Simon Peacock

British Columbia and Washington state encompass the northern extent of the Cascadia subductionzone and the southern termination of the Queen Charlotte Transform fault. Convergencebetween the North American plate and oceanic Juan de Fuca (JdF), Explorer, and Pacificplates is accommodated by subduction, underthrusting, and transform faulting and producesabundant seismicity that is unevenly distributed along the margin. This thesis probes severalearthquake-dense regions in Cascadia that include a unique cluster of seismicity south of TexadaIsland in BC, the highly active Puget Lowlands in Washington state, and the offshore NootkaFault zone (NFZ) region that separates the JdF from the Explorer microplate. High-resolutionrelative earthquake locations and new seismic velocity models strongly suggest that these regionsof dense seismicity are influenced by fluids. Fluids enter the subduction system throughthe oceanic plate and are expelled by increasing pressure and temperature conditions. Subsequentfluid redistribution likely influences the distribution of seismicity. The Texada cluster ofseismicity is revealed to manifest ∼20 km long faults within the JdF slab, which are inferred tobe structures inherited from a propagator wake that exhibits increased levels of hydration. Inthe Puget Lowlands, robust, tomographically-derived measurements of Poisson’s ratio suggestthat the Washington forearc is dominated by the accreted Siletzia terrane, and that this >10km thick unit of flood basalts inhibits infiltration of slab-derived fluids into the overriding crust,thereby concentrating earthquakes beneath it. A complex configuration of oceanic lithosphereat the NFZ is revealed by newly determined seismic velocity models, and the association oflarge earthquakes and abundant microseismicity with plate bending faults suggests that fluidingress lowers effective normal stresses and promotes brittle failure producing Vancouver Island’slargest earthquakes in recent decades. The penultimate chapter of this thesis presentsa new methodology for determining changes in seismic velocity from seismic coda. Repeatingearthquakes and ambient noise correlations from Haida Gwaii are utilized to demonstrate thatthe velocity of the shallow crust decreased following the 2012 Mw 7.8 earthquake, which weinterpret as due to earthquake-induced changes in fluid-filled porosity.

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