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Topic

Seismic localization and observation of tectonic tremor in the northern Cascadia subduction zone

School of Earth and Ocean Sciences

Date & location

• Wednesday, December 4, 2024
• 1:30 P.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Stan Dosso, School of Earth and Ocean Sciences, University of Victoria (Co-Supervisor)
• Dr. John Cassidy, School of Earth and Ocean Sciences, UVic (Co-Supervisor)
• Dr. Honn Kao, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Michel Lefebvre, Department of Physics and Astronomy, UVic (Outside Member)

External Examiner

• Dr. Abhijit Ghosh, Department of Earth and Planetary Sciences, University of California, Riverside

Chair of Oral Examination

• Dr. Mauricio Garcia-Barrera, Department of Psychology, UVic

Abstract

Convergent tectonic boundaries release accumulated stress on a spectrum of slip modes including large megathrust earthquakes with high societal impact, and slow slip events (SSEs), which are not felt by humans. In N. Cascadia, SSEs are regularly accompanied by seismic signals of tectonic tremor, such that the combined phenomena are referred to as episodic tremor and slip (ETS). This dissertation presents three related studies that characterize and localize tremor to better understand its generative processes and geophysical sources. I use a continuous 17-year record of tremor events to define episodes containing different numbers of events. This catalogue of episodes is used to demonstrate that major episodes (considered to represent ETS) and minor episodes exhibit distinct spatial distributions. Specifically, I provide the first observational evidence that the location of the forearc mantle corner controls whether major and minor episodes are segmented or colocated along dip. Minor episodes, which have been previously understudied, are ubiquitous along the margin but exhibit location-specific tendencies for episode sizes and recurrence intervals. To improve tremor localization, I developed the differential traveltime Bayesian inversion (DTBI) method, which prioritizes well-constrained localizations over comprehensive detection. DTBI localizes sources as 3D posterior probability distributions from which rigorous quantitative uncertainties (95% credibility intervals) are estimated. A test performed on small earthquakes supports the effectiveness of the method by relocating events within 4 km horizontally (100% of events) and 10 km vertically (90% of events) of their original locations. In addition, tremor localization exhibits less scatter than previous catalogues, indicating improved accuracy. I use the DTBI method to localize tremor sources during 20 episodes in N. Cascadia. This catalogue contains average horizontal and depth uncertainties of 4 km and 8 km, respectively. These uncertainties, which are computed for every tremor source, represent a significant improvement over previously-existing regional tremor catalogues. The enhanced constraints in the DTBI catalogue support detailed depth information that facilitates exploration of the tremor source region, which is often assumed to be the megathrust fault. I find that tremor is vertically-distributed within the deep accretionary complex up to 10 km shallower than the top of the oceanic crust (usually inferred to represent the megathrust fault). The compositional heterogeneity and structural complexity of the deep accretionary complex, as inferred from paleo-geologic studies, suggest that tremor may be generated by multiple mechanisms driven by high fluid pressures and elevated strain from subduction at deeper depths.