I. Dynamics of Subduction Initiation and II. Constraining Sedimentary Basin Structure with Seismic Ambient Noise

Citation

Li, Yida (2024) I. Dynamics of Subduction Initiation and II. Constraining Sedimentary Basin Structure with Seismic Ambient Noise. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/y3z2-z538. https://resolver.caltech.edu/CaltechTHESIS:05302024-182513713

Abstract

Subduction initiation, the inception of a subduction zone, heralds dramatic changes in tectonic plate kinematics and dynamics. In the first half of the thesis, I focus on understanding the dynamics of the subduction initiation process through a synthesis of numerical computations and theoretical frameworks. In Chapter 2, we employ force balance analysis and 2D geodynamic models to yield an analytical solution on the force evolution of the subducting plate. This formulation illuminates a pivotal phase in subduction initiation —- the compression-to-extension transition of plate forces -— as a defining milestone. In Chapter 3, we extend this analytical framework into a sliced 3D context (2.5D) while incorporating the influence of strike-slip motion. Modified from Chapter 2, the analytical solution validates that strike-slip motion facilitates subduction initiation by accelerating the process of weakening. Chapter 4 ventures into 3D geodynamic modeling, focusing on the Puysegur trench -— a living example of subduction initiation. The models demonstrate a capability to match multiple geophysical and geological observations quantitatively with mechanical models. With a parametric search, we discover the best-fitting models require a relatively fast strain weakening rate, which can be explained by pore-pressure weakening at shallow depths and grain-size reduction at greater depths.

The second part of this thesis transitions to ambient seismic noise correlation. In Chapter 5, we conduct an ambient noise tomography in northern Los Angeles basins with a newly obtained, dense seismic data set. The new shear wave velocity model exhibits a lower velocity in the basins than previous community models, which can potentially resolve the inconsistency between observed and calculated ground motions. In Chapter 6, we introduce a new method to identify the near-field noise sources from the spurious arrivals in ambient noise correlations. The correlation between the inverted noise sources and geological features in northern LA basins suggests the viability of this technique as a novel means of identifying geological structures, including faults.

Thesis Files