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

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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.