UBC Theses and Dissertations
Studies of forearc seismicity and structure in British Columbia and Washington State Merrill, Reid
British Columbia and Washington state encompass the northern extent of the Cascadia subduction zone and the southern termination of the Queen Charlotte Transform fault. Convergence between the North American plate and oceanic Juan de Fuca (JdF), Explorer, and Pacific plates is accommodated by subduction, underthrusting, and transform faulting and produces abundant seismicity that is unevenly distributed along the margin. This thesis probes several earthquake-dense regions in Cascadia that include a unique cluster of seismicity south of Texada Island in BC, the highly active Puget Lowlands in Washington state, and the offshore Nootka Fault zone (NFZ) region that separates the JdF from the Explorer microplate. High-resolution relative earthquake locations and new seismic velocity models strongly suggest that these regions of dense seismicity are influenced by fluids. Fluids enter the subduction system through the oceanic plate and are expelled by increasing pressure and temperature conditions. Subsequent fluid redistribution likely influences the distribution of seismicity. The Texada cluster of seismicity is revealed to manifest ∼20 km long faults within the JdF slab, which are inferred to be structures inherited from a propagator wake that exhibits increased levels of hydration. In the Puget Lowlands, robust, tomographically-derived measurements of Poisson’s ratio suggest that the Washington forearc is dominated by the accreted Siletzia terrane, and that this >10 km 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 lithosphere at the NFZ is revealed by newly determined seismic velocity models, and the association of large earthquakes and abundant microseismicity with plate bending faults suggests that fluid ingress lowers effective normal stresses and promotes brittle failure producing Vancouver Island’s largest earthquakes in recent decades. The penultimate chapter of this thesis presents a new methodology for determining changes in seismic velocity from seismic coda. Repeating earthquakes and ambient noise correlations from Haida Gwaii are utilized to demonstrate that the velocity of the shallow crust decreased following the 2012 Mw 7.8 earthquake, which we interpret as due to earthquake-induced changes in fluid-filled porosity.
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