Upper crustal anisotropy in the eastern Himalayan syntaxis
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摘要: 本文利用雅鲁藏布江下游台阵的16个台站2016年度的近震数据，通过横波窗内的横波分裂测量，在各台站总计得到369个有效的横波分裂参数对，分析得出喜马拉雅东构造结上地壳各向异性特征。空间上，各台站的快波偏振优势方向整体上自西向东由近EW向，转为NE向，然后转向近NS或NNE向，最后转向NW向。大部分靠近或位于活动断裂带上的台站的快波偏振优势方向与断裂的走向一致，主要体现在墨竹工卡断裂上的ZOS台，雅鲁藏布江断裂上的WOL，NYG，ZIB和DOJ台站，墨脱断裂上的BEB和DEX台站，以及迫龙—旁辛断裂上的BAX和DAM台站；而距离雅鲁藏布江断裂西段和东段各有一定距离的LAD和YIG台站，以及位于雅鲁藏布江断裂东段与嘉利断裂交会处的TOM台的快波偏振优势方向与断裂走向存在一定角度，但其与喜马拉雅东构造结主压应力场方向NNE向基本一致。上地壳各向异性整体体现了结构控制和应力控制的特征，但各台站的横波分裂参数并未表现出随时间的规律变化特征，这可能与2016年研究区地震活动强度较弱有关。研究区各台站间存在较大的横波分裂参数差异和自身离散度，反映出东构造结复杂的构造特征和剧烈的变形作用。Abstract: Based on the local seismic recordings of 16 seismic stations from the lower Yarlung Zangbo River array in 2016, we present 369 valid shear wave splitting parameter pairs by analyzing shear wave particle motion in the shear wave window so as to analyze the upper crust anisotropy characteristics in the eastern Himalayan syntaxis. We analyze the spatio-temporal characteristics of the shear wave splitting parameter. The spatial distribution of fast polarization directions shows a clear change from west to east that are oriented near EW to NE, to near NS or NNE, and then to NW. The fast polarization directions of most stations are close to or locate in the active fault zone are consistent with the strikes of the faults, which are mainly reflected in the station ZOS in the Mozhugongka fault, four stations （WOL, NYG, ZIB and DOJ） in the Yarlung Zangbo River fault, two stations （BEB and DEX） in the Motuo fault, and two stations （BAX and DAM） in Polong-Pangxin fault. At the stations mentioned above, the fast polarization directions are consistent with the strikes of the faults, which demonstrates upper crustal seismic anisotropy is mainly controlled by the structure. The fast polarization directions at two stations （LAD and YIG） far away from the Yarlung Zangbo River fault and the station TOM located at the intersection of the east section of the Yarlung Zangbo River fault and the Jiali fault are inconsistent with the fault strike. However, the fast polarization directions at the above three stations （LAD, YIG and TOM） are consistent with the maximum horizontal compressive stress directions NNE in the eastern Himalayan syntaxis, which reflects that upper crustal anisotropy is also controlled by the local stress field. With regard to the temporal variation, the shear wave splitting parameters at each station do not show the regular change characteristics with time, which is related to the weak intensity of seismicity in the study area in 2016. The large difference of shear wave splitting parameters and the large dispersion of each station in the study area reflect the strong deformation and complicated structure in eastern Himalayan syntaxis.
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Keywords:
- eastern Himalayan syntaxis /
- shear wave splitting /
- crustal stress /
- active fault
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图 1 喜马拉雅东构造结区域构造（邓起东等，2002）及台站和2016年近震的分布
F1：雅鲁藏布江断裂；F2：墨脱断裂；F3：嘉黎断裂；F4：迫龙—旁辛断裂；F5：阿帕龙断裂；F6：主边界断裂；F7：墨竹工卡断裂；F8：班公错—怒江断裂。左上角为GPS计算的青藏高原地壳运动速度场（Gan et al，2007），其中黑框为研究区
Figure 1. The regional geologic setting （Deng et al，2003） and distribution of local earthquakes in 2016 andstations in the eastern Himalayan syntaxis
F1：Yarlung Zangbo River fault；F2：Motuo fault；F3：Jiali fault；F4：Polong-Pangxin fault；F5：Apalon fault；F6：Main boundary fault；F7：Mozhugongka fault；F8：Bangongcuo-Nujiang fault. Inset shows the surface velocity field determined from GPS observations in the Tibetan Plateau （Gan et al，2007），and black frame is the studied region
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图 2 BAX台站的一次近震波形记录的横波分裂分析示例
（a） 原始横波的三分量波形记录；（b） 图（a）中P波初动窗口放大后的波形记录；（c） 两水平分量的波形记录；（d） 两水平分量的横波质点运动；（e） 旋转到快、慢波方向的水平分量波形；（f） 去除各向异性校正后快、慢波水平分量；（g） 校正后的快、慢波水平分量的质点运动图；（h）用于横波分裂分析的近震震源深度分布
Figure 2. An example of shear wave splitting analysis for a recording at station BAX
（a） Three-component records of original seismic waveform；（b） Amplified three-component records of P wave in Fig.（a）；（c） Two horizontal components of seismograms；（d） Shear wave particle motion in horizontal；（e） Seismograms rotated to the fast and slow shear wave directions；（f） Corrected seismograms in fast-slow coordinate system；（g） Partical motion of the corrected phases；（h） The focal depth distribution of the earthquakes used in shear wave splitting
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图 3 喜马拉雅东构造结各台站快波偏振方向等面积投影玫瑰图分布
Figure 3. The distribution of the homolographic projection rose diagrams of the fast directions at the stations in the eastern Himalayan syntaxis
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图 4 各台站快波偏振方向和快慢波到时延迟随时间的变化图（a—d）
Figure 4. Temporal variations of the fast polarization directions and the normalized delay times at the stations （a−d）
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图 4 各台站快波偏振方向和快慢波到时延迟随时间的变化图（e—l）
Figure 4. Temporal variations of the fast polarization directions and the normalized delay times at the stations （e−l）
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图 4 各台站快波偏振方向和快慢波到时延迟随时间的变化图（m—p）
Figure 4. Temporal variations of the fast polarization directions and the normalized delay times at the stations （m−p）
表 1 各台站基本参数与横波分裂参数
Table 1 Station parameters and results of shear-wave splitting
台站代码 东经/° 北纬/° 快波偏振
方向/°快波偏振方向
标准差/°到时延迟
/（ms·km−1）到时延迟标准差
/（ms·km−1）有效记
录条数BAX 95.4 29.6 177 17 2.04 0.65 31 BEB 95.2 29.2 41 4 1.35 0.60 2 DAM 95.5 29.5 165 29 1.83 0.50 12 DEX 95.3 29.3 26 33 1.91 0.92 21 DOJ 94.8 30.0 55 25 1.57 0.68 54 LAD 93.1 29.0 1 12 2.26 0.49 27 NYG 94.2 29.1 60 9 1.07 0.39 3 QID 95.6 30.1 108 9 1.67 0.47 26 SAM 97.0 28.4 165 22 1.87 0.47 59 SCY 96.7 28.8 156 32 0.99 0.25 7 TOM 95.1 30.1 18 13 1.36 0.40 27 WOL 93.7 29.1 96 10 1.48 0.13 3 YIG 94.8 30.2 12 17 1.68 0.66 4 ZIB 94.9 29.6 38 16 0.97 0.28 60 ZOB 96.3 29.6 122 13 2.23 0.46 4 ZOS 93.3 29.8 89 21 1.61 0.32 29 
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