Shear wave splitting of local earthquakes in Weiyuan area，Sichuan
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摘要: 基于布设在四川威远地区的流动地震台站2019年11月至2020年5月记录的近震波形数据，利用剪切波分裂分析方法获得了研究区内23个地震台站的剪切波分裂参数。结果显示：威远地区地震台站快波偏振方向多数为北西向，与区域主压应力方向一致；有个别台站可能由于受到局部构造环境的影响，快波偏振方向为北东向。而在地壳速度变化明显的威远背斜附近，有6个台站结果显示为两个快波偏振优势方向，表明该地区快波偏振方向是区域应力场和局部构造环境共同作用的结果，威远地区慢波延迟时间均值为4.43 ms/km，威远背斜南侧区域的慢波延迟时间值普遍大于其北部区域，表明在威远地区南部区域的各向异性强度强于北部区域。Abstract: In this paper, the shear wave splitting parameters of 23 seismic stations in Weiyuan area, Sichuan are obtained using the shear wave splitting analysis method, base on the local earthquake waveform data recorded by the temporary seismic stations from November 2019 to May 2020 in the study area. The results show that the polarization directions of fast shear wave at most stations in Weiyuan area are northwest, which is consistent with the direction of regional principal compressive stress. And the polarization directions of a few stations are northeast, which may be affected by the local tectonic environment. Near Weiyuan anticline with obvious differences in crustal velocity, there are six stations with two dominant polarization directions of fast wave, indicating that the polarization direction of fast wave in this area is influenced by the combined effects of regional stress field and local tectonic environment. The average delay time of slow wave in Weiyuan area is 4.43 ms/km. And the delay time of slow wave in the south of Weiyuan anticline is generally greater than that in the north, suggesting that the anisotropy intensity in the south of Weiyuan area is stronger than that in the north.
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图 1 2010年以来威远地区地震M-t 图（a）和月频次（b）图
Figure 1. M-t （a） and monthly frequency （b） diagrams of the earthquakes in Weiyuan area
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图 2 威远地区构造背景、台站和2019年11月—2020年5月地震分布
F1：牛颈垇断层；F2：大垭口断层；F3：东兴场断层；F4：高桥断层；F5：资中断层；F6：墨林场断层；F7：重滩断层；F8：黄桷坡断层；F9：长岩断层，下同
Figure 2. Tectonic settings，the distribution of seismic stations and the epicenters between November 2019 and May 2020 in Weiyuan region
F1：Niujing’ao fault；F2：Dayakou fault；F3：Dongxingchang fault；F4：Gaoqiao fault；F5：Zizhong fault；F6：Molinchang fault；F7：Chongtan fault； F8：Huangjuepo fault；F9：Changyan fault，the same below
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图 3 地震波形及偏振分析示例
（a） 台站SC38记录的2020-12-06 17:05:08发生的ML2.1地震波形；（b） 台站SC12记录的2021-05-11 21:40:46发生的ML1.5地震波形；（c） 台站SC06记录的2021-04-07 11:26:42发生的ML1.9地震波形。左图为三分量地震波形，t1，t2，t3为用于偏振分析的剪切波时间段；右图为三个时段剪切波质点偏振图，S1和S2分别为快剪切波和慢剪切波的到时
Figure 3. Seismic waveforms and diagrams of the polarization analysis
（a） Seismic waves of the ML2.1 earthquake at 17:05:08 on December 6，2020 recorded by the station SC38；（b） Seismic waves of the ML1.5 earthquake at 21:40:46 on May 11，2021 recorded by the station SC12；；（c） Seismic waves of the ML1.9 earthquake at 11:26:42 on April 7，2021 recorded by the station SC06 ；and the left panels are the three-component records of seismic waveform. t1，t2 and t3 are the time periods of shear wave for polarization analysis，respectively. The right panels are the particle trails of the shear waves in three time periods. S1 and S2 are the arrival times of fast and slow shear waves，respectively.
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图 4 研究区域内23个地震台站的快波偏振方向等面积投影玫瑰图
Figure 4. The homolographic projection rose diagrams of the polarization directions of the fast shear wave splitting for the 23 seismic stations in the studied region
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图 5 各台站平均快波偏振方向空间分布
Figure 5. Spatial distribution of polarization directions of fast shear wave at seismic stations
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图 6 威远地区慢波延迟时间空间分布
Figure 6. Spatial distribution of delay time of slow shear wave in Weiyuan area
表 1 威远地区各台站剪切波分裂参数结果
Table 1 The results of shear wave splitting parameters at the seismic stations in Weiyuan area
台站
代码有效事件
个数优势方向
个数快波偏振
方向/°慢波延迟时间
/（ms·km−1）台站
代码有效事件
个数优势方向
个数快波偏振
方向/°慢波延迟时间
/（ms·km−1）SC01 34 1 −39±17.8 4.85±1.21 SC15 78 1 −42±18.3 5.23±1.72 SC02 32 1 −34±30.1 5.12±1.04 SC16 43 1 −46±15.7 4.66±1.38 SC03 5 1 −55±4.0 3.74±0.85 46 2 51±13.2 4.73±1.19 SC05 17 1 −46±20.8 3.98±1.01 SC17 35 1 −41±16.8 4.49±1.54 SC06 38 1 −49±10.9 4.13±1.28 SC19 15 1 −17±23.7 3.31±0.36 11 2 37±18.3 3.99±1.06 SC20 7 1 −39±11.7 3.41±0.71 SC07 36 1 20±14.6 4.35±1.13 SC21 6 1 −36±12.9 3.48±0.76 SC08 41 1 −46±15.3 4.76±1.42 SC22 31 1 −33±25.6 3.65±1.03 12 2 15±9.9 4.21±1.16 SC27 67 1 −48±14.1 4.87±1.20 SC09 16 1 −39±11.6 4.40±1.02 SC28 48 1 −44±18.2 4.34±0.68 7 2 25±17.5 3.89±0.88 SC30 5 1 −57±13.5 4.92±1.38 SC10 13 1 −43±16.1 3.37±1.18 SC38 53 1 −45±14.7 4.52±1.15 SC11 15 1 −31±20.5 4.24±1.61 14 2 56±11.8 4.33±1.07 SC12 24 1 −42±15.8 5.06±1.80 SC39 47 1 −36±22.3 4.37±1.06 17 2 44±13.2 4.62±1.21 
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