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取决于地震波的方向。通过使用计算模拟复制这些观测结果，我们可以了解大量有关地球内部物质属性的信息。了解这些物质特性对于帮助科学家预测地球作为一个物体将如何变化和预测自然灾害至关重要。该奖项支持与台湾台北中科院地球研究所的Eh Tan博士和其他专家合作，模拟附近琉球俯冲带地震波的方向相关性。这一合作将使PI能够与俯冲带建模专家和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，使上地幔变形，并在地幔内的矿物橄榄石中产生晶格择优取向(LPO)时，就会发生俯冲。地幔主要由橄榄石组成，橄榄石晶体根据其晶轴的不同具有不同的地震速度。因此，LPO形成各向异性组构，导致地震波速度具有方向性。剪切波的方向相关性被用来推断全球地幔流动，这在很大程度上与板块运动相一致。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的横向变化不能用二维地幔流动模型中各向异性组构的发展来解释。为了约束地震各向异性对板块几何形状的依赖关系，我们将发展一个具有真实琉球几何形状的三维地幔对流模式，并计算LPO随时间的发展。这一NSF EAPSI奖项支持一名美国研究生的研究，并与台湾科技部合作提供资金。

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.