Characteristics of shear-wave splitting in the crust in Shanxi region
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摘要: 通过对山西数字地震台网2000年6月—2012年12月的波形记录资料的分析， 使用剪切波分裂系统分析方法， 即SAM综合分析方法， 获得了山西地区18个数字地震台站的快剪切波偏振结果. 结果表明： 位于活动断裂上的台站的快剪切波偏振优势方向与活动断裂的走向基本一致； 个别距离断裂较远的台站的快剪切波偏振优势方向与震源机制解及GPS主压应变方向完全一致； 少数位于几条断裂交汇处的台站的快剪切波偏振优势方向则较为复杂， 与活动断裂的走向和GPS主压应变方向均不一致， 反映了该地区断裂背景和应力分布特征的复杂性.Abstract: Based on the analysis of seismic waveform data recorded by the Shanxi Digital Seismic Network from June 2000 to December 2012, the dominant polarization directions of fast shear-waves at 18 stations are obtained by use of the systematic analysis method (SAM) of shear-wave splitting in this study. The results show that for the stations located on active faults, the dominant polarization directions of fast shear-waves are mainly consistent with the strike of active faults; for individual stations further away from active faults, the dominant polarization directions are completely consistent with focal mechanism solutions and directions of regional principal compressive strains inferred from GPS data. While for a few stations located at the junction of several faults, the polarization of fast shear-wave is inconsistent with both the strike of active faults and the direction of principal compressive strain from GPS, indicating that faulting background and distribution characteristics of regional stress field are complex in Shanxi region.
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Keywords:
- Shanxi region /
- fast shear-wave /
- polarization /
- principal compressive stress /
- active fault
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图 1 山西地区测震台站与断裂分布图
① 口泉断裂； ② 六棱山北麓断裂； ③ 恒山北麓断裂； ④ 恒山南麓断裂； ⑤ 云中山山前断裂； ⑥ 五台山北麓断裂； ⑦ 系舟山北麓断裂； ⑧ 交城断裂； ⑨ 太谷断裂； ⑩ 霍山山前断裂； ⑪ 罗云山山前断裂； ⑫ 峨嵋台地北缘断裂； ⑬ 中条山北麓断裂
Figure 1. Distribution of seismic stations and faults in the Shanxi region
① Kouquan fault； ② Liulengshan northern piedmont fault； ③ Hengshan northern piedmont fault； ④ Hengshan southern piedmont fault； ⑤ Yunzhongshan piedmont fault； ⑥ Wutaishan northern piedmont fault； ⑦ Xizhoushan northern piedmont fault； ⑧ Jiaocheng fault； ⑨ Taigu fault； ⑩ Huoshan piedmont fault； ⑪ Luoyunshan piedmont fault； ⑫ Northern margin fault of Emei terrace； ⑬ Zhongtiaoshan northern piedmont fault
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图 2 两水平方向剪切波偏振图
(a) 剪切波质点运动轨迹(偏振图)， S1, S2分别表示快、 慢剪切波震动的起始位置， 以箭头示出； (b) 南北(NS)分量的剪切波波形； (c) 东西(EW)分量的剪切波波形. 横坐标上两条短竖线给出了 偏振图中剪切波的波形范围， 虚线段为剪切波的开始位置
Figure 2. Two horizontal shear-wave polarization diagrams
(a) Particle motion traces of shear-waves where S1 and S2 indicate the start points of fast shear-wave and slow shear-wave, respectively; (b) Shear waveform in NS direction; (c) Shear waveform in EW direction. In figs. (b) and (c), two vertical short bars indicate the range of shear waveforms, and vertical dotted lines indicate the start points of shear-waves
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图 3 偏振分析检验
(a) 快剪切波(F)和慢剪切波(S)经过时间延迟校正后的偏振图； (b) 快剪切波(F)波形； (c) 慢剪切波 (S)波形． 横坐标上两条短竖线表示偏振图中剪切波的波形范围， 虚线段分别表示快、 慢剪切波的初至
Figure 3. Polarization analysis and check
(a) Diagram of particle motion traces of fast shear-wave (F) and slow shear-wave (S), in which the effect of time delay has been removed. (b) Waveform of fast shear-wave； (c) Waveform of slow shear-wave. In Figs. (b) and (c), two vertical short bars indicate the range of shear waveforms, which are equivalent to move the slow shear wave ahead of one time-delay unit, and the effect of time delay is removed. Two dashed lines indicate the first arrival of fast and slow shear waves, respectively
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图 4 山西地区18个台站快剪切波偏振方向的空间分布及其等面积投影玫瑰图
红色线段表示地震台下方的平均快剪切波偏振方向，其长度代表慢波相对延迟时间的大小； 蓝色线段表示 2001年11月至2008年5月期间山西地区地震震源机制的最大主压应力方向(王凯英等，2012); 玫瑰图中台站代码后面括号中的数字为用于分析的地震事件个数； 灰色三角形表示无有效观测数据的台站
Figure 4. Spatial distribution of average fast shear-wave polarization directions and their homolographic projection rose graphs beneath 18 stations in the Shanxi region
The red lines show the average fast shear-wave polarization directions beneath the stations，the length of each line indicates the value of the slow shear-wave time-delay. The blue lines show the direction of maximum principal compressive stress axis of focal mechanism in Shanxi region during November 2001 to May 2008(after Wang et al，2012). The number in parenthesis after the station code in the rose graphs is the number of seismic events used for analysis， and the gray triangles denote the stations without effective observation data
表 1 山西数字测震台网18个台站的剪切波分裂参数
Table 1 Parameters of shear-wave splitting for 18 seismic stations of the Shanxi Digital Seismic Network
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