EAPSI: Modeling the Directional Dependence of Seismic Waves at the Ryukyu Subduction Zone

观测表明，地震波穿过地球上地幔的速度可能取决于地震波的方向。通过使用计算机模拟来复制这些观测结果，我们可以了解到很多关于地球内部物质特性的信息。了解这些材料的特性对于帮助科学家预测地球作为一个整体将如何变化和预测自然灾害至关重要。该奖项支持与Eh Tan博士和台湾台北中央研究院地球研究所的其他专家合作，模拟附近琉球俯冲带地震波的方向依赖关系。这种合作将使PI能够与建模俯冲带的专家以及熟悉俯冲带本身的学者合作。当一个构造板块冲到另一个构造板块之下时，就会发生俯冲，使上地幔变形，并在地幔内的矿物橄榄石中产生晶格优先取向（LPO）。地幔主要由橄榄石组成，橄榄石晶体有不同的地震速度，这取决于它们的晶体轴。因此，LPO形成各向异性结构，使地震波的速度具有方向依赖性。横波的方向依赖性被用来推断全球地幔流动，这在很大程度上与板块运动一致。然而，在俯冲带，地震各向异性观测显示出复杂的三维模式，这表明流场也必须是三维的。在琉球俯冲带，观测到的地震各向异性横向变化不能用二维地幔流动模型中各向异性结构的发育来解释。为了约束地震各向异性对板块几何形状的依赖，我们将建立一个具有现实琉球几何形状的三维地幔对流模型，并计算LPO随时间的发展。该奖项由美国国家科学基金会EAPSI与台湾科学技术部合作资助，支持一名美国研究生的研究。

Observations have shown that the speed in which a seismic wave travels through the Earth's upper mantle can depend on the wave's direction. By replicating these observations using computational simulations we can learn a great deal about the material properties of the Earth's interior. Understanding these material properties is essential in helping scientists predict how the Earth will change as a body and anticipate natural hazards. This award supports research to collaborate with Dr. Eh Tan and other experts at Academia Sinica's Earth Institute in Taipei, Taiwan to simulate the directional dependence of seismic waves at the nearby Ryukyu Subduction Zone. This collaboration will allow the PI to work with experts in modeling subduction zones and with scholars familiar with the subduction zone itself. Subduction occurs when a tectonic plate thrusts under another tectonic plate, deforming the upper mantle and generating a lattice preferred orientation (LPO) in the mineral olivine within the mantle. The mantle is mostly comprised of olivine, and olivine crystals have different seismic velocities, dependent on their crystal axis. Therefore LPO develops anisotropic fabrics causing the velocity of seismic waves to have a directional dependence. The directional dependence of shear waves is used to infer global mantle flow, which largely aligns with plate motion. However at subduction zones, seismic anisotropy observations show a complex 3-D pattern, suggesting that the flow field must also be 3-D. At the Ryukyu subduction zone, observed lateral variations in the seismic anisotropy cannot be explained by the development of anisotropic fabrics in 2-D mantle flow models. To constrain the dependence of seismic anisotropy on slab geometry, we will develop a 3-D mantle convection model with realistic Ryukyu geometry and calculate the development of LPO over time. This NSF EAPSI award supports the research of a U.S. graduate student and is funded in collaboration with the Ministry of Science and Technology of Taiwan.