SUMMARY

We report finite-frequency imaging of the global 410- and 660-km discontinuities using boundary sensitivity kernels for traveltime measurements made on SS precursors. The application of finite-frequency sensitivity kernels overcomes resolution limits in previous studies associated with large Fresnel zones of SS precursors and their interferences with other seismic phases. In this study, we calculate the finite-frequency sensitivities of SS waves and their precursors based on a single-scattering (Born) approximation in the framework of travelling-wave mode summation. The global discontinuity surface is parametrized using a set of triangular gridpoints with a lateral spacing of about 4°, and we solve the linear finite-frequency inverse problem (2-D tomography) based on singular value decomposition (SVD). The new global models start to show a number of features that were absent (or weak) in ray-theoretical back-projection models at spherical harmonic degree l > 6. The thickness of the mantle transition zone correlates well with wave speed perturbations at a global scale, suggesting dominantly thermal origins for the lateral variations in the mantle transition zone. However, an anticorrelation between the topography of the 410-km discontinuity and wave speed variations is not observed at a global scale. Overall, the mantle transition zone is about 2–3 km thicker beneath the continents than in oceanic regions. The new models of the 410- and 660-km discontinuities show better agreement with the finite-frequency study by Lawrence & Shearer than other global models obtained using SS precursors. However, significant discrepancies between the two models exist in the Pacific Ocean and major subduction zones at spherical harmonic degree >6. This indicates the importance of accounting for wave interactions in the calculations of sensitivity kernels as well as the use of finite-frequency sensitivities in data quality control.

中文翻译：

---

使用SS前体对全球410公里和660公里间断点进行有限频率成像

概要

我们报告了使用边界灵敏度核对SS前驱体进行的走时测量，对全球410公里和660公里不连续面的有限频率成像。有限频率敏感度内核的应用克服了先前研究中与SS前体的大菲涅耳带及其干扰其他地震相有关的分辨率极限。在这项研究中，我们基于行波模式求和框架中的单散射（玻恩）近似来计算SS波及其前体的有限频率灵敏度。使用一组横向间隔约为4°的三角形网格点对全局不连续表面进行参数化，并且我们基于奇异值分解（SVD）解决了线性有限频率逆问题（2-D层析成像）。新的整体模型开始显示出在球谐度l> 6时，在射线理论反投影模型中不存在（或较弱）的许多特征。地幔过渡带的厚度与整体上的波速扰动密切相关规模，表明地幔过渡带横向变化的主要热成因。但是，在全球范围内未观察到410 km不连续的地形与波速变化之间的反相关性。总体而言，大陆架下的地幔过渡带比海洋地区厚约2-3 km。与使用SS前兆获得的其他全球模型相比，新的410和660 km不连续点模型显示出与Lawrence＆Shearer的有限频率研究更好的一致性。然而，在太平洋和主要俯冲带中，这两种模式之间存在明显的差异，球谐度> 6。这表明在敏感性核的计算中考虑波相互作用的重要性以及在数据质量控制中使用有限频率敏感性的重要性。