华北上地幔各向异性研究

Upper mantle anisotropy beneath North China

对华北地震科学台阵的200个宽频带和甚宽带地震台站所记录的远震SKS(SKKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和叠加分析方法求得每一个台站的SKS(SKKS)快波偏振方向和快、慢波的时间延迟,并结合已发表的固定台站的结果,获得了华北上地幔各向异性图像.从得到结果看,华北东部各向异性快波方向基本为NWW-SEE方向,而西部的快波方向转到NW-SE或NNW-SSE.快、慢波时间延迟范围是0.50～1.47s,华北西部的平均快、慢波时间延迟小于华北东部.在华北东部,快波方向与绝对板块运动(APM)方向基本一致,预示了NWW向的软流圈地幔流是引起该区域上地幔各向异性的主要原因,它使得上地幔橄榄岩等晶体的晶格优势取向沿地幔物质流动方向,从而导致了NWW趋向的快波方向.然而,在稳定的西部,快波方向既不与绝对板块运动方向一致,也不与构造走向一致,这种弱各向异性很可能是遗留在古老克拉通的厚的岩石圈内的"化石"各向异性.

Based on the polarization analysis of teleseismic SKS（SKKS） waveform data recorded at 200 broadband and very-broadband stations in the North China Seismic Array,the fast-wave polarization direction and the delay time between the fast and slow shear waves at each station were determined by using the grid searching method of minimum transverse energy and the stacking analysis method,and the published results of permanent stations also were cited,and then we acquired the image of upper mantle anisotropy in North China.The fast-wave directions are basically NWW-SEE in Eastern North China,while the fast-wave directions rotate to NW-SE and NNW-SSE in Western North China.The delay time falls into the interval [0.50 s,1.47 s].The average delay time of Western North China is less than that of Eastern North China.In Eastern North China,the fast polarization direction is consistent with the absolute plate motion（APM） direction,implying that the NWW-trending asthenospheric mantle flow driven by the APM plays a major role in observed upper mantle anisotropy beneath Eastern North China,and makes the alignment of upper mantle peridotite lattice parallel to the deformation direction,and thus generates consistently the NWW-trending fast polarization direction.However,in the stable Western Block,the weak anisotropy observed is possibly attributed to the ＂fossil＂ anisotropy frozen in the thick lithosphere of ancient cratonic NCC.