This dissertation focuses on the seismic constraints on the Alaska Peninsula section of the Alaska subduction zone and the central part of the Tonga subduction zone, leveraging the latest onshore and offshore seismic station deployments. I utilized multiple seismic body-wave tomography methods to build 3D P-wave (Vp), S-wave (Vs), and compressional to shear wave ratio (Vp/Vs) models at different scales, providing comprehensive insights into key geological processes in subduction zones.Chapter 1 provides an overview of the geologic background and outlines the motivations for investigating the subduction zone structures at the Alaska Peninsula and Tonga. This chapter highlights the distinct regional tectonics and earthquake distributions that characterize these areas, setting the stage for the detailed analyses in the following chapters. Chapter 2 focuses on along-strike variations of the megathrust slip behaviors and structural heterogeneities at the Alaska Peninsula. I present 3D Vp and Vp/Vs models of the Alaska Peninsula to 50 km depth, derived from body-wave double-difference tomography using a newly acquired arrival-time dataset from the Alaska Amphibious Seismic Experiment (AACSE). This chapter also introduces a new slip deficit model obtained from geodetic modeling that is compared with seismic tomography images. The inferred fluid content from the Vp/Vs model reveals distinct differences at the plate interface and in the overriding plate across the Shumagin, Chignik, Chirikof, and Kodiak segments. High fluid content is inferred at both the plate interface and in the overriding plate at the Shumagin segment, where a low slip deficit is observed. In contrast, low fluid content is inferred at the plate interface and in the overriding plate at the Chignik and Chirikof segments, which display moderate-to-high slip deficit. In the Kodiak segment characterized by the highest slip deficit, the plate interface is fluid-rich while the overriding plate is relatively dry. The notable anti-correlation of the fluid content in the overriding plate and slip deficit distribution highlights the critical role of the overriding plate in controlling along-strike variations of slip behaviors in the Alaska Peninsula. Chapter 3 focuses on the deep part of the Alaska subduction from ~50 to 700 km depths, including the mantle wedge and the subducted slab in the upper mantle. I introduce two sets of 3D velocity models of the Alaska Peninsula at different scales: large-scale Vp and Vs models extending to 700 km, and local-scale Vp and Vp/Vs models focusing on structures within the top 200 km. The large-scale models reveals slab gaps in the mantle transition zone (MTZ) beneath the Chirikof segment, while the local-scale models exhibit strong along-strike variations, correlating with seismic activity and sub-arc melting controlled by slab dehydration. The multi-scale velocity models presented in Chapter 3, together with those from Chapter 2, provide a comprehensive investigation of structural variations in the Alaska Peninsula. They offer valuable insights into a variety of subduction-related processes, ranging from shallow ones including megathrust and interseismic slip to deeper processes such as sub-arc melting and slab deformation in the MTZ. Chapter 4 investigates the deep slab morphology and mantle wedge structures in the Tonga subduction zone. Two sets of Vp and Vs models were developed using arrival-times from manual picks and machine learning detection, showing notable consistency. The subducted Pacific Plate flattens out in the MTZ north of 19° S, while it penetrates the 660-km discontinuity south of 19° S. A slab tear at ~19° S implies strong slab deformation in the MTZ. Sub-slab low-velocity anomalies below 300 km suggest the presence of subducted asthenosphere or the influence of the Samoan plume from the north. The deep slab morphology and mantle structures revealed by body-wave tomography provide crucial insights into the complex tectonic processes in the Tonga subduction zone, where significant back-arc spreading and plume interactions are involved.
Read
