Geophysical and geomorphological signatures of accretionary wedge faulting at the Cascadia subduction zone

Abstract

For thousands of years, people on Earth have been living with earthquakes and tsunamis – in both harmony and destruction. Millions of people living along the active western margin of the North American continent are at risk of potentially devastating earthquakes and tsunamis. As geoscientists, we must do more than simply increase our understanding of the hazards posed to society by active tectonic margins – we must also effectively communicate our scientific findings with the general public and work together with communities to adapt to coastal hazards and change. Here I present my dissertation research that applies a range of geophysical and geomorphological analysis methods to modern high-resolution geophysical datasets of the offshore Cascadia accretionary wedge to better understand earthquake and tsunami hazards at the Cascadia subduction zone. In Chapters 1-3, I find that accretionary wedge faults leave a distinct geophysical and geomorphic signature within the subsurface and at the seafloor. My work provides new insights into the tectonic evolution of the Cascadia margin and the likelihood for future fault rupture behavior and tsunamigenesis. In Chapter 1, using modern seismic reflection and multibeam bathymetric datasets of varying resolutions, I find that there is no evidence for an active margin-spanning megasplay fault at the Cascadia subduction zone. Instead, I find that out-of-sequence accretionary wedge splay faulting is highly variable and segmented along strike, consistent with the segmentation of other physical and mechanical properties at the Cascadia subduction margin. Building from regional fault mapping completed in Chapter 1, in Chapter 2, I use ultra-high-resolution sub-bottom seismic and multibeam bathymetric data to map the Albatross fault system, a system of en echelon Holocene-active right-lateral strike-slip faults spanning 45 kilometers from 45-46°N offshore Oregon. Applying a Riedel shear model, I propose that oblique plate convergence at the central Cascadia subduction zone is accommodated through distributed shearing on multiple conjugate strike-slip faults within the accretionary wedge. In Chapter 3, I apply subaerial topographic analysis tools to submarine topography of the offshore Cascadia accretionary wedge. I find that the longitudinal profiles of submarine channels respond to active subsurface faulting characterized in Chapters 1 and 2. This finding enables me to improve upon and extend previous fault mapping based on the morphologic expression of faulting in submarine channels and at the seafloor. To conclude, in Chapter 4, I share my experience applying the Riverways Model to engage with the Quileute Tribal School and the Quileute Tribe of La Push, Washington through the Quileute Culture & geoScience Exchange Program. I explore how this work contributes to increasing coastal community resilience to earthquakes and tsunamis through building educational partnerships between scientific and Indigenous communities and creating place-based and culturally sustaining K-12 geoscience curriculum. Overall, through my dissertation work, I find that we can become more resilient as a society when we break down barriers (e.g., cultural, geographical, institutional) between scientists and the broader community. It is my mission, through this dissertation and in my career ahead, to continue working towards this goal; recognizing that one person has the power to influence the actions of others towards protecting life along Earth’s active tectonic margins.