UBC Theses and Dissertations

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UBC Theses and Dissertations

A collection of earthquake studies throughout western British Columbia Littel, Geena


The tectonic regime of western British Columbia is largely governed by convergence of the offshore oceanic Pacific and Juan de Fuca plates with the continental North American plate. Offshore British Columbia, the Juan de Fuca plate, a remnant of the Farallon plate, subducts beneath Vancouver Island at the Cascadia subduction zone. The Pacific plate obliquely slips along the Queen Charlotte fault, a major translational fault offshore Haida Gwaii extending to southeast Alaska. These zones are joined by the Queen Charlotte triple junction, a tectonically complex region that is rapidly evolving over geologic timescales. Western mainland British Columbia experiences residual transpressional motion from oblique Pacific plate convergence, and volcanic hot spot activity. I use earthquakes to better understand the underlying tectonic processes of these distinct regions, with each presenting its own unique challenges and scientific insights. Offshore, from analysis of a new earthquake catalog I suggest that the triple junction can be modelled as a migrating pull-apart basin under regional transpression. Furthermore, breakup of the Explorer microplate likely occurs as seismicity along a predetermined strength fabric in the crust that was generated during its formation at a spreading ridge. Western British Columbia is generally seismically quiet, save for a unique concentration of earthquakes near the intersection of a major shear zone and a volcanic hot spot trend called the Anahim Volcanic Belt. I investigate these earthquakes by determining accurate hypocenters, focal mechanism solutions, and principal stresses, and by analyzing their spatio-temporal behavior. I interpret them to be a result of reactivated fault strands under northeast-directed extension in weakened lithosphere of the volcanic belt. In the Cascadia subduction zone, I apply an improved detection method to small earthquakes that accompany slow slip along the plate boundary called low-frequency earthquakes. By synthesizing accurate locations of these earthquakes with previous geophysical observations, I draw a direct link between these earthquakes and material transfer in the subduction zone from the downgoing oceanic plate to the overriding continental crust. Finally, I describe an approach to ascertaining the origin of the interesting frequency characteristics of these small earthquakes using a new seismic array on southern Vancouver Island.

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