan MS7.0 earthquake sequence
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摘要: 本文测定了2013年4月20日芦山MS7.0地震震源区及其附近台站的S波分裂参数，包括快波偏振方向和慢波延迟时间，最终得到了40个台站的S波分裂结果．结果显示：在地震主破裂区内观测到的快波优势取向为NE向，与余震分布的长轴方向一致；位于双石—大川断裂以西台站的快波偏振优势方向为NW向，与区域最大主压应力轴方向一致；位于荥经断裂附近台站的快波偏振优势方向为NW向，与该断裂走向一致．快波偏振优势方向随时间的变化结果显示：主震前位于地震破裂区附近的TQU和BAX台站的快波偏振优势方向均呈NE向；主震后TQU台站的快波偏振优势方向为近EW向，而BAX台站的快波偏振优势方向则不突出，反映出芦山地震主震前快波偏振方向受控于龙门山断裂带，而主震后受构造应力场的作用更加明显．此外，各台站的慢波延迟时间为1.25—5.40ms/km，在余震覆盖密集区域，台站的慢波延迟时间均大于3.0ms/km，反映出震源区的各向异性程度较强．芦山主震后，各台站的延迟时间随时间变化持续减小，反映出震源区地壳应力随余震活动逐渐减小．Abstract: This paper measured the shear-wave splitting parameters, the polarization direction of fast shear-wave and the time delay of slow shear-wave, by using the seismic data recorded by the stations in the focal region and its adjacent areas of the MS7.0 Lushan earthquake on April 20, 2013. And then the shear-wave splitting results from 40 seismic stations were obtained, which indicates the crustal anisotropy in the studied area. The results show that the dominant polarization directions of fast shear-waves are in NE in the rupture zone, which is consistent with the strike of the major axis of aftershocks distribution. The dominant polarization directions of fast shear-waves in the west of Shuangshi--Dachuan fault are in NW in agreement with the directions of the regional principal compressive stress. And the dominant polarization directions of fast shear-waves, near Yingjing fault are in NW, which is consistent with the strike of Yingjing fault. It is also shown that the dominant polarization directions of fast shear-waves at the stations TQU and BAX around the rupture zone are in NE direction before the main shock, and the dominant polarization directions at the station TQU are almost in EW after the main shock, and the variation of polarization directions before and after the main shock is not obvious at the station BAX, which indicates the polarization directions of fast shear-waves are controlled by Longmenshan fault zone before the main shock, and mainly affected by tectonic stress field after the main shock. Furthermore, the time delay of slow shear-wave is in the range of 1.25—5.40 ms/km for all the stations, but more than 3.0 ms/km in the region with dense aftershocks coverage, reflecting strong anisotropy in the source area. After the main shock, the delay time decreased continuously, suggesting the stress field became stable gradually in the source region.
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图 1 研究区内主要断裂和芦山主震、余震及地震台站分布图
F1：大邑—名山断裂；F2：双石—大川断裂；F3：盐井—五龙断裂；F4：金汤弧形断裂；F5：荥经断裂；F6：大渡河断裂
Figure 1. Distribution of the main faults，main shock(star)and aftershocks(black dots) of Lushan earthquake sequence and seismic stations in the studied area
F1：Dayi-Mingshan fault；F2：Shuangshi-Dachuan fault；F3：Yanjing-Wulong fault；F4：Jintang arc fault；F5：Yingjing fault；F6：Daduhe fault. Red and blue triangles represent the permanent and portable stations，respectively
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图 2 S波分裂过程分析实例
(a)，(b)，(c)分别为TQU，L5530和SANJ台站记录的地震．第一行为三分量地震波形图，两虚线间的波形用 来绘制质点偏振图，Δ为震中距，d为震源深度；第二行为截取的部分波形质点振动图，S1和S2分别为快、慢波到时，两根短线间隔为0.01 s；第三行为快(F)、慢(S)波波形图，两虚线分别标记快、慢波到时
Figure 2. Examples for the process analysis of shear-wave splitting
(a)-(c)are three earthquakes recorded by station TQU，L5530 and SANJ，respectively. The first line represents the three-component waveforms. Two vertical dashed lines mark the scope of waveform shown in polarization figure. Δ represents the epicentral distance，d represents the focal depth. The second line represents the trails of particle of the cut waveform. S1 and S2 represent the arrival time of fast shear-wave and slow shear-wave，the time interval between two short lines is 0.01 s. The third line represents the waveforms of fast wave and slow wave. The two vertical dashed lines represent the arrival time of slow shear-wave and fast shear-wave in turn，respectively
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图 3 40个台站的快波偏振方向等面积极射投影与等面积玫瑰图
黑色短线的方向表示快波偏振方向
Figure 3. Equal-area rose diagrams(lower hemispherical project)of fast shear-wave polarizations at the forty stations
The direction of short line represents the polarization direction of fast shear-wave
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图 4 研究区内40个台站的快波偏振方向等面积投影玫瑰图及芦山主震、余震分布图
Figure 4. The homolographic projection rose diagrams of polarization directions of fast shear-wave at forty stations and the distribution of main shock(star) and aftershocks(black dots)in the studied area
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图 5 3个固定台站的快波偏振方向等面积投影玫瑰图
蓝色和红色分别表示芦山主震前、后的快波偏振方向
Figure 5. The homolographic projection rose diagrams of the polarization directions of the fast shear-wave for the three permanent stations
The blue and red represent the polarization directions of fast shear-wave before and after Lushan main shock，respectively
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图 6 10个台站的快波偏振方向随时间的变化
Figure 6. The temporal variation of fast shear-wave polarization directions at ten stations
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图 7 芦山地区慢波延迟时间的空间分布图
Figure 7. The spatial distribution of time delay of slow shear-wave in Lushan area
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图 8 10个台站的归一化慢波延迟时间随时间的变化
Figure 8. The temporal variation of the normalized time delay of slow shear-wave at ten stations
表 1 40个台站的快波偏振方向
Table 1 The polarization directions of fast shear-waves for forty stations
编号 台站 观测时段 快波偏振优势 方向/° 快波偏振优势 方向标准差/° 有效记录条数 1 L131 2013-04-21—2013-06-09 79 26 169 2 L132 2013-04-21—2013-06-09 55 17 151 120 25 18 3 L134 2013-04-21—2013-06-08 54 15 488 4 L135 2013-04-22—2013-05-03 58 13 229 148 28 14 5 L136 2013-04-23—2013-06-11 46 19 14 6 L5530 2013-04-23—2013-06-05 132 23 47 7 L5531 2013-04-22—2013-05-02 48 14 11 8 L5533 2013-04-23—2013-05-22 150 19 133 9 BAX 2011-02-01—2014-07-31 814 10 TQU 2009-09-01—2014-07-31 91 27 891 11 MDS 2008-05-01—2014-07-31 57 10 187 12 GZA 2008-05-01—2014-07-31 147 9 198 13 AJWA 2013-06-25—2014-07-31 75 16 10 14 BABU 2013-06-25—2014-07-31 137 14 11 57 17 6 15 CYUT 2013-06-25—2014-07-31 82 20.4 12 167 5 3 16 DACH 2013-06-25—2014-07-31 24 14 9 17 DARY 2013-06-25—2014-07-31 105 18 11 18 FXGU 2013-06-25—2014-07-31 46 10 7 94 0 1 19 HEMN 2013-06-25—2014-07-31 38 12 5 20 LCGO 2013-06-25—2014-07-31 9 21 LIES 2013-06-25—2014-07-31 100 14 19 22 LING 2013-06-25—2014-07-31 57 13 302 23 LLAN 2013-06-25—2014-07-31 87 18 95 24 LMEN 2013-06-25—2014-07-31 93 28 180 25 LOND 2013-06-25—2014-07-31 142 19 76 35 15 8 26 LUSA 2013-06-25—2014-07-31 72 19 138 27 MINL 2013-06-25—2014-07-31 178 13 40 28 MUJI 2013-06-25—2014-07-31 35 10 25 29 SANJ 2013-06-25—2014-07-31 63 30 SUSI 2013-06-25—2014-07-31 52 12 297 31 TAIP 2013-06-25—2014-07-31 24 10 82 32 TTSA 2013-06-25—2014-07-31 40 13 12 105 6 3 33 WANG 2013-06-25—2014-07-31 46 11 21 162 8 7 34 WEJI 2013-06-25—2014-07-31 5 35 WOLN 2013-06-25—2014-07-31 55 29 67 36 XIGO 2013-06-25—2014-07-31 175 12 20 37 XINL 2013-06-25—2014-07-31 50 12 5 38 YANJ 2013-06-25—2014-07-31 150 16 333 39 ZHLI 2013-06-25—2014-07-31 71 29 52 40 ZISG 2013-06-25—2014-07-31 136 26 50 
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