The S-velocity structure of the East Asian mantle from inversion of shear and surface waveforms
The S-velocity structure of the East Asian mantle from inversion of shear and surface waveforms
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DOI：
10.1046/j.1365-246x.2003.01869.x
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发表时间：
2003-04
影响因子：
2.8
通讯作者：
W. Friederich
中科院分区：
文献类型：
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作者：
W. Friederich
摘要利用地震横波和面波的全波形研究了东亚800公里深度以下的地幔结构。震中距离限制在40°以内。与以往的波形反演方法不同，我们避免了射线理论或路径积分方法。取而代之的是，我们使用(1)准确的3-D波形灵敏度核，它正确地反映了偏离路径的灵敏度和菲涅尔区的存在。我们应用了(2)基于耦合模式、多重前向散射方法的精确的三维正演模拟技术，允许我们(3)通过几个三维模型迭代反演过程，以及(4)评估三维模型的数据和合成材料之间的真实失配。在路径覆盖率较好的地区，模式的平均横向分辨率在整个上地幔为400公里。在100~250公里的深度范围内，横向分辨率甚至接近200公里。由于考虑了波前愈合，速度扰动的幅度比其他层析模型要大。此外，波形敏感核提供了模型的内在物理平滑，稳定了反演。最后，由于使用了精确的灵敏度，使用一组给定的地震记录可以获得更好的分辨率。模型的主要特征是西太平洋俯冲带上的高速俯冲板块停滞在660公里的不连续处，西太平洋弧后地区80-250公里深度之间的强低速带，贝加尔裂谷带南端下方延伸到过渡带的羽状低速特征，天山下连接过渡带的低速区，以及西藏下方厚的地壳，最大厚度约为80公里，接近35度纬线。藏南岩石圈地幔的速度非常快，表明印度岩石圈处于俯冲状态。相反，西藏北部的上地幔从莫霍面到660公里的不连续面呈现出中慢值。在塔里木盆地地下再次观测到很高的S速度。藏南高速地幔岩石圈与青藏高原西端喀喇昆仑山脉下方的塔里木连接。
SUMMARY The structure of the mantle beneath East Asia down to 800 km depth is investigated using full waveforms of seismic shear and surface waves. Epicentral distances are limited to less than 40°. In contrast with previous waveform inversions, we avoid ray-theoretical or path-integral approaches. Instead, we use (1) exact 3-D waveform sensitivity kernels that correctly reflect off-path sensitivity and the existence of Fresnel zones. We apply (2) an accurate 3-D forward modelling technique based on a coupled-mode, multiple-forward-scattering approach, allowing us (3) to iterate the inversion procedure through several 3-D models and (4) to evaluate the true misfit between the data and the synthetics for the 3-D model. Average lateral resolution of the model in regions with good path coverage is 400 km throughout the upper mantle. In the depth range from 100 to 250 km the lateral resolution even approaches 200 km. Since wave front healing is taken into account, the amplitudes of velocity perturbations are larger than in other tomographic models. Moreover, the waveform sensitivity kernels provide an intrinsic physical smoothing of the model, stabilizing the inversion. Finally, owing to the use of exact sensitivities, better resolution can be achieved with a given set of seismograms. Major features of the model are high-velocity subducting slabs along the West Pacific subduction zones stagnating at the 660 km discontinuity, a strong low-velocity zone between 80 and 250 km depth in the West Pacific backarc regions, a plume-like low-velocity feature beneath the southern tip of the Baikal rift zone extending into the transition zone, a low-velocity region under the Tien Shan with connection into the transition zone and thick crust under Tibet reaching its maximum thickness of approximately 80 km close to the 35th parallel. The lithospheric mantle underneath southern Tibet is very fast indicating underthrusting of the Indian lithosphere. In contrast, the upper mantle beneath northern Tibet exhibits average-to-slow values from the Moho down to the 660 km discontinuity. Very high S velocities are again observed beneath the Tarim Basin. The high-velocity mantle lithospheres under southern Tibet and the Tarim join beneath the Karakorum range at the western tip of the Tibet Plateau.