On the trade-off between seismic anisotropy and heterogeneity: Numerical simulations and application to Northeast Japan
On the trade-off between seismic anisotropy and heterogeneity: Numerical simulations and application to Northeast Japan
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DOI：
10.1002/2014jb011784
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发表时间：
2015-05-01
影响因子：
3.9
通讯作者：
Liu, Xin
中科院分区：
文献类型：
--
作者：
Huang, Zhouchuan;Zhao, Dapeng;Liu, Xin
P波层析成像最近已被用于研究俯冲带和大陆区域的三维方位和径向各向异性。然而，关于各向同性和各向异性结构之间的权衡的基本问题仍然不清楚。本研究采用综合试验的方法对这一问题进行了系统的研究。我们的研究结果表明，良好的射线覆盖的方位角（方位各向异性）和入射角（径向各向异性）是需要确定可靠的各向异性模型。各向同性和各向异性结构是强耦合的，并且当用于反演的射线被限制在小范围的方位角或入射角时，拖尾效应是显著的。因此，我们在每个网格节点处绘制射线方位角和射线入射椭圆，并建议使用短轴的归一化长度（即，短轴和长轴长度的比值）来定量估计射线覆盖。将我们的新方法应用于大量高质量的本地浅中深度地震的到达时间数据，我们获得了日本东北部3-D P波方位角和径向各向异性的新层析图像。方位角和径向各向异性的结果可靠地确定为研究区域的浅部，而涂抹效应是显着的地幔楔和俯冲板的较深的部分。研究结果表明，地幔楔以沟槽法向和垂直快向各向异性为主，而俯冲板片以沟槽平行和水平快向各向异性为主，表明俯冲带不同区域的动力学特征不同。
P wave tomography has been recently used to study 3-D azimuthal and radial anisotropy of subduction zones and continental regions. However, the fundamental issue about the trade-off between the isotropic and anisotropic structures is still unclear. In this study, we investigate this issue systematically with comprehensive synthetic tests. Our results indicate that good ray coverage in the azimuth (for azimuthal anisotropy) and incidence (for radial anisotropy) is required for determining reliable anisotropic models. The isotropic and anisotropic structures are strongly coupled, and smearing effects are significant when the rays used in the inversion are limited in a small range of azimuth or incidence. We therefore plot ray azimuth and ray incidence ellipses at every grid nodes and propose to use the normalized length of the short axis (i.e., the ratio of the short-axis and long-axis lengths) for estimating the ray coverage quantitatively. Applying our novel approach to a large number of high-quality arrival time data of local shallow- and intermediate-depth earthquakes, we obtained new tomographic images of 3-D P wave azimuthal and radial anisotropy in Northeast Japan. Both the azimuthal and radial anisotropy results are determined reliably for the shallow parts of the study region, whereas the smearing effects are significant in the deeper part of the mantle wedge and the subducting slab. Our results show dominant trench-normal and vertical-fast anisotropy in the mantle wedge while trench-parallel and horizontal-fast anisotropy in the subducting slab, which indicates different dynamics in different domains of the subduction zone.