Constraining Interseismic Strain Release at the Cascadia Subduction Zone with Geodetic Data

Abstract

In this dissertation, I study the style and distribution of interseismic strain release along a major convergent plate boundary to better understand how faults accommodate the relative motion between tectonic plates. Focusing on the Cascadia Subduction Zone, I utilize geodetic data, specifically in the form of Global Navigation Satellite System (GNSS) data, to measure crustal deformation and relate this deformation to slip processes along the megathrust and crustal faults. In particular, I focus on characterizing the spatiotemporal pattern of slow slip events (SSEs) across the margin and examine how conditions along the plate interface may influence the distribution of these events. I also analyze how strain is accommodated by crustal forearc faults in southern Cascadia near the Mendocino triple junction (MTJ) and consider how slip on these faults reduces the buildup of strain along the megathrust fault. The release of interseismic strain via SSEs and slip on crustal faults impacts the distribution of accumulated strain within the ‘locked zone’, a portion of the megathrust fault that is capable of producing large catastrophic earthquakes. Therefore, by characterizing SSE behavior and slip rates on crustal faults, this work provides important insights on the state of strain along the megathrust and how it relates to seismic hazards in Cascadia. Furthermore, this work serves as a case study for strain release in subduction zone systems and broadens our understanding of seismic and aseismic deformational processes within these environments. Chapter 1 provides an introduction to the themes and concepts that are discussed in subsequent chapters. In Chapter 2, I focus on a mode of strain release associated with long-duration SSEs. I conduct a systematic analysis of 13 years of GNSS time series data from 2006 to 2019 and present evidence of at least one low-amplitude long-term SSE on the Cascadia subduction zone, with the possibility of others that are less resolved. This 1.5-year transient is observed in southern Cascadia, and the data are modeled as a Mw 6.4 slow slip event occurring at 15–35 km depth on the plate interface, just updip of previously recognized short-term slow slip and tremor. The event shares many characteristics with similar long-term transient events on the Nankai subduction zone. However, the maximum horizontal surface displacements and total fault slip amplitudes for this event are an order of magnitude smaller than most other long-term SSEs at other subduction zones. Therefore, I propose that the frequency and size of long-term SSEs in Cascadia is limited by the width of the semi-frictional zone along the plate interface. In Chapter 3, I examine how plate boundary strain is partitioned across shallow crustal faults in the forearc of the Cascadia subduction zone. I construct elastic block models of the MTJ region and leverage GNSS velocity data to constrain deformation on the Little Salmon fault, Mad River fault zone (MRFZ) and Grogan fault, which are all identified as quaternary-active crustal structures. I evaluate models with various forearc fault structures and apply a bootstrap analysis to provide histograms of the long-term slip rate for each fault. Model results indicate that the Little Salmon fault zone is an important structure that accommodates both thrust and right-lateral shear motion in the forearc. Appreciable right-lateral slip rates on the MRFZ indicate that this system plays an important role in facilitating translation of the forearc and may serve as an extension of the northern San Andreas fault system. In contrast, the models place very little strike-slip motion on the Grogan fault and prefer this structure to mainly host reverse slip. The highest slip rates are observed on fault structures immediately to the north of the MTJ, indicating significant strain on the Bear River fault zone or nearby fault strands. Overall, these results help to constrain the seismic hazards associated with crustal faults in this region. In Chapter 4, I characterize strain release along the plate boundary in a special setting where slow slip and tremor are observed simultaneously during episodic tremor and slip (ETS) events. I use tremor and GNSS time series data to identify nineteen of the largest ETS events in southern Cascadia between 2016.5-2022 and document source properties. Distributed slip models for these events show that cumulative fault slip along the megathrust reaches a maximum near 40.5° N latitude and that large ETS events accommodate up to 80% of plate convergence at this location on the plate interface. However, ETS fault slip and tremor terminate farther to the south near ~40° N latitude, some 50 km before the southern lateral edge of the subducting Gorda plate. I explore possible controls on the distribution of strain release from ETS in southern Cascadia, including changes in the slab geometry and thermal gradient near the southern edge of the subduction zone. After exploring possible controls on the distribution of ETS, I propose that the heating of the downgoing slab edge and complex slab geometry inhibit ETS behavior in southernmost Cascadia. In particular, I demonstrate that seismic deformation in the form of tremor does not take place along the plate interface in the southernmost 50 km of Cascadia below 35 km depth, which is distinct from the rest of the subduction zone. In Chapter 5, I summarize the primary results and highlight the significance of my work. I also identify what future study is needed to resolve any lingering scientific questions and test outstanding hypotheses.