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

俯冲板片强地震各向异性导致的剪切波分裂预测的验证

• 批准号: 2027150
• 负责人: Yingcai Zheng
• 金额: $ 31.41万
• 依托单位: University of Houston
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-09-01 至 2024-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2027150&HistoricalAwards=false
• 关键词: Verification; predicted; shear; wave; splitting

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.

在日本、阿拉斯加和华盛顿发现的最大震级的地震都发生在俯冲带。在这些地区，一个构造板块潜入另一个板块之下的地幔。当两个板块相互滑动和摩擦时，浅源地震--以及发生在深度大于~60 km（最深可达~700 km）的深源地震--会沿着下行板块沿着产生。深源地震发生的机制仍然不清楚，并正在争论中。在这里，研究人员分析了日本俯冲带深地震产生的地震波。地震波通过地幔和地壳传播，携带着关于震源（地震）和岩石特性的信息，沿着它们的路径沿着到达地表。该团队专注于俯冲板块的属性，即其各向异性。地幔中的流动使本构岩石变形，从而形成岩石组构，如晶体择优取向和排列的包裹体。当岩石呈现出强组构时，其地震波传播特性往往表现出一定的各向异性。这意味着它们取决于岩石内波传播方向的取向。 通过分析地震剪切波的特征，并将其与模型预测进行比较，研究人员推断了俯冲板块界面的岩石组构位置。它们逐渐揭示了俯冲带的结构和动力学。 该项目为休斯顿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。地震波各向异性不仅影响地震波辐射图像，而且影响横波分裂。在这里，研究小组使用深地震辐射模式（又名矩张量）来绘制板层内部的高分辨率各向异性。研究人员还通过对通过各向异性板传播的剪切波的分裂模式进行向前预测，验证了推断的各向异性的存在。 然后将预测与观察进行比较。他们使用传播矩阵方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，上述各向异性俯冲板的地震台站，预测的快速S偏振可以是沟槽平行或沟槽垂直的S波的入射角的依赖，也已观察到在地震数据的模式。研究结果为认识俯冲板片各向异性岩石组构和深源地震提供了新的思路。他们还为地球动力学、矿物学、地球化学和岩石学提供了一个关键元素，以研究所观察到的组构的性质。该奖项反映了NSF的法定使命，并被认为是值得支持的，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。

项目成果

Episodic Magma Hammers for the 15 January 2022 Cataclysmic Eruption of Hunga Tonga‐Hunga Ha'apai

• DOI: 10.1029/2023gl102763
• 发表时间: 2023-04
• 期刊: Geophysical Research Letters
• 影响因子: 5.2
• 作者: Yingcai Zheng;Hao Hu;F. Spera;M. Scruggs;G. Thompson;Yuesu Jin;T. Lapen;S. McNutt;K. Mandli;Zhigang Peng;D. Yuen

Earthquake Stress Drop for a Circular Crack in an Anisotropic Medium

各向异性介质中圆形裂纹的地震应力降

• DOI: 10.1785/0120220075
• 发表时间: 2022
• 期刊: Bulletin of the Seismological Society of America
• 影响因子: 3
• 作者: Tang, Shuhang;Zheng, Yingcai;Zhou, Hua-Wei;Hu, Hao
• 通讯作者: Hu, Hao