Seismic investigation of massive submarine landslide deposits offshore Oregon and subducting North American plate along the Puerto Rico Trench

This thesis focuses on subduction zones, where oceanic plates plunge beneath continental plates, giving rise to geohazards such as earthquakes, landslides, and tsunamis. My investigation centers on two subduction margins, introduced in Chapter 1, that have been seismically quiescent over the last 200 to 300 years: the Cascadia Subduction Zone (Chapter 2) and the Puerto Rico Trench (Chapter 3), both of which are critical to understanding and mitigating risks posed to nearby coastal communities. Chapter 2 explores large mass-transport events along the underwater slopes of the Cascadia accretionary wedge, revealing basin-scale deposits from ancient submarine landslides. Utilizing recently acquired 2D seismic reflection data from the Cascadia Seismic Imaging Experiment (CASIE21), my research maps these massive deposits in detail and investigates impacts of these events on the morphotectonic evolution of the accretionary wedge. The analysis method encompasses detailed mapping of the deposits, estimate of their dimensions, inferences about slide-motion, and determination of the timing of slope failures through integration with regional drilling data. The analysis shows 10 individual submarine landslides occurred along the wedge slopes, all within the last 1.6-Ma. The mass-movements involved 100s to >1000-km³ of material from the wedge and were likely influences on morphotectonic evolution of the wedge itself. Chapter 3 examines the Puerto Rico Trench, focusing on the incoming plate and shallow plate interface imaged by a legacy seismic reflection dataset. The 19 single-channel seismic reflection profiles, acquired during the EW9605 cruise, provide constraints on the deformation of the North American Plate as it subducts beneath the Caribbean Plate. My research highlights significant variations in shallow plate-boundary structure along the trench, which I divide into four-segments with distinct deformation patterns within the incoming plate and shallow plate-interface geometries. The findings improve our understanding of the complex geological processes at play in the Puerto Rico Trench. These chapters are thematically linked through utilization of seismic reflection imaging, shedding light on the intricate geological processes within subduction zones. The following research contributes insights which may be used in future evaluations for the safety and resilience of coastal communities along the western United States and the island of Puerto Rico.