南迦巴瓦地区近震剪切波分裂研究

Study on shear wave splitting of local earthquakes in Namche Barwa

南迦巴瓦地区位于喜马拉雅山脉构造结东端,地壳各向异性具有其特殊性和复杂性.本研究收集了布设在南迦巴瓦地区的流动地震台站和固定台站于2017—2019年记录的近震波形数据,利用剪切波分裂偏振分析方法得到了研究区内10个地震台站的剪切波分裂参数快波偏振方向和慢波延迟时间,分析了剪切波分裂参数在空间上和时间上的分布,探讨了南迦巴瓦地区上地壳地震各向异性特征,以及2017年11月18日米林MS6.9地震发生后地壳应力状态的变化.结果显示,南迦巴瓦地区快波偏振优势方向有两个,即北西向和北东向.位于米林断裂、嘉黎断裂、墨脱断裂和雅鲁藏布江断裂附近的台站,快波偏振方向基本平行于断裂走向.研究区域上地壳各向异性方向受区域应力场的控制,另外还受到断裂构造的影响.南迦巴瓦地区慢波延迟时间均值为4.13 ms·km^(-1).距离米林地震主震震中位置较近的L0230台站慢波延迟时间较大.本研究没有观测到快波偏振方向在时间上的规律性变化.L0230台站的慢波延迟时间在米林MS6.9地震之后表现出明显的降低,反映了大地震之后地壳应力的释放与调整.

Namche Barwa is located in the eastern end of the Himalayan tectonic knot,and the anisotropy in the crust has its particularity and complexity.In this study,the local seismic waveform data recorded by the temporary and permanent seismic stations in the Namche Barwa area from 2017 to 2019 were collected.The shear wave splitting parameters including the polarization direction of fast shear wave and delay time of slow shear wave from 10 seismic stations in the study area were obtained by using the polarization analysis method of shear wave splitting.The spatial and temporal distribution of shear wave splitting parameters of seismic stations was analyzed.The anisotropic characteristics of the upper crust in the Namche Barwa area,and the characteristic changes of stress field after the Mainling MS6.9 earthquake on November 18,2017 were discussed.The results show that there are two dominant polarization directions of fast shear wave in the Namche Barwa area,which are northwest and northeast.The polarization directions of fast waves at the seismic stations near the Mainling fault,Jiali fault,Motuo fault and Yarlung Zangbo river fault,are basically parallel to the strike of the faults.The anisotropic direction of the upper crust in the study area is controlled by the regional stress field,and is also affected by the fault structure.The average value of the delay time in the Namche Barwa area is 4.13 ms·km^(-1).The delay time at seismic station L0230,which is close to the epicenter of main shock of Mainling MS6.9 earthquake,is relatively large.In this study,the polarization direction of fast shear wave does not show regular variations.The delay time of slow wave at station L0230 shows a significant decrease after the Mainling MS6.9 earthquake,which reflects the release and adjustment of the stress in the crust after the strong earthquake.