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Structure and Mechanics of the Subducted Gorda Plate: Constrained by Afterslip Simulations and Scattered Seismic Waves

Jianhua Gong, Ph.D., 2021
Jeff McGuire, Co-Advisor
Daniel Lizarralde, Co-Advisor

Subduction zones host the greatest earthquakes on earth. The largest slip in megathrust earthquakes often occurs in the 10–50 km depth range, yet seismic imaging in this region is difficult. This thesis focuses on developing methods to utilize high-frequency seismic waves scattered from the megathrust plate interface to constrain its fine-scale velocity structures and to investigate the relationship between velocity structures and megathrust slip behaviors. Chapter 2 investigates the locking condition of the subducted Gorda plate by simulating afterslip caused by the stress changes from offshore strike-slip earthquakes. Chapter 3 develops array analysis methods to identify P-to-S and S-to-P converted phases converting at the subducted Gorda plate interface from local earthquakes and uses them to constrain the geometry and material properties of the plate-boundary fault between 5–20 km depth. Chapters 4 and 5 use a dense nodal array and numerical simulations to study the guided waves propagating along the thin low velocity layer at the boundary of the subducted Gorda plate. Taken together, our results indicate that material properties of the subduction plate-boundary fault is highly heterogeneous and the plate-boundary fault is potentially contained in a low-velocity layer with significant porosity and fluid content at seismogenic depths.