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

Tarlow Scott H, Redondo Beach CA

Investigators

Abstract

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.