Verification of predicted shear wave splitting due to strong seismic anisotropy in subducting slabs

在日本、阿拉斯加和华盛顿发现的最大震级地震发生在俯冲带。在这些地区，一个构造板块潜入另一个板块下面的地幔。当两个板块相互滑动和摩擦时，浅层地震——以及发生在深度大于~60公里（深度可达~700公里）的深地震——沿着下沉的板块产生。深层地震发生的机制仍然不清楚，而且还在争论中。在这里，研究人员分析了日本俯冲带深处地震产生的地震波。地震波穿过地幔和地壳，沿着地震波到达地表的路径携带着有关震源（地震）和岩石性质的信息。研究小组关注的是俯冲板块的性质，即其各向异性。地幔中的流体使本构岩发生变形，形成晶体优先取向和排列包裹体等岩石结构。当岩石呈现出坚固的结构时，其地震波传播的特性往往表现出一些各向异性。这意味着它们取决于岩石内部波传播方向的方向。通过分析地震横波的特征，并将其与模型预测结果进行比较，研究人员推断出俯冲板块界面岩石结构的位置。它们逐渐揭示了俯冲带的结构和动力学。该项目为休斯敦大学的两名研究生和一名博士后提供支持和培训。它还为K-12学生和公众提供教育推广。地震各向异性既会影响地震的地震波辐射模式，也会影响横波分裂。在这里，研究小组使用深地震辐射模式（又名矩张量）来绘制高分辨率的板内各向异性。研究人员还通过对横波在各向异性板上传播的分裂模式进行正演预测，验证了推断各向异性的存在。然后将预测与观测结果进行比较。他们使用传播矩阵法和三维全波各向异性弹性有限差分代码进行预测。初步结果表明，在各向异性俯冲板上的台站，根据S波入射角的不同，预测的快S极化可以是槽平行的，也可以是槽垂直的；在地震资料中也观察到这种模式。工作成果对认识俯冲板块各向异性岩石组构及深部地震有新的启示。它们还为地球动力学、矿物学、地球化学和岩石学研究观察到的织物的性质提供了关键的元素。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The largest magnitude earthquakes such as those found in Japan, Alaska, and Washington, occur at subduction zones. In these areas, one tectonic plate dives into the Earth’s mantle underneath another plate. As the two plates slip and grind against each other, shallow earthquakes - as well as deep earthquakes occurring at depths greater than ~60 km (and up to ~700-km depths) - are produced along the down-going slab. The mechanism by which deep earthquakes occur is still unclear and being debated. Here, the researchers analyze seismic waves produced by deep earthquakes in a subduction zone in Japan. Seismic waves travel through the mantle and crust, carrying information about their sources (the earthquakes) and the properties of rocks along their paths to the surface. The team focuses on the properties of the subducting slab, namely its anisotropy. Flows in the Earth’s mantle deform the constitutive rocks which can develop rock fabrics such as crystal preferred orientations and aligned inclusions. When a rock presents a strong fabric, its properties for the propagation of seismic waves often show some anisotropy. It means that they depend on the orientation of the wave propagation direction within the rock. By analyzing the characteristic of seismic shear waves, and comparing them with modeled predictions, the researchers infer the location of the rock fabric for the subducting plate interface. They gradually unveil the structure and dynamics of subduction zones. This project provides support and training for two graduate students and a postdoctoral associate at University of Houston. It also provides educational outreach to K-12 students and the public. Seismic anisotropy can influence both earthquake seismic-wave radiation patterns and shear wave splitting. Here, the team uses deep earthquake radiation patterns (aka moment tensors) to map for high-resolution anisotropy inside the slab. The researchers also verify the existence of the inferred anisotropy by making forward predictions for the splitting pattern of a shear wave propagating through the anisotropic slab. Predictions are then compared with observations. They make the predictions using a propagator-matrix method and a 3D full-wave anisotropic elastic finite-difference code. Preliminary results show that for a seismic station above an anisotropic subducting slab, the predicted fast-S polarization can be either trench-parallel or trench-perpendicular depending on the incident angle of the S wave; a pattern which has also been observed in seismic data. The work outcomes shed new light on the understanding of the anisotropic rock fabric in the subducting slab and the deep earthquakes. They also provide a pivotal element for geodynamics, mineralogy, geochemistry, and petrology in their study of the nature of the observed fabrics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.