按方位叠加接收函数分析青藏高原东南缘的地壳各向异性

在各向异性地壳中,来自Moho的P-to-S转换波(Pms)的到时不仅取决于入射角和地壳厚度,而且还随地震事件方位角而变化.地处青藏高原东南缘的川滇地区,地壳变形十分强烈.本文利用川滇地区的108个固定台站记录的远震三分量地震波形数据提取台站下方的P波接收函数,并把接收函数被校正到了同一参考震中距处(例如67°).然后按后方位角10°为间隔将接收函数叠加成一道信号以增强信噪比,并从叠加信号里拾取不同后方位角对应的Pms相的观测到时.在快波极化方向和分裂时间构成的解的平面上,能使观测到时与理论到时之差最小的点即为所求的分裂参数的位置.合成地震图和实际观测数据的实验表明,这个方法不但稳定性较好,而且误差估计也较小.我们从108个台中获得了96个Pms相的分裂参数,结果表明,川滇地区地壳各向异性十分强烈,Pms相分裂时间在0.05s±0.06s到1.27s±0.10s之间,平均值为0.54s±0.12s.地壳各向异性的快波极化方向与地表GPS速度场的差异性表明,印支块体的上下地壳之间是解耦的,而川滇菱形块体北部、松藩—甘孜和四川盆地的上下地壳之间是耦合的.然而,川滇菱形块体南部,地壳变形主要受控于小江断裂和金沙江—红河断裂.

Analysis for crustal anisotropy beneath the southeastern margin of Tibet by stacking azimuthal receiver functions

The continental collision between India and Eurasia in the Cenozoic has resulted in significant crustal shortening across Asia and uplifting of the Tibetan Plateau. Lithosphere that has undergone deformation may record this process in the form of fabric such as foliation and lineation from ductile deformation, as well as non-horizontal interfaces between materials with different properties within the crust or at the bottom of the crust (Moho). As a proxy for deformation, seismic anisotropy plays an important role in constraining the mode and location of the Earth's deformation. Crustal seismic anisotropy has been reported to exist in many active tectonic regions such as the southeastern margin of Tibet, where the crustal thickness almost increases to the twice from the southern Yunnan to the Songpan-Garzê (SG) fold system and the northern part of the Sichuan-Yunnan diamond-shaped block (SYDSB). In general, seismic anisotropy in the Earth's upper crust is caused by stress-induced alignment of cracks, while it in the lower crust and mantle is usually attributed to strain-induced lattice-preferred orientation of the minerals in the crust and mantle. In the southeastern margin of Tibet, besides of crustal thickening, distinctly different mechanisms have been suggested to accommodate the huge convergences caused by the continental collision between India and Eurasia. Since the early 1990s, some core shear phases, such as SKS and SKKS are widely used to probe the mantle anisotropy, leading to seismic anisotropy being observed in many tectonic domains. However, the splitting could be induced by one or more anisotropic layers anywhere along the ray path between the locations of the shear waves generated and received, SKS/SKKS phases splinting have excellent lateral resolution but limited vertical resolution. Unlike the SKS/SKKS phases converted at the CMB, the Moho P-to-S phase (Pms) is converted at the crust-mantle boundary; therefore, the source region inducing anisotropy can be exactly confined within the crust. Thus, the Pms splitting provides an opportunity to isolate the anisotropy of the crust from that of the deep mantle, and also gives clues as to the deformations within the whole crust in the past and/or present. Within anisotropy crust, the arrival time of P-to-S conversion at Moho (Pms) not only depends on incident angle and crustal thickness, but also on the azimuth of seismic event. The crustal deformation beneath Sichuan and Yunnan, which is located at the southeastern margin of Tibet, is very strong. In this study, the 3-components teleseismic data, which is recorded at 108 stations located in Sichuan and Yunnan provinces, is used to extract P receiver functions, and these P receiver functions with different epicentral distance are moveout corrected to a reference distance of 67°. Then, in order to enhance signals-to-ratio, the P receiver functions are stacked in 10° bin along the back azimuth direction so that the observation arrival time of Pms can be picked up in the stacked trace corresponding different back-azimuth. On the solution surface composed of splitting time and fast orientation, the expected splitting parameters are located at the point which minimizes the difference between observation and theoretic arrival time of Pms. The experiments on synthetic and real waveforms confirmed that this approach is stable and significantly reduces uncertainty. We obtained 96 splitting parameters of Pms phase from 108 stations, the results indicated that the anisotropy in crust is very strong in Sichuan and Yunnan region, and the splitting time of Pms phase varies from 0.07 s±0.07 s to 1.27 s±0.10 s,with an average of 0.54 s±0.12 s. The comparison between GPS vectors and fast orientations indicates that the upper crust is decoupled from lower crust beneath Indochina block, but that it is coupled on the northern SYDSB, SG fold system and Sichuan basin. However, on the southern SYGSB, the crustal deformation is primarily controlled by Xiaojiang fault and Jinshajiang-Red River fault.