In situ Seismic Anisotropy in the Source Region of Global Deep Earthquakes

深度超过60公里的地震发生在俯冲带的下行岩石圈中，称为板块。根据它们发生的深度范围，它们被称为中等深度地震（可能对人类社会造成破坏）或深源地震。解释这些事件的发生是非常具有挑战性的，因为正常的地幔岩石变得具有韧性，原则上，这应该抑制地震的发生。然而，全世界超过20%的地震发生在60公里或更深的地方。此外，已经观察到，大部分深源地震辐射的地震波不同于从浅源地震的双力偶机制所预期的地震波。这些证据往往导致人们相信，深源地震的物理性质与浅源地震有着根本的不同。本研究的研究目标是通过考虑发生深源地震的俯冲洋板块的地震结构来了解深源地震及其地震辐射的物理学。 本项目的重点是阐明为什么许多深源地震在矩张量震源分析结果中显示出很大的非双偶分量。震源机制解常被用来推断重要地震的震源机制。具体而言，本研究测试以下精心制定的假设为一个特定的俯冲带：可以解释在该地区所包含的所有地震中观察到的非双力偶分量，而不需要调用异常震源过程的板内的一个小体积中的均匀的地震各向异性层的存在？这种各向异性是预期的，因为由于板下沉，在板界面处应该发生显著的剪切。首先，本研究利用正演模拟，验证了利用地震辐射图像直接反演地震周围各向异性的方法。然后分析了汤加、马里亚纳、日本、千岛群岛、南美洲和爪哇等6条全球俯冲带的22个深震群。其次，对于22组中的每一组，研究人员都反演了一个共同的各向异性。初步结果表明，原位层压各向异性织物（即，倾斜的横向各向同性）的地震周围可能会产生所观察到的非双力偶分量的地震辐射图案使用传统的双力偶机制，由于断层。在所有俯冲板片中，无论深度如何，倒置的各向异性组构都平行于俯冲板片界面。典型的反向剪切波各向异性很大，约25%，在世界各地的板。推断的各向异性的一致性相对于板的几何形状和大的各向异性值在许多俯冲带填补了一个重要的知识空白板的内部结构所产生的应力和化学变质作用。这些新的结果可以提供一个重要的协同联系，为广泛的深板片过程中了解板片动力学和板块构造。

Earthquakes deeper than ~60km occur at subduction zones in the downgoing lithospheres known as slabs. Depending on the depth range at which they occur, they are known as either intermediate-depth, which can be destructive to human society, or deep-focus earthquakes. It is very challenging to explain the occurrence of these events since normal mantle rocks become ductile and this, in principle, should inhibit the occurrence of earthquakes. Yet more than 20% of the earthquakes worldwide occur at 60 km or larger depths. Moreover, it has been observed that a large proportion of deep earthquakes radiate seismic waves differently than that expected from the double-couple mechanism of shallow earthquakes. These lines of evidence often lead to believe that the physics of deep earthquakes are fundamentally different from that of shallow earthquakes. The research goal of this study is to understand the physics of deep earthquakes and their seismic radiation by taking into account the seismic structure of the subducting oceanic slab where they occur. The focus of this project is to elucidate why many deep earthquakes show a large non-double-couple-components in the results of moment tensor source analysis. The source focal mechanism solution is often used to infer important earthquake source mechanics. Specifically, this study tests the following well-formulated hypothesis for a particular subduction zone: could the presence of a uniform seismic anisotropic layer in a small volume inside the slab explain the non-double-couple component observed in all earthquakes contained in that region without the need of invoking anomalous earthquake source processes? Such anisotropy is expected since significant shearing should occur at the plate interface due to slab sinking. First, by using forward modeling, this study validates the approach of using earthquake radiation patterns to directly invert for in situ anisotropy around the earthquakes. Then 22 deep-earthquake groups in six global subduction zones: Tonga, Mariana, Japan, Kurile, South America, and Java subduction zones are analyzed. Second, for each of the 22 groups, the researchers invert for a single common anisotropy. Preliminary results show that the existence of the in situ laminated anisotropic fabric (i.e., tilted transverse isotropy) around the earthquakes could produce the observed non-double couple component in earthquake radiation pattern using earthquakes with conventional double-couple mechanisms due to faulting. In all subducting slabs, the inverted anisotropic fabric is parallel to the subducting slab interfaces, regardless of depth. The typical inverted shear-wave anisotropy is large, ~25%, in slabs worldwide. The consistency of the inferred anisotropy with respect to the slab geometry and large anisotropy values across many subduction zones fill an important knowledge gap regarding the slab internal structure produced by stress and chemical metamorphism. These new results can provide a critical synergistic link for a wide range of deep slab processes in understanding slab dynamics and plate tectonics.