Southern Vancouver Island lies on the forearc of the Cascadia subduction zone, north of a concave bend in the plate boundary centred around the Olympic Mountains. The bend in the margin coincides with a significant decrease in northward-directed trench-parallel forearc migration, and a network of active crustal faults in the Puget Lowland east of the Olympic Mountains accommodates permanent north-south shortening and transpression. The nature of forearc deformation on southern Vancouver Island is less well constrained, due in part to the unknown extent and kinematics of active crustal faulting. Recent work has shown that a brittle fault zone associated with the Eocene terrane-bounding Leech River fault has produced at least two surface-rupturing earthquakes in the late Quaternary. I use LiDAR-derived topographic data, slip-sense indicator analysis of slickenlines on fault planes, electrical resistivity tomography (ERT), and ground-penetrating radar (GPR) to investigate the geometry, kinematics, and Quaternary activity along the eastern half of the active, brittle Leech River fault zone. My mapping reveals a complex, near-vertical zone up to 1 km wide and 25 km long that exhibits many characteristics of a strike-slip fault. Displaced Quaternary deposits are observed directly at two sites in the western 8 km of the study area, and inferred through geophysical imagery, topographic data, and liquefaction features to extend to the eastern terrestrial extent of the fault zone towards previously mapped active faults in the Juan de Fuca Strait and the Darrington-Devil’s Mountain fault zone in western Washington. I use fault kinematics and geometry to show that the eastern Leech River fault zone has been reactivated as a right-lateral strike-slip fault that accommodates forearc deformation within the modern stress field north of the Olympic Mountains. / Graduate / 2018-12-01

At the northern extent of the Cascadia subduction zone, the subducting Explorer and Juan de Fuca plates interact across a translational deformation zone, known as the Nootka fault zone. The Seafloor Earthquake Array Japan-Canada Cascadia Experiment (SeaJade) was designed to study this region. In two parts (SeaJade I and II, deployed from July – September 2010 and January – September 2014), seismic data from the SeaJade project has led to several important discoveries. Hypocenter distributions from SeaJade I and II indicate primary and secondary conjugate faults within the Nootka fault zone. Converted phase analysis and jointly determined seismic tomography with double-difference relocated hypocenters provide evidence to several velocity-contrasting interfaces seaward of the Cascadia subduction front at depths of ~4-6 km, ~6-9 km, ~11-14 km, and ~14-18 km, which have been interpreted as the top of the oceanic crust, upper/lower crust boundary, oceanic Moho, and the base of the highly fractured and seawater/mineral enriched veins within oceanic mantle. During SeaJade II, a MW 6.4 mainshock and subsequent aftershocks, known as the Nootka Sequence, highlighted a previously unidentified fault within the subducted Explorer plate. This fault reflects the geometry of the subducting plate, showing downward bending of the plate toward the northwest. This plate bend can be attributed to negative buoyancy from margin parallel mantle flow induced by intraslab tearing further northwest. Seismic tomography reinforces the conclusions drawn from the Nootka Sequence hypocenter distribution. Earthquakes from the entire SeaJade II catalogue reveal possible rotated paleo-faults, identifying the former extent of the Nootka fault zone from ~3.5 Ma. / Graduate

In the late Pleistocene, most of British Columbia and northern Washington was covered by the Cordilleran ice sheet. The weight of the ice sheet caused up to several hundred metres of depression of the Earth’s crust. This caused relative sea level to be higher in southwestern British Columbia despite lower global eustatic sea level. After deglaciation, postglacial rebound of the crust caused sea level to quickly drop to below present levels. The rate of sea-level fall is used here to determine the rheology of the mantle in southwestern British Columbia. The first section of this study deals with determination of the postglacial sea-level history in the Victoria area. Constraints on sea-level position come from isolation basin cores collected in 2000 and 2001, as well as from previously published data from the past 45 years. The position of sea-level is well constrained at elevations greater than -4 m, and there are only loose constraints below that. The highstand position in the Victoria area is between 75-80 m. Sea level fell rapidly from the highstand position to below 0 m between 14.3 and 13.2 thousand calendar years before present (cal kyr BP). The magnitude of the lowstand position was between -11 and -40 m. Though there are few constraints on the lowstand position, analysis of the crustal response favours larger lowstand. Well constrained sea-level histories from Victoria, central Strait of Georgia and northern Strait of Georgia are used to model the rheology of the mantle in southwestern British Columbia. A new ice sheet model for the southwestern Cordillera was developed as older models systematically underpredicted the magnitude of sea level in late glacial times. Radiocarbon dates are compiled to provide constraints on ice sheet advance and retreat. The Cordillera ice sheet reached maximum extent between 17 and 15.4 cal kyr BP. After 15.4 cal kyr, the ice sheet retreated, and by 13.7 cal kyr BP Puget Sound, Juan de Fuca Strait and Strait of Georgia were ice free. By 10.7 cal kyr BP, ice was restricted to mountain glaciers at levels similar to present. With the new ice model, and using an Earth model with a 60 km lithosphere, asthenosphere with variable viscosity and thickness, and transitional and lower mantle viscosity based on the VM2 Earth model, predicted sea level matches the observed sea level constraints in southwestern British Columbia. Nearly identical predicted sea-level curves are found using asthenosphere thicknesses between 140-380 km with viscosity values between 3x10^18 and 4x10^19 Pa s. Predicted sea level is almost completely insensitive to the mantle below the asthenosphere. Modeled present day postglacial uplift rates are less than 0.5 mm yr^-1. Despite the tight fit of the predicted sea level to observed late-glacial sea level observations, the modelling was not able to fit the early Holocene rise of sea level to present levels in the central and northern Strait of Georgia.

The primary focus of this thesis is to examine the detailed seismic structure of the northern Cascadia margin, including the Cascadia basin, the deformation front and the continental shelf. The results of this study are contributing towards understanding sediment deformation and tectonics on this margin. They also have important implications for exploration of hydrocarbons (oil and gas) and natural hazards (submarine landslides, earthquakes, tsunamis, and climate change). The first part of this thesis focuses on the role of gas hydrate in slope failure observed from multibeam bathymetry data on a frontal ridge near the deformation front off Vancouver Island margin using active-source ocean bottom seismometer (OBS) data collected in 2010. Volume estimates (∼ 0.33 km^3) of the slides observed on this margin indicate that these are capable of generating large (∼ 1 − 2 m) tsunamis. Velocity models from travel time inversion of wide angle reflections and refractions recorded on OBSs and vertical incidence single channel seismic (SCS) data were used to estimate gas hydrate concentrations using effective medium modeling. Results indicate a shallow high velocity hydrate layer with a velocity of 2.0 − 2.1 km/s that corresponds to a hydrate concentration of 40% at a depth of 100 m, and a bottom simulating reflector (BSR) at a depth of 265 − 275 m beneath the seafloor (mbsf). These are comparable to drilling results on an adjacent frontal ridge. Margin perpendicular normal faults that extend down to BSR depth were also observed on SCS and bathymetric data, two of which coincide with the sidewalls of the slump indicating that the lateral extent of the slump is controlled by these faults. Analysis of bathymetric data indicates, for the first time, that the glide plane occurs at the same depth as the shallow high velocity layer (100±10 mbsf). In contrast, the glide plane coincides with the depth of the BSR on an adjacent frontal ridge. In either case, our results suggest that the contrast in sediments strengthened by hydrates and overlying or underlying sediments where there is no hydrate is what causing the slope failure on this margin. The second part of this dissertation focuses on obtaining the detailed structure of the Cascadia basin and frontal ridge region using mirror imaging of few widely spaced OBS data. Using only a small airgun source (120 cu. in.), our results indicate structures that were previously not observed on the northern Cascadia margin. Specifically, OBS migration results show dual-vergence structure, which could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Understanding the physical and mechanical properties of the basal layer has important implications for understanding earthquakes on this margin. The OBS migrated image also clearly shows the continuity of reflectors which enabled the identification of thrust faults, and also shows the top of the igneous oceanic crust at 5−6 km beneath the seafloor, which were not possible to identify in single-channel and low-fold multi-channel seismic (MCS) data. The last part of this thesis focuses on obtaining detailed seismic structure of the Vancouver Island continental shelf from MCS data using frequency domain viscoacoustic full waveform inversion, which is first of its kind on this margin. Anelastic velocity and attenuation models, derived in this study to subseafloor depths of ∼ 2 km, are useful in understanding the deformation within the Tofino basin sediments, the nature of basement structures and their relationship with underlying accreted terranes such as the Crescent and the Pacific Rim terranes. Specifically, our results indicate a low-velocity zone (LVZ) with a contrast of 200 m/s within the Tofino basin sediment section at a depth 600 − 1000 mbsf over a lateral distance of 10 km. This LVZ is associated with high attenuation values (0.015 − 0.02) and could be a result of over pressured sediments or lithology changes associated with a high porosity layer in this potential hydrocarbon environment. Shallow high velocities of 4 − 5 km/s are observed in the mid-shelf region at depths > 1.5 km, which is interpreted as the shallowest occurrence of the Eocene volcanic Crescent terrane. The sediment velocities sharply increase about 10 km west of Vancouver Island, which probably corresponds to the underlying transition to the Mesozoic marine sedimentary Pacific Rim terrane. High attenuation values of 0.03 − 0.06 are observed at depths > 1 km, which probably corresponds to increased clay content and the presence of mineralized fluids. / Graduate / 0373 / 0372 / 0605 / subbarao@uvic.ca

Seafloor compliance is a non-intrusive geophysical method sensitive to the shear modulus of the sediments below the seafloor. A compliance analysis requires the computation of the frequency dependent transfer function between the vertical stress, produced at the seafloor by the ultra low frequency passive source-infra-gravity waves, and the resulting displacement, related to velocity through the frequency. The displacement of the ocean floor is dependent on the elastic structure of the sediments and the compliance function is tuned to different depths, i.e., a change in the elastic parameters at a given depth is sensed by the compliance function at a particular frequency. In a gas hydrate system, the magnitude of the stiffness is a measure of the quantity of gas hydrates present. Gas hydrates contain immense stores of greenhouse gases making them relevant to climate change science, and represent an important potential alternative source of energy. Bullseye Vent is a gas hydrate system located in an area that has been intensively studied for over 2 decades and research results suggest that this system is evolving over time. A partnership with NEPTUNE Canada allowed for the investigation of this possible evolution. This thesis describes a compliance experiment configured for NEPTUNE Canada’s seafloor observatory and its failure. It also describes the use of 203 days of simultaneously logged pressure and velocity time-series data, measured by a Scripps differential pressure gauge, and a Güralp CMG-1T broadband seismometer on NEPTUNE Canada’s seismic station, respectively, to evaluate variations in sediment stiffness near Bullseye. The evaluation resulted in a (- 4.49 x10-3± 3.52 x 10-3) % change of the transfer function of 3rd October, 2010 and represents a 2.88% decrease in the stiffness of the sediments over the period. This thesis also outlines a new algorithm for calculating the static compliance of isotropic layered sediments.

Seafloor compliance is a non-intrusive geophysical method sensitive to the shear modulus of the sediments below the seafloor. A compliance analysis requires the computation of the frequency dependent transfer function between the vertical stress, produced at the seafloor by the ultra low frequency passive source-infra-gravity waves, and the resulting displacement, related to velocity through the frequency. The displacement of the ocean floor is dependent on the elastic structure of the sediments and the compliance function is tuned to different depths, i.e., a change in the elastic parameters at a given depth is sensed by the compliance function at a particular frequency. In a gas hydrate system, the magnitude of the stiffness is a measure of the quantity of gas hydrates present. Gas hydrates contain immense stores of greenhouse gases making them relevant to climate change science, and represent an important potential alternative source of energy. Bullseye Vent is a gas hydrate system located in an area that has been intensively studied for over 2 decades and research results suggest that this system is evolving over time. A partnership with NEPTUNE Canada allowed for the investigation of this possible evolution. This thesis describes a compliance experiment configured for NEPTUNE Canada’s seafloor observatory and its failure. It also describes the use of 203 days of simultaneously logged pressure and velocity time-series data, measured by a Scripps differential pressure gauge, and a Güralp CMG-1T broadband seismometer on NEPTUNE Canada’s seismic station, respectively, to evaluate variations in sediment stiffness near Bullseye. The evaluation resulted in a (- 4.49 x10-3± 3.52 x 10-3) % change of the transfer function of 3rd October, 2010 and represents a 2.88% decrease in the stiffness of the sediments over the period. This thesis also outlines a new algorithm for calculating the static compliance of isotropic layered sediments.